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Probing the electronic structure of dioxygen as a ligand : using x-ray absorption spectroscopy to quantify… Covelli, Danielle Sarah 2011

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PROBING THE ELECTRONIC STRUCTURE OF DIOXYGEN AS A LIGAND: USING X-RAY ABSORPTION SPECTROSCOPY TO QUANTIFY BACKBONDING by  Danielle Covelli  A THESIS SUBMITTED FOR PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in THE DEPARTMENT OF GRADUATE STUDIES (Chemistry)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  August 2011 © Danielle Covelli, 2011 ii  ABSTRACT The search for more economical and environmentally friendly oxidation catalysts for organic functional group transformations is currently one of the most prevalent research areas in chemistry. The coordination and activation of dioxygen by transition metal complexes holds particular promise and has thus been thoroughly investigated. Although work has predominantly focused on synthesis of new transition metal dioxygen complexes, determining how the dioxygen ligand interacts with transition metals is also particularly critical. The nature of the M-O2 bond should have important implications on both the chemical and structural properties of the complexes. This thesis focuses on developing and exploring the spectroscopic characteristics of a series of newly reported M-O2 with highly unusual bonding. Such complexes, best described as singlet dioxygen adducts, represent a new class of metal dioxygen complexes with characteristics that are very different from typical metal-superoxo and peroxo complexes. The electronic properties of a variety of rhodium and ruthenium complexes were explored utilizing a combination of synchrotron-based X-ray Absorption Spectroscopy (XAS) techniques in conjunction with Density Functional Theory (DFT) calculations. To quantitatively investigate the complexes, a new fitting methodology was developed, and is described herein. For several of the rhodium dioxygen complexes, the Rh L2,3-edge data provided evidence that no formal oxidation occurred at the metal center upon dioxygen coordination. The data extracted from the experimental spectra provided the first quantitative π-backbonding information for second row transition metal complexes known thus far. Both Rh K-edge XAS and DFT results corroborate the findings from the L-edge spectra. A set of complementary ruthenium complexes, thought to have similar M-O2 binding characteristics, were studied in an analogous manner. Ultimately, this thesis provides the first example of utilizing second row transition metal L2,3-edge XAS data to experimentally determine π-backbonding. Although the research described herein focuses on π-backbonding between dioxygen and transition metals, it provides a basis for applying similar experimental strategies for investigating π-backbonding interactions in other systems of catalytic interest. iii  TABLE OF CONTENTS ABSTRACT .................................................................................................................... ii TABLE OF CONTENTS ................................................................................................ iii LIST OF TABLES .......................................................................................................... vi LIST OF FIGURES ........................................................................................................vii LIST OF EQUATIONS .................................................................................................... x LIST OF SCHEMES ...................................................................................................... xi LIST OF ABBREVIATIONS ..........................................................................................xii ACKNOWLEDGEMENTS ............................................................................................xiv Chapter 1 : INTRODUCTION ........................................................................................ 1 1.1 GENERAL OUTLINE ....................................................................................... 1 1.2 DIOXYGEN AS A LIGAND ................................................................................... 3 1.2.1. The Electronic Structure of Molecular Oxygen .............................................. 3 1.2.2 Dioxygen Bound To Transition Metals ............................................................ 6 1.2.3 Oxidation Catalysts Using Molecular Oxygen as the Oxidant ........................10 1.2.4 Backbonding in Transition Metal Complexes .................................................14 Chapter 2 : METHODS .................................................................................................20 2.1: GENERAL INTRODUCTION TO XAS ................................................................20 2.1.1 Synchrotron Radiation ...................................................................................21 2.1.1.1 Dipole Magnets and Insertion Devices ................................................23 2.2 XAS SPECTRA ....................................................................................................24 2.2.1 XAS Transitions and Selection Rules ............................................................26 2.3: LIGAND K-EDGE XANES ..................................................................................28 2.3.1 Pre-edge Peak Intensity – Metal-ligand Covalency .......................................29 2.4 TRANSITION METAL K-EDGE XANES ..............................................................32 2.5 TRANSITION METAL L-EDGE XANES ..............................................................34 iv  2.6 EXPERIMENTAL SETUP ....................................................................................36 2.6.1 Beamline 7-3 .................................................................................................36 2.6.2 Beamline 4-3 .................................................................................................38 2.7 Density Functional Theory Calculations ..........................................................39 2.7.1 Basics of DFT ................................................................................................39 2.7.2 Relevant Types of DFT Calculations .............................................................43 Chapter 3 : XAS INVESTIGATIONS OF RHODIUM-DIOXYGEN COMPLEXES .........45 3.1 INTRODUCTION ..................................................................................................45 3.1.1 Rhodium Complexes Examined ....................................................................51 3.2 EXPERIMENTAL .................................................................................................53 3.2.1 Collection of Rhodium XAS Data ...................................................................53 3.3 PROCESSING OF RHODIUM XAS DATA ..........................................................54 3.3.1 Rhodium L2,3-Edge XAS Data Processing Steps ...........................................55 3.4 DATA ANALYSIS AND DISCUSSION ................................................................61 3.4.1 Results and Discussion of DFT Calculations .................................................68 3.4.2 Quantifying π-Backbonding ...........................................................................79 Chapter 4 : XAS INVESTIGATIONS OF RUTHENIUM DIOXYGEN COMPLEXES .....82 4.1 INTRODUCTION ..................................................................................................82 4.1.1 Ruthenium Complexes Investigated ..............................................................84 4.2 EXPERIMENTAL .................................................................................................85 4.2.1 Data Collection and Processing .....................................................................85 4.3 DATA ANALYSIS AND DISCUSSION ................................................................91 4.3.1 Discussion of Complexes 6-Cl, 6-O2, 7-Cl and 7-O2 ......................................91 4.3.2 Discussion of 8-O2 and 8-CO Complexes ......................................................97 4.4 COMPARING DIOXYGEN BACKBONDING .....................................................105 Chapter 5 : CONCLUSIONS AND FUTURE WORK ..................................................107 v  REFERENCES ............................................................................................................111 APPENDIX A: Fitting Results of Rhodium Complexes ...........................................119 APPENDIX B: Fitting Results of Ruthenium Complexes ........................................129 APPENDIX C: X-ray Crystallography Compared with DFT Results for Geometries .....................................................................................................................................140        vi  LIST OF TABLES Table 1.1. Generally accepted parameters for O2 ions and peroxo-bound and superoxo- bound metal complexes.9,10,18 ......................................................................................... 8 Table 2.1.  Energy of importance for XAS edges examined in this thesis work.61 .........20 Table 3.1. Quantitative parameters extracted from fully fitted spectra of 4 and 5. .........63 Table 3.2. Hirshfeld and Voronoi charges for the Rhodium metal centers extracted from single point DFT calculations. ........................................................................................73 Table 3.3. Tabulated O-O bond distances and O-O stretching frequencies from DFT calculations. ...................................................................................................................77 Table 3.4. Relevant fit parameters from fits of complexes 1-5. ......................................80 Table 4.1. Important parameters extracted from fitted Ru L-edge spectra. ....................94 Table 4.2. Important parameters extracted from fully fitted L2,3-edge XAS spectra of 8- CO and 8-O2. .................................................................................................................99 Table 4.3. Various extracted parameters related to backbonding for 1, 2, and 8-O2. ..106           vii  LIST OF FIGURES Figure 1.1. Qualitative Molecular Orbital diagram of molecular oxygen representing its ground state electronic configuration (3Σg). ..................................................................... 4 Figure 1.2. Energy state diagram indicating the relative energies of the three lowest lying states of molecular oxygen. .................................................................................... 5 Figure 1.3. Four major transition metal-dioxygen structural bonding motifs, as proposed by Vaska. ........................................................................................................................ 7 Figure 1.4. Depiction of the current bonding motifs of transition metal-dioxygen complexes. ..................................................................................................................... 8 Figure 1.5. Proposed catalytic cycle for the Wacker oxidation process. ........................12 Figure 1.6. Illustration of Dewar-Chatt-Duncanson model for olefin-transition metal backbonding. .................................................................................................................15 Figure 2.1. Schematic diagram representing important components of a synchrotron facility. ............................................................................................................................23 Figure 2.2.  A typical XAS spectrum highlighting the two distinct regions: XANES AND EXAFS. (Spectrum shown is of a Vanadium cyclam complex). .....................................25 Figure 2.3 XANES spectrum showing the pre-edge (pink) and the edge jump (red). (Spectrum is of Cs2CuCl4) .............................................................................................26 Figure 2.4.  An example of a Cl K-edge XAS spectrum. (Spectrum of Cs2CuCl4) .........30 Figure 2.5. A schematic representation of a Cl K pre-edge transition. ...........................31 Figure 2.6. An example of a vanadium metal K-edge XAS spectrum for a vanadium cyclam complex. The inset shows a close-up of the pre-edge region for clarification. (Note the pre-edge region is intense due to a strong V=O bond). .................................33 Figure 2.7. A schematic illustration of high energy beamline 7-3 at SSRL. ...................38 Figure 3.1. Rh L-edge XAS of RhCl3 (Rh(III) reference) and Rh(IPr)(OAc)(CO)2 (Rh(I) reference) compounds (adapted from reference 1, where IPr is 1,3-bis(2,6- diisopropylphenyl)imidazol-2-ylidene). ...........................................................................48 Figure 3.2. Simplified ligand field picture of d orbital splittings for Rh(III) (octahedral) and Rh(I) (square planar) complexes. ..................................................................................48 Figure 3.3. Rh L3-edge XAS of Rh-O2 complex (black) compared to the Rh(I) (red) and Rh(III) (blue) references (adapted from reference 1). ....................................................49 Figure 3.4. Rhodium complexes examined by Rh L2,3-edge and Rh K-edge XAS. ........52 viii  Figure 3.5. Generic model used to quantitatively fit experimental Rh L2,3-edge XAS spectra of complexes 1-5. ..............................................................................................59 Figure 3.6. Normalized and background subtracted Rh L3-edge XAS spectra of 4 (Rh (I) reference, Rh(IPr)2H2Cl) and 5 (Rh(III) reference, Rh(IPr)(OAC)(CO)2). .......................62 Figure 3.7. Normalized and background subtracted Rh L3-edge XAS spectra of 1, 4 and 5. ....................................................................................................................................64 Figure 3.8. Normalized and background subtracted Rh L3-edge XAS spectra of 1, 2, 4 and 5. .............................................................................................................................65 Figure 3.9. Normalized and background subtracted Rh L3-edge XAS spectra of 3, 4 and 5. ....................................................................................................................................66 Figure 3.10. Normalized and background subtracted Rh K-edge XAS spectra for 1, 2 and 3. .............................................................................................................................67 Figure 3.11. A valence MO diagram of 1 constructed from single point DFT calculation (Hydrogens left out for clarity and contour of 0.05 used to represent orbitals). ..............69 Figure 3.12. A valence MO diagram of 2 constructed from single point DFT calculation (Hydrogens left out for clarity and contour of 0.05 used to represent orbitals). ..............70 Figure 3.13. A valence MO diagram of 3+ constructed from single point DFT calculation (Hydrogens left out for clarity and contour of 0.05 used to represent orbitals). ..............71 Figure 3.14. A proposal of the two limiting cases of rhodium dioxygen bonding exhibited. .......................................................................................................................74 Figure 3.15. An example of the N-Methyl derivative of 1 used for linear transit DFT calculations. ...................................................................................................................74 Figure 3.16. The percentage of Rh d, Cl p, and O p character present in the LUMO as the Rh-Cl bond is shortened in a linear transit DFT calculation. ....................................76 Figure 3.17. A list of N-methyl-derivatives of 1 where the ligand trans to the dioxygen was altered for DFT calculations....................................................................................77 Figure 3.18. Graphical DFT results showing the affect of the trans ligand on RhO2 backbonding. .................................................................................................................78 Figure 4.1. Ruthenium-dioxygen complex reported by Whittlesey group in 2009. .........83 Figure 4.2. Representation of the six ruthenium complexes that were studied in this thesis. ............................................................................................................................85 ix  Figure 4.3. The Cl K-edge XAS spectrum of Cs2CuCl4, used as a reference compound to calibrate all ruthenium data. The intense pre-edge feature used for calibration purposes is highlighted. .................................................................................................87 Figure 4.4. An example of a Ru L2,3-edge spectrum with the Cl K-edge highlighted to show its close proximity to the Ru L3-edge. ...................................................................89 Figure 4.5. The generic model used to simultaneously fit Ru L2,3-edge and Cl K-edge XAS data. ......................................................................................................................90 Figure 4.6. A comparison of the normalized Ru L3-edge XAS spectra of 6-Cl and 6-O2. .......................................................................................................................................92 Figure 4.7. A comparison of the normalized Ru L3-edge XAS spectra for 7-Cl and 7-O2. .......................................................................................................................................93 Figure 4.8. Putative ligand field splittings for the ruthenium d6, d5, and d4 electron configurations, assuming a change from pseudo-octahedral geometry to square pyramidal geometry. ......................................................................................................95 Figure 4.9. A comparison of the normalized Ru K-edge XAS spectra of 6-Cl and 6-O2. 96 Figure 4.10. A comparison of the normalized Ru K-edge XAS spectra of 7-Cl and 7-O2. .......................................................................................................................................97 Figure 4.11. A comparison of the normalized Ru L2-edge XAS spectra of 8-CO and 8- O2. .................................................................................................................................98 Figure 4.12. A comparison of the normalized Ru K-edge XAS spectra of 8-CO and 8-O2. .....................................................................................................................................100 Figure 4.13. An illustration exemplifying how the different strengths of the ligand-metal bonds can affect the energy position of the backbonding orbital. (Note the RuCOπ* orbital could be slightly above the Ru 4dζ*, but the representation shown is in agreement with DFT predictions). ................................................................................102 Figure 4.14. A valence MO diagram extracted from single point DFT calculation of 8-CO (H atoms left out for clarity). .........................................................................................103 Figure 4.15. A valence MO diagram extracted from a single point DFT calculation of 8- O2 (H atoms left out for clarity). ....................................................................................104 Figure 5.1. An illustration of transition metal-dioxygen synergistic bonding, similar to DCD model for alkenes. ...............................................................................................107  x  LIST OF EQUATIONS Equation 2.1.  The Fermi Golden Rule69 ........................................................................27 Equation 2.2.  The Fermi Golden Rule re-written as a function of initial states.69 ..........27 Equation 2.3.  Approximated Fermi Golden Rule to describe XAS spectral shape.69 ....28 Equation 2.4.  General wave function for acceptor orbital.76 ..........................................31 Equation 2.5.  General equation for ligand pre-edge intensity.76 ...................................32 Equation 2.6. Electron correlation functional in terms of density.104 ..............................40 Equation 3.1. Model used to fit experimental Rh L2,3-edge XAS spectrum of 1. ............58 Equation 4.1. General equation used to fit Ru L-edge XAS data. ..................................91    xi  LIST OF SCHEMES Scheme 1.1. Latimer diagram for the stepwise reduction of molecular oxygen under acidic and basic conditions (adapted from reference 6). ................................................. 5 Scheme 1.2. Wacker Oxidation Reaction. .....................................................................12 Scheme 3.1. Synthesis, by Milstein and coworkers, of Rh-O2 pincer complex with short O-O bond length. ...........................................................................................................46 Scheme 3.2. An example of the Crudden group synthesis of novel Rh-O2 complex. ....47 Scheme 4.1. The synthetic route utilized by the Whittlesey group for the synthesis of this ruthenium-dioxygen complex. .................................................................................83  xii  LIST OF ABBREVIATIONS ADF  Amsterdam Density Functional BarF -               Tetrakis (3,5-bis(trifluoromethyl)phenyl)borate CDA  Charge Decomposition Analysis CIF  Crystallographic Information File CT  Charge Transfer DCD  Dewar-Chatt-Duncanson DFT  Density Functional Theory DOC  Differential Orbital Covalency EDA  Energy Decomposition Analysis EPR  Electron Paramagnetic Resonance ETS  Extended Transition State EXAFS Extended X-ray Absorption Fine Structure GGA  Generalized Gradient Approximation GTOs  Gaussian-type Orbitals HOMO Highest Occupied Molecular Orbital IMes  1,3-bis(2,4,6-trimethyl)(phenyl)imidazol-2-ylidene  IPr   1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene IR  Infrared KS  Kohn-Sham LDA  Local Density Approximation LMCT  Ligand-to-Metal Charge Transfer xiii  LUMO  Lowest Unoccupied Molecular Orbital MeCN  Acetonitrile MLCT  Metal-to-Ligand Charge Transfer MO  Molecular Orbital NBO  Natural Based Orbitals NEXAFS Near Edge X-ray Absorption Fine Structure NHC  N-heterocyclic carbene NMR  Nuclear Magnetic Resonance rR  Resonance Raman RT  Room Temperature SCF  Self Consistent Field SSRL  Stanford Synchrotron Radiation Lightsource STOs  Slater-type Orbitals t-Bu  tert-Butyl TZP  Triple-Zeta with Polarization XAS  X-ray Absorption Spectroscopy XANES X-ray Absorption Near Edge Spectroscopy xiv  ACKNOWLEDGEMENTS I want to begin by trying to express my sincere gratitude to Pierre Kennepohl, my research supervisor. Pierre is one of the most easy-going, friendly and encouraging people I have ever had the pleasure of knowing. It was these characteristics, as well as many others, that brought me into this wonderful field of synchrotron research when I was only a third year undergraduate. Without his support and guidance, I feel the results presented in this thesis work would not have been accomplished. His enthusiasm and nurturing character have made my time here at UBC positive and memorable. Thank you Pierre, you are truly wonderful and it has been a pleasure working for you. I really appreciate all the time and effort you have spent helping support and educate me throughout the years. I am also very thankful to Mario Delgado, former Kennepohl group member. Mario is responsible for the creation of the Matlab based fitting program used to process and analyze most of the data presented in this thesis work. His flexibility on designing the program and listening to changes I suggested were greatly appreciated. He is also responsible for developing the methodology that was essential for fitting all of the complicated XAS spectra presented in this thesis. His support in the lab on the whole, including numerous conversations explaining and discussing abstract and complicated concepts were greatly appreciated and essential to my success. I would also like to express my appreciation to the staff at SSRL for their assistance in running various XAS experiments, in particular Serena DeBeer, Allyson Aranda, Mathew Latimer, and Erik Nelson. Additionally, I would like to thank my various collaborators, Cathleen Crudden and coworkers at Queen‟s University for supplying all of the rhodium complexes and Michael Whittlesey and coworkers at the University of Bath for all of the ruthenium complexes. I would also like to thank Dr. Michael Wolf for allowing me to use his non-aqueous electrochemistry set up, and Glen Bremner for helping me use it. Also, I would like to thank Dr. Dan Bizzotto for many helpful electrochemistry discussions and for allowing me to perform some aqueous electrochemistry in his laboratory. His graduate students, Jannu Casanova Moreno and Amanda Richer were also great support and company while I worked. xv  I would also like to thank Jakie Thomson and Tulin Okbinoglu for helping improve grammatical and structural aspects of my thesis. Additionally, I would like to thank Edwige Otero for her suggestions on the thesis and her support and encouragement throughout both my Masters and PhD degrees. In addition, I feel the need to thank the many people in my life who have provided me with the encouragement and support I feel were essential to my success here at UBC. This includes former and current coworkers in the Kennepohl lab, all my dear friends here in Vancouver, who helped me de-stress with laughter and good times, my friends back in Ontario for always giving me reasons to make trips home, my landlords and roommates for making my living arrangements so comfortable and positive and finally my family back in Ontario for supporting and encouraging me throughout this journey. Finally, I would like to thank all of the funding support which made the thesis work possible. The research carried out was funded by NSERC (Canada). Much of the experimental work was conducted at SSRL, which is a national user facility operated by Stanford University on behalf of the United States Department of Energy. Also, portions of the computational studies presented in the thesis were performed through the Center for Higher Order Structure Elucidation (CHORSE) of the Chemistry Department. Additionally, I would like to thank MEC for awarding me a sustainability scholarship which provided additional funding for one year.         1  Chapter 1 : INTRODUCTION 1.1 GENERAL OUTLINE This thesis provides a comprehensive description of the efforts made to investigate the electronic structure of a new class of transition metal dioxygen complexes. The main goal of this thesis is to provide evidence of an experimental tool capable of directly probing π-backbonding interactions in rhodium and ruthenium complexes in order to extract quantitative information.  X-ray Absorption Spectroscopy (XAS) was the major spectroscopic method used to evaluate bonding and Density Functional Theory (DFT) calculations were utilized as support for the experimental findings. A new methodology for XAS data analysis, within the context of a Matlab code developed by Dr. Mario Delgado, was implemented for the first time herein. The application of this methodology for analyzing XAS data to quantify π-backbonding in second row transition metal complexes was a collaborative project. This dissertation provides evidence of a previously unanticipated binding mode of O2, specifically the formation of adducts of singlet dioxygen with transition metal centers.1  This new dioxygen coordination mode will be referred to as side-on singlet dioxygen throughout the thesis. Additionally, this research identifies one of the first experimental tools used to directly probe π- backbonding in transition metal complexes via X-ray spectroscopic transitions to the antibonding M-O2π* orbital. The work further explores the ability of this methodology to quantitatively assess π-backbonding for such systems by means of providing intensities with error bars that are responsible for the backbonding interaction. Although this thesis focuses on metal-dioxygen π-backbonding, the ideas and methodology presented herein could quite straight-forwardly be extended to the investigation of other π- backbonding ligands with alternate transition metals in the future. The current chapter introduces three major concepts which are pertinent to the underlying motivation for my research. The chapter begins with a discussion of the basics of the electronic structure of molecular oxygen and the resultant implications for transition metal-dioxygen bonding, and its relevance to catalytic processes. Subsequently, the attractiveness of dioxygen as a “benign” oxidant in oxidation catalysis is explored. This includes a brief overview of oxidation catalysis in general, as well as 2  certain important highlights from the literature in the development of more practical oxidation catalysts.  The chapter concludes with an examination of the importance of π- backbonding in transition metal chemistry. In particular, currently used experimental methods for evaluating π-backbonding are discussed in relation to its importance in defining the nature of the metal-ligand bond. The case is made that a method that can directly probe the orbital π-backbonding interactions between a transition metal and a ligand is an important asset in properly defining the nature and reactivity of transition metal complexes. This is used to exemplify the necessity of developing a new method whereby π-backbonding can be directly probed experimentally, in a manner allowing for quantification, which is the basis of the work in this dissertation. Chapter 2 focuses on the fundamentals of the predominant experimental technique used for this thesis work, XAS, as well as the computational density functional theory method utilized for support. This chapter begins by providing a brief introduction to the technique and the basic features of the high energy facilities where the experiments were conducted.  It continues with a comprehensive overview of the technique as a whole, including the various selection rules involved with the absorption process. Chapter 2 outlines the three major categories of XANES experiments performed, and highlights the challenges present with the metal L-edge experiments. A detailed description of the various beamlines utilized for all experimental work presented in this thesis is also provided. The chapter concludes with a general overview of the foundation of DFT in addition to a brief description of the type of calculations performed. Chapter 3 introduces the experiments performed to investigate a set of rhodium- dioxygen complexes.  This chapter begins by describing the complexes studied and discusses the experimental XAS spectra of them. This includes a detailed analysis of the spectra using the new methodology from Dr. Delgado, as well as a general description of the fitting methodology. Theoretical support from various DFT calculations is provided to corroborate experimental findings. The first quantitative experimental data on second row transition metal-ligand π-backbonding interactions is presented and details of the analysis are discussed. Chapter 4 describes the investigation of several ruthenium-dioxygen complexes, purported to have a similar M-O2 bonding motif as that described in chapter 3. The 3  complexes investigated and their corresponding experimental XAS spectra are discussed. DFT calculations are utilized to aid in the interpretation of the XAS data. The analysis of the XAS data is performed with the same methodology as was used for the rhodium complexes in chapter 3. It is determined that only certain examples of these complexes can be ascribed as side-on singlet dioxygen complexes. Chapter 5 provides a summary of the overall work presented in this dissertation. The major conclusions from the work are emphasized and key findings are highlighted. The dissertation concludes with several suggestions for future work that would be useful extensions from the research described herein. 1.2 DIOXYGEN AS A LIGAND Throughout this thesis, the term molecular oxygen will be used to refer to the gaseous state of O2; when it is bound to a transition metal the term dioxygen will be utilized. Before dioxygen can be discussed as a ligand in transition metal complexes, it is important to have an understanding of the basic properties and electronics of molecular oxygen on its own. Therefore, this section begins with a brief description of molecular oxygen, which will progress into a discussion of dioxygen as a ligand in transition metal chemistry. 1.2.1. The Electronic Structure of Molecular Oxygen Oxygen is the most abundant element on earth and is essential for human existence. Its discovery can be credited to both Joseph Priestley, an English chemist, and William Scheele, a Swedish chemist, around the year 1775, when papers on the independent discoveries were published.2 However, it was French chemist Antoine Laurent Lavoisier, who interpreted the role of oxygen in combustion and in 1777, named the molecule oxygen.3 Despite the apparent simplicity of this diatomic molecule, it exhibits unique chemical reactivity and many interesting properties including its magnetic behavior and spectroscopy that can be explained from its electronic structure. A qualitative molecular orbital (MO) diagram of molecular oxygen in its ground state is shown in Figure 1.1. From this figure, it is clear that the ground state is a triplet (with two unpaired electrons). It is the nature of this diradical and its open shell configuration, that dominates the 4  chemistry of molecular oxygen.4 The unpaired electrons that give rise to the unique electron configuration are a consequence of Hund‟s rule of spin multiplicity.  Figure 1.1. Qualitative Molecular Orbital diagram of molecular oxygen representing its ground state electronic configuration ( 3 Σg). The ground state electron configuration of molecular oxygen is given by (1ζg) 2(1ζ*u) 2(2ζg) 2(2ζ*u) 2(3ζg) 2(1πu) 4(1π*g) 2 ; all molecular orbitals are doubly occupied except the first two degenerate antibonding π*g,x and π*g,y orbitals. 5 This electronic configuration gives rise to three closely lying states, the ground state triplet (3Σg) that is shown in Figure 1.1, and two excited state singlets (1Σg, and 1Δg), which were first predicted by Mullikan.5 The closeness in states is demonstrated in Figure 1.2 with the 1Δg excited state lying 1 eV above the ground state 3Σg, and the 1Σg excited state lying only 0.6 eV above the 1Δg excited state. 6 5   Figure 1.2. Energy state diagram indicating the relative energies of the three lowest lying states of molecular oxygen. The triplet nature of the ground state of molecular oxygen is of great importance for understanding its reactivity. As can be seen from the Latimer diagram for O2 (Scheme 1.1), a non-spontaneous one electron reduction is required to weaken the O-O double bond. Once the unfavorable reduction has occurred, molecular oxygen becomes a powerful oxidant, capable of reacting with many different substrates. For reactivity involving only molecular oxygen itself or molecular oxygen and hydrogen, the thermodynamics can be estimated using tables of standard reduction potentials.7  Scheme 1.1. Latimer diagram for the stepwise reduction of molecular oxygen under acidic and basic conditions (adapted from reference 6). When molecular oxygen is reacted with organic substrates, the kinetic barrier required for reactivity is high because of the triplet ground state configuration of the oxygen. Typically, organic molecules, including many biologically relevant ones, have singlet ground states with no unpaired electrons.7 Triplet to singlet spin conversions are 6  quantum mechanically forbidden and thus these reactions are significantly slower (kinetically unfavorable) than comparable spin-allowed reactions and normally require some form of catalyst or elevated temperature to ensure reactivity.7 Since many transition metals and transition metal systems have open shell configurations they can often react more readily with molecular oxygen. 1.2.2 Dioxygen Bound To Transition Metals The study of transition metal complexes with molecular oxygen has been of considerable interest for many years due to its important applications in both chemistry and biology. The first study can be dated back to an 1852 paper by Frémy examining oxygenated ammoniacal salts of cobalt.8 Much research has been undertaken to elucidate geometric and electronic structural information of a wide variety of metal- dioxygen complexes.6,9,10 A wide range of structural studies of metal-dioxygen complexes have been carried out in order to understand the similarities and differences present in the various geometric structures. Such systematic studies can be traced back to the pioneering work of Vaska. In general, it was found that nearly all transition metals bind dioxygen and the formal oxidation states of the metals have been found to range from + 2 to + 6.9 By far, the best method utilized to obtain geometric information of such complexes has been single crystal X-ray diffraction; however, there are a few problems with using this method when dioxygen-bound species are under assessment. When the dioxygen is closely bound to a heavy transition metal, the exact locations of the oxygen atoms are difficult to pinpoint and various structures of the same complex do not always agree.8 In addition, disorder in the dioxygen ligand can further complicate the evaluation of both bond distances and bond angles.8 For this reason, other methods such as electron paramagnetic resonance (EPR) spectroscopy studies have been used in conjunction with X-ray diffraction to help classify the nature of different metal-dioxygen species.11 Other commonly used experimental methods for assessing metal-dioxygen coordination include Infrared (IR) and resonance Raman (rR) spectroscopy. Traditionally both of these techniques have been utilized as powerful probes of metal- ligand bonding.12 The O-O stretching frequencies obtained in IR spectroscopy have been useful for assessing the strength of the O-O bond in many systems.13,14 However, 7  sometimes the vibrational O-O stretch is weak or very complex (overlapping bands, hard to decipher which one belongs to O-O), and IR spectroscopy is not overly useful.1,12 In these cases, the use of rR spectroscopy becomes important.12 Resonance Raman spectroscopy uses laser excitation in a region where resonance with a strong electronic absorption band can occur. This greatly enhances vibrational bands in the spectrum making them more distinguishable. The 1976 review by Vaska placed every transition metal-dioxygen complex known at the time into one of four structural categories. Vaska noted that if considering ligated dioxygen alone, nearly all of the known structures could be divided into two categories based on the dioxygen ligand; superoxo (type I) and peroxo (type II).9 Each category could be further divided based on whether the dioxygen ligand was coordinated to one (subcategory a) or two (subcategory b) metal atoms.9 A structural representation of these four categories is shown in Figure 1.3. The four categories were based on the findings from many structural studies of metal-dioxygen complexes. With only a few exceptions, remarkably, they could all be confined to such a limited number of structural descriptions. This unified view of metal-dioxygen complexes is still considered to be quite accurate for the overwhelming majority of complexes investigated. The major exceptions involved the discovery of ɳ1 :ɳ2  and   ɳ2 : ɳ2  bridging complexes.8,15-17 An up-to-date description of transition metal-dioxygen complexes uses a more generic description whereby the complexes are first separated into two structural categories based on whether they exhibit end-on or side-on bonding.18 These two structural motifs are further divided into two categories, similarly to Vaska‟s electronic interpretation of the dioxygen ligand, either peroxo-bound or superoxo-bound.18 An illustration of the more recent categorization is provided in Figure 1.4.  Figure 1.3. Four major transition metal-dioxygen structural bonding motifs, as proposed by Vaska. 8   Figure 1.4. Depiction of the current bonding motifs of transition metal-dioxygen complexes. To determine whether a complex is considered peroxo or superoxo, structural information of the complex must be established and compared with values of the free superoxo and peroxo ions. Table 1.1 lists accepted bond lengths and stretching frequencies for molecular oxygen, the superoxide anion, and the peroxide anion, as well as coordinated dioxygen in its peroxo and superoxo bound forms. Coordinated dioxygen does not exhibit identical structural properties to the free ions as can be expected since it is altered by coordination to the metal center. For instance, peroxo compounds tend to have O-O bond distances that are shorter than free peroxide, and superoxo compounds have properties closer to, but not the same as superoxide. The latter portion of Table 1.1 depicts the more general values that can be expected for metal-dioxygen complexes. Stretching parameters have been included due to Vaska‟s observation that stretching frequencies of the O-O vibrations are related to the type of structure exhibited by the complexes.9 Currently, O-O stretching frequencies (from IR and rR spectroscopy) and O-O bond distances (from X-ray crystallography) remain the most common type of physical data utilized for structural classification of metal- dioxygen complexes. Molecule  O-O distance (Å) O-O stretch (cm-1) O2  1.21 1580 O2 -  1.33 1097 O2 2-  1.49 802 Superoxo-bound  1.2 – 1.3 1050 – 1200 Peroxo-bound  1.4 – 1.5 800 - 930  Table 1.1. Generally accepted parameters for O2 ions and peroxo-bound and superoxo-bound metal complexes. 9,10,18  9  After the large increase in structural analysis of metal-dioxygen complexes, interest in the electronic structures of the complexes grew immensely. This was in large part due to the wide variety of metal-dioxygen complexes present in biologically relevant species. Some of the important examples include the oxygen-carrying proteins myoglobin, hemoglobin, hemerythrin, and hemocyanin.10 Interest also stemmed from the widespread applications of oxidation catalysis and attempts at trying to create more sufficient models for the bonding of O2 with catalytic surfaces. 10 A unifying result for all complexes investigated showed that the binding of dioxygen to a transition metal occurs with both reduction of the number of unpaired electrons on dioxygen as well as a transfer of charge density to the dioxygen ligand.8 Thus, the transition metal-dioxygen bond results in the kinetic barrier of activation being lifted as the dioxygen is no longer in a triplet state when bound to the metal center and the dioxygen ligand develops nucleophilic character.8  Many different types of reactivity have been explored with molecular oxygen and a number of different transition metals. General overviews of these studies are presented in reviews by Valentin and Gubelmann.8,19 Importantly, the electronic nature of transition metal-dioxygen species (e.g. peroxo versus superoxo complexes) are generally determined solely on the basis of structural (crystallography) and vibrational (IR) data. This fits with the standard model of dioxygen as a ligand, which suggests that electron transfer occurs upon bonding i.e. when molecular oxygen binds to a transition metal center, the metal is oxidized by one or two electrons while the dioxygen is subsequently reduced by the same one or two electrons. Although there is significant evidence for this throughout the literature, this work uncovers a general bonding mode where the stretching frequencies and O-O bond distances are insufficient for determining the electronic nature of the M-O2 bond. Thus the complexes examined herein may be anomalies to typical bonding in transition metal-dioxygen species. Chapter 3 will discuss in detail the rhodium-dioxygen complex that was discovered which opened the path for the research undertaken in this thesis. To provide more detailed information regarding the electronic nature of the M-O2 bond in the complexes herein, XAS was used. As described in chapter 2, metal L-edge XAS provides a way of directly probing metal d orbital occupation, thereby evaluating 10  the specifics of the metal d orbital distribution. In essence, XAS can be used as a probe of electron density distribution, and thus provides a way of evaluating electronic structure. 1.2.3 Oxidation Catalysts Using Molecular Oxygen as the Oxidant  Selective oxidation reactions for functional group manipulation are of essential importance in chemistry. Selective activation of carbon-carbon and carbon-hydrogen bonds in alkanes and unactivated alkyl groups to form useful functional groups is one of the most critical challenges in chemical industry.20,21 These reactions are of particular importance because the principal source of the world‟s industrial organic chemicals emerge from petroleum that is mainly composed of reduced hydrocarbons.22 Oxidations are of particular significance due to the commercial importance of organic chemicals such as alcohols, ketones, aldehydes and acids. Aldehydes and ketones are both important precursors and intermediates for chemical industry as they are readily used in the production of flavors, fragrances, biologically active compounds and pharmaceuticals.23  Traditionally, these organic derivatives are produced by the selective oxidation of alcohols using stoichiometric amounts of oxidizing agents such as chromium salts, oxalyl chloride and hypervalant iodines which are mostly toxic and/or hazardous.23,24 These reactions are a major environmental concern as they produce both heavy metal waste and generate undesirable byproducts. Thus the search for environmentally friendly or “green” oxidation processes is in great demand and a wealth of research is currently dedicated to this area of chemistry. The quintessential green oxidant to be used for functional group transformations in chemical synthesis is molecular oxygen. It is available in abundance, with relatively no cost and is environmentally benign.21 Despite these obvious advantages, employing molecular oxygen as the sole oxidant has rarely been utilized as there are several problems associated with its use. The first of these problems was previously addressed when discussing the electronic ground state of the molecule. Since molecular oxygen has a triplet configuration, as indicated in section 1.2.1, it does not react readily with organic molecules due to the high kinetic barrier involved with spin flips. In addition to this problem, using molecular oxygen as an oxidant is often difficult to control and uncatalyzed reactions most commonly result in combustion.23 Additionally, in most 11  cases only one of the two oxygen atoms is retained following oxidation (i.e. monooxygenase activity) and therefore an over-stoichiometric amount of co-reductant is required for these oxidation processes.23 Given these unfortunate circumstances, the necessity to find greener oxidation catalysts, which use molecular oxygen as the oxidant, is an important modern issue in the chemical community and is therefore being thoroughly investigated. Some of the most promising systems for homogeneous oxidation of alkanes have employed platinum catalysts. An informative review on many of these systems and reactions is provided by Stahl and coworkers.25 Despite the promising oxidation reactions, the usefulness of the chemistry is hampered by the use of expensive and less environmentally friendly oxidants. Many attempts at utilizing platinum species that can react directly with molecular oxygen have been investigated. One of the more intriguing studies involved the reaction of a Pt(IV) dialkyl hydride species with molecular oxygen to produce a stable Pt(IV) hydroperoxide complex where insertion of O2 into the Pt-H bond was observed rather than reactions of the O2 with the alkyl group on the metal.26 This paper was particularly notable as it was the first account of a transition metal alkyl hydride cleanly reacting with molecular oxygen to generate a hydroperoxide product. This could be a significant development in the search for a viable homogeneous catalytic alkane oxidation using molecular oxygen as the sole oxidant.26 Several years later a similar finding, where a Pt(II) dimethyl compound was reacted with molecular oxygen to form a Pt(II) methylperoxo species was reported by Grice and Goldberg.27 The discovery of the insertion of O2 into a Pt-alkyl bond was fascinating as there have been several reports of Pt complexes forming stable metal-alkyl complexes through alkane activation.27 These two important papers represent examples of recent studies ongoing in search of Pt catalyzed systems capable of alkane oxidation using molecular oxygen as the sole oxidant. Palladium catalyzed oxidations may be more promising for selective alkane oxidation reactions than platinum systems. Palladium is probably the most active and useful transition metal currently being employed in organic synthesis for both oxidative and nonoxidative transformations and has revolutionized the synthetic approach to many natural products and pharmaceuticals.22 Of  particular note are cross coupling reactions 12  (nonoxidative transformations) that have become some of the most efficient methods whereby carbon-carbon and carbon-heteroatom bonds have been formed.22 Despite the expansive use of cross coupling reactions, the Wacker process is historically the most important palladium catalyzed reaction for industrial applications.28 The Wacker process, which involves the oxidation of an olefin to a carbonyl compound, has been used in chemical industry for more than forty years.22,29 The Wacker oxidation  involves the Pd(II) catalyzed reaction of ethylene with water to form acetaldehyde, which typically utilizes a PdCl2/CuCl2 catalyst mixture with hydrochloric acid and an oxidizing agent (normally molecular oxygen).29  This reaction, shown in Scheme 1.2, demonstrates that the Pd(II) is reduced to Pd(0) and subsequently re- oxidized by a Cu(II) co-catalyst (CuCl2, as it goes to CuCl), while the molecular oxygen serves to return the Cu(I) back to Cu(II), completing the catalytic cycle.29 The catalytic mechanism for the overall Wacker process is shown in Figure 1.5.  Scheme 1.2. Wacker Oxidation Reaction.   Figure 1.5. Proposed catalytic cycle for the Wacker oxidation process. 13   Despite the great success of the Wacker oxidation reaction, the synthetic potential of other palladium catalyzed oxidation reactions has not yet been realized. Although this process is important and useful, some of its features are quite restrictive causing problems for similar reactions with other organic substrates. For instance, many organic molecules are not very soluble in water, and the cocatalysts do not function effectively in organic solvents, thereby requiring alternative oxidants such as benzoquinone or Cu(II) salts to be used in place of molecular oxygen.21  Consequently these reactions have generally not been applicable to industry due to added costs, increased waste and more complicated product isolation.21 Thus a search for a new method of palladium oxidation chemistry has been in high demand. Recently, the development of homogeneous palladium catalyzed oxidation reactions that undergo direct dioxygen-coupled catalytic turnover without redox active cocatalysts have been discovered. In these reactions, catalyst regeneration occurs by the direct reaction of molecular oxygen with reduced palladium.21,22 This area of exploration has grown because of the discovery of the palladium acetate-dimethyl sulfoxide [Pd(OAc)2/DMSO] catalyst system in the early 1990s. 30,31 Following this discovery, the use of nitrogen-donor ligands with palladium grew popular, and newly developed systems performed better than the [Pd(OAc)2/DMSO] catalyst system. 32,33 Closely following the discovery of this system, new ligands for use on palladium metal were founded. Employing nitrogen-donor ligands in palladium catalyst systems showed increased stability, reactivity and selectivity, most likely due to the stable binding of the ligand to the metal center.21 The discovery of many of these systems represents a step towards discovering more practical catalysts on the path for “green” oxidation catalysts. In addition to the palladium and platinum metals, there has been research involving other transition metals for homogeneous oxidation catalysis. Similar to the discovery of the new nitrogen-donor ligands used for palladium catalysts, the use of N-heterocyclic carbenes (NHCs) as ligands for transition metals has led to an increase in the number of new catalytic systems. The first stable N-heterocyclic carbene compound was reported in 1988.34 However, it was not realized until much later that NHCs would be an important group of ligands in organometallic chemistry, especially for their use in catalysis. 14  The discovery of NHCs has been an important development in the field of inorganic chemistry. The study of the ligands themselves as well as their coordination to many transition metals has led to a field of its own. The recognition that these ligands could be of use in catalysis has led to the development of many important catalytic systems. The synthesis of new compounds utilizing these ligands is important to this thesis work as all of the compounds studied herein are based on novel compounds containing NHC ligands coordinated to either rhodium or ruthenium metal centers. The synthesis of the compounds by our collaborators may have begun by searching for more promising hydrogenation, dioxygen-activation, or homogeneous oxidation catalysts, however, the focus of this thesis work arose from the unusual and intriguing properties exhibited by the different transition metals coordinating to dioxygen. 1.2.4 Backbonding in Transition Metal Complexes Understanding the chemical bonding between transition metals and the ligands coordinated to them is of central importance to understanding the reactivity and stability of transition metal complexes in chemistry. This is generally well established when the bonding is comprised of covalent ζ bonds from donation of ligand-based lone pairs to empty orbitals on the metal center.35 However, the bonding situation becomes much more complicated when π-backbonding is involved. The idea of π-backbonding was originally suggested by Dewar in 1951, when he introduced metal-ligand orbital interactions in terms of an alkene bound to a transition metal. Dewar proposed two parts to the metal-ligand orbital interactions; ζ-donation from the ligand to the metal, reciprocated by π-back-donation from the metal to the ligand.36  This paper was very specific and thus it wasn‟t until Chatt and Duncanson used the ideas presented in Dewar‟s 1951 paper and applied them to a systematic study of metal-olefin complexes, that led to the formation of a powerful bonding description for transition metal chemistry.37 The combination of Dewar‟s initial proposal and Chatt and Duncanson‟s later work led to the formation of the “Dewar-Chatt- Duncanson” (DCD) model used to describe the two part bonding interactions between a metal and a ligand in transition metal chemistry still currently accepted and used today.38 A pictorial representation, of a transition metal and an olefin, depicting the two components of the DCD model is illustrated in Figure 1.6. 15   Figure 1.6. Illustration of Dewar-Chatt-Duncanson model for olefin-transition metal backbonding. The DCD model is a valuable tool that depicts π-backbonding in transition metal complexes and is effective in descriptively explaining the interactions between transition metals and π-backbonding ligands. However, this model simply provides an explanation for the bonding that can occur with ligands capable of π-backbonding (carbonyls (CO), cyanides (CN-), and alkenes) to transition metal centers. The use of experimental techniques to probe these interactions is needed in order to provide insight into the strength of various metal-ligand π-backbonding interactions. Currently, experimental techniques that can provide insightful information concerning transition metal-ligand π-backbonding are scarce. One experimental tool commonly employed is X-ray crystallography. For instance, when examining π-backbonding in transition metal-olefin complexes, the C=C bond lengths obtained from the crystal structure have been analyzed.39  The bond lengths of the olefin in the complex can be compared to that of the free alkene, and the shorter the C=C bond length in the complex, the lesser the degree of π-backbonding.39 It is important to note that this technique provides indirect evidence of π-backbonding as it assumes that a weakening of the C=C bond (and the resultant increase in bond length) is only caused by π- backbonding. It does not provide information regarding the allocation of electron density. The extent of π-backbonding between copper and disulfide (S2 2-) ligands was examined using similar methods. A study compared Cu-S bond strengths and S-S bond strength using Cu-S and S-S bond lengths determined experimentally by X-ray crystallography in combination with S-S vibrational stretching frequencies from rR spectroscopy.40 They correlated shorter Cu-S distances and lower S-S stretching frequencies with stronger Cu-S bonds and shorter S-S bonds using the explanation of 16  greater π-backbonding from a copper d orbital into the S2 2- ζ* orbital.40 Despite their combined use of several experimental techniques, the π-backbonding is still only being examined in a qualitative and indirect manner as no evidence of localized electron density is revealed. Infrared spectroscopy is one of the most common physical methods used to examine the extent of π-backbonding in various complexes, especially when carbonyl ligands are present. This dates back to the early 1960s when the donation effect of different phosphine ligands was investigated through the study of IR stretching frequencies of the carbonyl ligands present in the complexes.41-43  Since then, it has become common practice to use the vibrational frequency of carbonyls in transition metal complexes as a means to probe π-backbonding. The vibrational frequency of a free carbonyl molecule is compared to the vibrational frequency of a carbonyl in a complex. If it is lower in the complex it is assumed to be because of π-backbonding from a metal d orbital into the π* orbital of the carbonyl ligand.44 Although this method can provide valuable insight into the nature of the metal-ligand bond, in particular with reference to π-backbonding, it does not provide a means to indicate the actual electronic structure. Although IR stretching frequencies are most commonly employed when assessing π- backbonding using IR spectroscopy, the stretching force constants would be a much more accurate measure.45 Several papers have assessed the strength of various force constants with changes in carbonyl and nitrosyl π-backbonding, and good correlation was made with respect to the changes in force constant in relation to the changes in π- backbonding. 45,46 Comparing force constant values among different complexes is a more accurate measure of π-backbonding strength than using IR stretching frequencies because they are essentially uncoupled to any other vibrations in the molecule.47 Despite the increased accuracy, utilizing relative force constant values as a measure of π-backbonding strength is not a common practice. In addition to several experimental techniques that have been utilized, significant efforts have been invested in computational studies exploring π-backbonding. Over the past few decades, there has been progress in computational chemistry, which calculates measureable molecular properties such as geometries, vibrational frequencies in addition to many others. There have also been developments of 17  theoretical methods that are aimed at calculating electronic structure. An overview of the developments in computational chemistry is presented in the Encyclopedia of Computational Chemistry.48 Theoretical models striving to calculate electronic structure are used to qualitatively relate molecular properties to reactivity. This is of central importance for chemistry in general, but understanding the electronics between a transition metal and the ligands attached can provide useful insight into the reactivity of the complex on the whole. Some of the most important models designed for examining the nature of the bonding between transition metals and the ligands bound to them are Frenking‟s Charge Decomposition Analysis (CDA), the natural based orbital (NBO), energy decomposition analysis (EDA) and extended transition state (ETS) methods.49-52 Frenking‟s CDA is a three part partitioning scheme for breaking down donor-acceptor interactions in a complex, where the total amount of donation, back donation and charge polarization in the system is calculated from the sum of the three fractions.49 The NBO method partitions the electronic charge distribution in a molecule that is useful because it allows for the relative energies of the intramolecular orbital interactions to be estimated.38 Both the EDA and ETS methods partition the energy of a chemical bond in different contributions which makes it possible to calculate energy contributions from ζ and π interactions separately.38 An overview of these methods, in conjunction with a summary of them applied to various transition metal complexes is presented in a review by Frenking and Fröhlich.38 Recently, a modification to the original NBO partitioning method has been made that allows it to provide a quantitative estimation of π-backbonding in transition metal complexes. The implementation of these alterations to the NBO method when combined with DFT calculations were first alluded to in 2005, but not fully established until 2007 in a collaboration by Harvey and Leyssens.35,53 The extension to the original NBO method uses second-order pertubative NBO analysis within DFT calculations, which permits the estimation of metal-ligand π-backbonding interaction energies. Since this methodology was established in 2007 it has been used in several other studies to provide a theoretical quantitative estimation of π-backbonding in a variety of transition metal and organic complexes.44,54 Despite these promising developments, the results 18  are from computation only, due to the lack of experimental techniques which can probe the π-backbonding interactions directly. Thus, it is highly desirable to find an experimental method that can be compared to DFT predictions. The most notable efforts in finding an experimental technique which can directly probe π-backbonding in transition metal complexes was developed by Solomon and coworkers. In a 2003 paper, they developed a methodology based on multiplet simulations to analyze experimental iron (Fe) L-edge XAS (the term L-edge will be explained in section 2.1).55 Their new methodology was an extension of an already existing ligand field theory-multiplet model, developed by Thole and coworkers in the early 1990s.56,57 Modification by Solomon and coworkers included the addition of charge transfer (CT) terms to account for ligand-to-metal charge transfer (LMCT) features present in various Fe L-edge spectra of Fe(II) and Fe(III) complexes. Using the new methodology, they were able to experimentally determine differential orbital covalency (DOC), which they defined as the difference in metal d character of the t2g and eg orbitals. 55 The original 2003 paper investigated systems where ligand-to-metal donation were the major interactions present; a follow up paper in 2006 applied this new methodology to systems of iron-cyanides. In 2006, Solomon and coworkers studied Fe L-edge XAS of a series of iron-cyanide complexes. Due to the use these cyanide ligands, donor interactions between occupied orbitals on the ligand (CN-), and unoccupied orbitals on the iron, in addition to acceptor interactions between the occupied orbitals of the iron and unoccupied orbitals of the ligand are assumed to be present as predicted by the DCD model. Therefore, Fe L- edge XAS should be very sensitive to both the LMCT and the metal-to-ligand charge transfer (MLCT) interactions.  Thus, they re-adjusted the multiplet simulations to account for LMCT (as in 2003) as well as MLCT interactions in this set of iron complexes.58  This new modification allowed for determination of DOC in the systems, which included the contributions to π-backbonding. The Fe L-edge XAS spectra of complexes with π-backbonding were found to have a weak shoulder at slightly higher energy than the main feature. This was attributed to additional transitions from ligand- based π* orbitals containing iron d character from the occupied iron t2g orbitals. 58 Using the methodology and multiplet simulations, they were able to quantify the shoulder 19  features, and were therefore the first to demonstrate the application of a direct experimental probe of metal-ligand π- backbonding that could provide quantitative data. Although the research carried out by Solomon and coworkers demonstrated an experimental tool capable of directly probing and quantifying π-backbonding in transition metal complexes, it was limited to first row transition metals that exhibit an extra feature in their XAS spectra because of π-backbonding. For the work carried out in this thesis, rhodium and ruthenium complexes were studied due to their increased use in catalysis.59,60  As will be discussed in section 2.5, second row transition metal L- edge XAS is rarely found in the literature due to complicated backgrounds and analysis. Also, because the L-edges of second row transition metals are at much higher energies than the first row transition metals, the energy resolution is poorer. Therefore, the multiplet calculations developed and implemented by Solomon and coworkers, which were used to determine DOC in iron systems, cannot be used for these second row transition metal systems. As will be seen in chapter 3, the majority of the rhodium complexes investigated are Rh(I) d8 species which tend to exhibit square planar geometries where multiplet calculations are not necessary. Additionally, the other complexes are all second row transition metals (Rh or Ru), the energy splittings are much larger than Δoct, and therefore, the effect of multiplets is much less prevalent in these systems.  As will be evident in chapters 3 and 4 of this thesis, the underlying goal of this research was to develop a method whereby π-backbonding in second row transition metal-dioxygen systems could be directly investigated through experiment and quantitative values for the π-backbonding interaction could be obtained. 20  Chapter 2 : METHODS This chapter provides a detailed account of the major experimental technique used for this thesis work, XAS. A general overview of the technique, in addition to a description of the high energy facilities in which the experiments were performed, and detailed descriptions of the different types of XAS experiments will be provided. The latter half of the chapter will include a brief explanation of DFT. It will conclude with discussions of the various types of DFT calculations performed within this thesis work. 2.1: GENERAL INTRODUCTION TO XAS XAS is a spectroscopic technique that involves the excitation of core level electrons. In a typical XAS experiment, if enough energy is supplied, a core level electron can be completely ionized from the molecule. However, more informative processes occur when sufficient energy is supplied to create a core excited state by providing the appropriate amount of energy to promote a core electron into an available unoccupied or partially filled molecular orbital. XAS is considered an element specific, core-level spectroscopy. The element specificity arises because core electrons in atoms have a specific and characteristic binding energy that is well separated from neighboring atoms.  For instance, the carbon 1s binding energy is accepted to be approximately 284.2 eV whereas the oxygen 1s binding energy is 543.1 eV.61  This makes XAS a valuable technique for the study of different ligation effects at the transition metal center of complexes. The elemental edges of interest for this work are indicated in Table 2.1. Major attractions of XAS include its element specificity, its ability to investigate solids, liquids or gaseous complexes, as well as very dilute non-crystalline samples. 62 Element  K-edge L3-edge L2-edge Rhodium  23220 eV 3004 eV 3146 eV Ruthenium  22117 eV 2838 eV 2967 eV Chlorine  2822.4 eV NA NA  Table 2.1.  Energy of importance for XAS edges examined in this thesis work. 61  The type of XAS experiment performed is indicated by a specific label as depicted in Table 2.1. The labels begin with a capital letter that corresponds to the shell in which 21  the excited electron originates. For instance, when looking at 1s electrons (n=1 shell), the type of XAS is termed K-edge spectroscopy. Similarly, when electrons are excited from the n=2 shell they give rise to L-edge spectra, and the n=3 shell corresponds to M- edge spectra. There is often a subscript number that goes along with the capital letter in the label. For instance, L1 spectroscopy refers to excitation of 2s core electrons whereas L2 and L3 edge spectroscopy comes from 2p1/2 and 2p3/2 electrons respectively. For the majority of this thesis work, the types of XAS spectra examined were metal K-edges (both rhodium and ruthenium) and metal L2,3-edges (both rhodium and ruthenium). Ligand K-edge data is also popular and will be discussed briefly but was not utilized for the work herein (the Cl K-edge data is mentioned as it was examined in conjunction with the Ru-L edge data due to the overlap in energy ranges). The differences in both data collection and analysis between the metal K-edges and the metal L-edges will be discussed in detail in sections 2.4 and 2.5 of this thesis. 2.1.1 Synchrotron Radiation As previously mentioned, XAS is often referred to as a synchrotron based form of core level spectroscopy. Therefore, this section of the thesis will provide background information regarding synchrotron radiation, including why it is needed for XAS and ways it is altered to provide different photon properties. Synchrotron radiation is defined as the electromagnetic radiation emitted by electrons or positrons moving at relativistic velocities along a curved trajectory with a large radius of curvature.63 All of the experimental work competed for this thesis was carried out at the Stanford Synchrotron Radiation Lightsource (SSRL), where the synchrotron radiation was produced by the excitation of electrons not positrons. The reasons why a synchrotron is so widely used in chemistry and physics have to do with the properties of the radiation emitted. Synchrotron radiation is released every time there is a change of trajectory in the accelerated electron path. The radiation emitted has a very broad range of photon energies. The chemically relevant range of energy spans from the IR to hard X-rays.63 This allows scientists to access a wide variety of energies in one facility, and with the use of a monochromator, the energies of interest can be selected and finely tuned to scan in various types of experiments. 22  There are two major components that comprise a synchrotron, the storage ring and the beamlines. The storage ring is responsible for keeping charged particles (electrons) circulating in a closed orbit under vacuum at relativistic speed.63 The electrons are first produced by an electron gun and accelerated by a linear accelerator. They are then injected into the storage ring by an injector system (often a booster ring is used to get the electrons to relativistic speeds before injecting them into the ring).63 The components comprising the storage ring include a vacuum chamber, a radiofrequency cavity and a number of different magnets. The vacuum chamber keeps the electrons under ultrahigh vacuum (UHV) circulating along a closed trajectory.63 The radiofrequency cavity system is responsible for restoring energy that the electrons lose when they release synchrotron radiation.63 The storage ring has straight sections where the electrons have a linear trajectory and travel in a collimated beam. Dipole bending magnets and insertion devices, both discussed in the next section, are located throughout the storage ring to change the trajectory of the electrons in order to accelerate them, which produces electromagnetic radiation.63 Beamlines are responsible for delivering the emitted synchrotron radiation to the experimental workstation. Beamlines also consist of a number of components, the first of which is a monochromator. The monochromator is responsible for selecting the appropriate photon energy to be used in the experiments. There are also a number of different vertical and horizontal mirrors utilized to allow proper focusing of the radiation. Additionally, harmonic rejection mirrors are important to allow data collection without interference from higher harmonics. A vacuum system is also necessary to keep the pressure in the beamline at levels suitable for its connection to the vacuum chamber of the storage ring. 63 Finally, the beamlines also consist of beamline controls (usually a computer) that the user can employ to change settings such as monochromator energy selection, sample position and mirror angles as well as experimental chambers that are different for each beamline depending on the type of experiment being conducted. A schematic diagram highlighting important aspects of the synchrotron is illustrated in Figure 2.1. 23   Figure 2.1. Schematic diagram representing important components of a synchrotron facility. 2.1.1.1 Dipole Magnets and Insertion Devices For circular electron storage rings to utilize synchrotron radiation, the use of dipole magnets is essential. Dipole magnets (often referred to as bending magnets) provide a wide spectral range with high intensity. They are dispersed throughout the storage ring to guide the trajectory of the electrons in the ring and cause the production of the electromagnetic radiation as the electron beam is deflected by the bending magnets. Quadrupole magnets are inserted into the spaces between the dipole magnets in order to prevent the electron beam from crashing into the edges of the storage ring (vacuum tube).64  In the earliest first generation synchrotrons, the storage ring was made of dipole and quadrupole magnets only. In the 1980s, the development and implementation of insertion devices greatly improved the quality of modern synchrotrons.65 Insertion device is a general term used to describe different magnetic elements which can be inserted into spaces between bending magnets in a synchrotron storage ring to enhance the properties of synchrotron radiation. The insertion devices are placed in the straight sections of the storage ring, free of bending magnets, and are used to alter the 24  characteristics of the electromagnetic synchrotron radiation emitted. There are two main types of insertion devices, wigglers and undulators.  Wigglers and undulators are very similar in that they both act to wiggle the electron beam trajectory many times. The wiggling is achieved using a periodic array of magnets that are designed to produce a series of oscillations for the electron beam.63 The wiggled electron beam also produces synchrotron radiation as a bending magnet but with enhanced properties.  When interference effects from the wiggles are neglected, the radiation emission spectrum is similar to that of bending magnets, but multiplied by two times the number of periods of the magnetic array, which usually only occurs in the limit of a strong magnetic field or a small number of periods.63 Undulators are not included in this limit, thus they either have weak magnetic fields or large number of periods and therefore interference effects are important. The interference effects produce radiation that is centered around one or a few wavelengths which can be cleverly exploited to produce very high brightness at specific photon energies.63 Therefore, the radiation produced by an undulator is very bright and generally concentrated in a narrow band around the fundamental wavelength of the insertion device.63  All experiments performed for this thesis work were at SSRL, which is a third generation synchrotron that utilizes both types of insertion devices. The beamlines used for the experimental work in this thesis were both wiggler beamlines, so the brightness of the beam is enhanced compared to older bending magnet beamlines, but the light is not as collimated as that from an undulator beamline. 2.2 XAS SPECTRA Generally an XAS spectrum can be divided into two major areas; the X-ray absorption near-edge spectroscopy (XANES) region and the extended X-ray absorption fine structure (EXAFS) region. The division of the two is illustrated in Figure 2.2. The XANES region of the spectrum ranges from several eV below the edge jump to approximately 30-50 eV above the edge; whereas, the EXAFS region extends from the end of the XANES region to significantly above the ionization edge.66 Generally, the XANES spectrum is attributed to excitations of core electrons into unoccupied or partially occupied molecular orbitals. The analysis of the XANES region is most commonly referred to as pre-edge or near-edge analysis and it often includes analysis 25  of the edge itself.  In contrast, EXAFS spectra result from backscattering of the ionized photoelectron off of its nearest neighbors. EXAFS analysis requires several involved fitting procedures of the data, and was not used for any work reported in this thesis.  Figure 2.2.  A typical XAS spectrum highlighting the two distinct regions: XANES AND EXAFS. (Spectrum shown is of a Vanadium cyclam complex). The type of information extracted from the two regions of an XAS spectrum is significantly different. Generally, XANES spectra are comprised of single or multiple small peaks which correspond to discrete transitions often referred to as pre-edge transitions. The intensities and positions of these peaks provide information regarding electronic environment of the absorbing atom as well as fractional d-electron density of the complex. The absorption edge, or edge jump, is often represented as an arctangent function, which represents the ionization of the core electron and its excitation into the continuum. The energy of the edge itself is conventionally used to provide oxidation state information. Typically, an increase of the inflection point of the edge of approximately 1 eV gives rise to an increase of one in oxidation state for the absorbing atom.67,68 An illustration indication the pre-edge and edge on a XANES spectrum is provided in Figure 2.3. 26   Figure 2.3 XANES spectrum showing the pre-edge (pink) and the edge jump (red). (Spectrum is of Cs2CuCl4) Since the EXAFS region occurs due to photoelectron backscattering of neighbouring atoms with both constructive and destructive interference, it has a very oscillatory structure. Therefore, very refined fitting procedures are used to extract geometric information from the EXAFS region. The EXAFS analysis can provide very accurate information regarding local atomic structure surrounding the source atom.66 This can be useful in cases where crystal structures cannot be obtained. However, for the purposes of this thesis, EXAFS was not used, and will therefore not be discussed further. 2.2.1 XAS Transitions and Selection Rules An XAS transition occurs when sufficient energy is supplied to excite a core electron into an unoccupied/partially occupied molecular orbital or when enough energy is supplied to completely ionize the core electron. Thus, XAS measures the absorption of X-rays as a function of X-ray energy. The optimal process occurs when the incident X- rays (hν) match the energy difference between the ground and excited state of the core electron. Generally, an XAS spectrum exhibits three main features: a decrease in X-ray absorption when the X-ray energy is increased, an abrupt rise (step-like) in absorption at the edge-jump, and a series of oscillations above the absorption edge.66 27  The first feature, the decrease in X-ray absorption with increasing energy, gives rise to the overall shape of the XAS spectrum. This feature is described by the Fermi Golden rule, given in Equation 2.1.69 PXAS ~ |<Φf|ê·r|Φi>| 2 δEf-Ei-ħω Where,         PXAS is the probability of the transition from Φi to Φf  (ê·r) is the dipole matrix element that couples to both the Φf  (final) and  Φi (initial) states  δEf-Ei-ħω  is the delta function (ensuring energy conservation) Equation 2.1.  The Fermi Golden Rule 69   Since a core electron has been excited in the final state, it can be described as a function of the initial state, with a core electron removed and a continuum electron added as shown in Equation 2.2 .69 PXAS ~ |<Φiϲε|ê·r|Φi>| 2 δEf-Ei-ħω Where,  ϲ represents the removed core electron  ε represents the continuum electron Equation 2.2.  The Fermi Golden Rule re-written as a function of initial states. 69  An important approximation that is made assumes that the matrix element can be written as a single-electron matrix element by removing electrons that are not involved with the transition.69 Therefore all electron rearrangements that occur when the core hole is excited to a continuum electron are neglected thereby making the series of delta functions identify with the density of states (ρ) and the new XAS spectral shape is described by Equation 2.3 .69   28  PXAS ~ |<ε|ê·r|ϲ>| 2 ρ Equation 2.3.  Approximated Fermi Golden Rule to describe XAS spectral shape. 69  This provides the rationale for the two important selection rules relevant to XAS. The first being the density of states has an orbital moment that differs from the core state by one (ΔL ± 1), with a spin which is conserved (ΔS = 0).69 This indicates that the most pronounced transitions in XAS will occur when the incident X-ray energy is consistent with a difference in energy between the ground and excited state of a core electron when the angular momentum only changes by 1 (s ← p, p ← d, etc.). The other important factor that contributes to the observed XAS spectra corresponds to the amount of core atom character present in the excited state orbital. Due to the fact that XAS is a core-level spectroscopic method, the transition is beginning from an orbital which is approximately an atomic nucleus.  Thus, the excited state must contain a significant contribution from the atomic nucleus of the originating electron or no transition will be possible. This requirement for XAS will become more apparent in the following sections where the three main types of XAS experiments are described in detail. 2.3: LIGAND K-EDGE XANES The bonding between metal centers and their ligands in transition metal complexes is of great importance for understanding both the physical properties and reactivity of the systems. The covalency in the metal-ligand bonds is responsible for many of the physical properties of the complexes as well as the system‟s reactivity.70-72 Due to these important factors, it is of essential importance to quantify covalency in metal-ligand bonds. Quantifying covalency has been done both theoretically and experimentally. One of the major problems with the theoretical quantification is obtaining reliable results. The Hartree-Fock method tends to be biased in favour of high spin states, low d-orbital occupations and ionic bonds.73,74 Whereas, approximate density functional methods have been found to be biased towards low spin states, high d-occupations and covalent bonds.73 Although some of the newer hybrid density functional methods tend to perform better for open transition metal systems, it is still very important to have experimental data to legitimize the actual covalency of the systems. 29  There are several experimental options available to quantify the covalency of metal- ligand bonds in transition metal systems. The most notable is the ground state study electron paramagnetic spectroscopy. In EPR, the three major pieces of information come from g-values, metal hyperfine coupling and ligand superhyperfine coupling. Of these, the most direct ground state probe of metal-ligand covalency is the ligand superhyperfine coupling.70,72 However, it is best measured by double resonance or pulsed EPR methods which are not facile, and require an EPR active site that makes it inapplicable to any molecular orbitals not containing unpaired electrons.70,72  Due to these restrictions, the use of ligand K-edge and metal L-edge XAS (Section 2.5) may be quite useful in quantifying the metal-ligand covalency in open shell transition metal complexes. Quantitative ligand K-edge XAS was introduced by Solomon and coworkers in 1990.75 This method was further developed and improved throughout the rest of the 1990s.73,76,77 It has now been extensively used to determine the covalency between metal-sulfur and metal-chlorine bonds in many organometallic, bioinorganic and transition metal complexes.72,73,78-81 A few other types of ligand K-edge XAS spectroscopy exist, the most notable are carbon K-edge (often referred to as Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy)82 and phosphorus K-edge XAS. The carbon K-edge spectroscopy with transition metals is often complicated by both the large number of carbon atoms present in the complexes as well as the inevitable carbon contamination within the beamline. These two factors make reliable quantification difficult. Phosphorus K-edge XAS is has not been overly utilized for quantification. 83 2.3.1 Pre-edge Peak Intensity – Metal-ligand Covalency Ligand K-edge XAS involves the excitation of a 1s electron on the ligand to unoccupied or partially occupied molecular orbitals of the complex. The electric dipole allowed transitions for K-edges are np ← 1s, which makes ligand K-edge XAS an excellent tool to use as a direct probe of metal-ligand bonding interactions. A typical chlorine K-edge spectrum is shown in Figure 2.4. The intense electric dipole allowed features that comprise the main edge jump are from 4p ← Cl 1s transitions.76  Pre-edge 30  transitions arise from transitions to molecular orbitals containing both metal and ligand character.  Figure 2.4.  An example of a Cl K-edge XAS spectrum. (Spectrum of Cs2CuCl4) This technique is advantageous because unlike excited state absorption methods, the donor orbital in ligand K-edge XAS is localized on the ligand and can therefore be thought of as a pure ligand orbital. This means that any pre-edge transitions that are visible must be occurring to molecular orbitals that contain a significant portion of ligand np character.76,77  A schematic representation of the pre-edge transition is shown in Figure 2.5. 31   Figure 2.5. A schematic representation of a Cl K pre-edge transition. A detailed mechanism to extract quantitative information regarding covalency in metal-ligand bonds for transitions metals, by analysis of pre-edge transitions in ligand K-edge spectroscopy, was by pioneered by Solomon and coworkers. First they developed a method for d9 metal systems, and then quickly followed up with a more general method for dn systems.76,77 Typically the pre-edge transition is occurring from the donor (ligand 1s electron) to the acceptor orbitals which are thought to be antibonding orbitals whose wave function can be described as indicated in Equation 2.4. Ψ* = (1-α2)1/2 |Mmd˃  – α |Lnp˃ Where,  Ψ* is wavefunction of the acceptor orbital α2  is the covalency in terms of the ligand |Mmd˃  is the wavefunction of the atomic orbital of the transition metal |Lnp˃  is the wavefunction of the atomic orbital of the ligand Equation 2.4.  General wave function for acceptor orbital. 76  32  Since the intensity visible from the pre-edge transition is due to the electric dipole allowed np ← 1s transition weighted by α2, it can be described as shown in Equation 2.5. I (Ψ* ← L1s ) = 1/3 α 2 I (Lnp ← L1s) Where,  I (Ψ* ← L1s ) is the intensity of the pre-edge transition  α2  is the covalency in terms of the ligand  I (Lnp ← L1s) is the intensity of the pure ligand-centered transition Equation 2.5.  General equation for ligand pre-edge intensity. 76  These equations provide a fairly facile interpretation of the pre-edge intensity of ligand K-edge XAS. In general, the intensity of the pre-edge peak is due to the intensity of a ligand centered Lnp ← L1s transition weighted by the covalent character of the ligand np orbitals into the acceptor molecular orbital. Therefore, this method provides a direct probe of the covalency between the ligand-metal bond. For the work presented in this thesis, ligand K-edge XAS was not utilized. The only ligand K-edge spectra that will be shown are chlorine K-edges from some of the ruthenium compounds that were only obtained due to the small difference in energy of the ruthenium L3-edge and the chlorine K-edge. The theme of this thesis was to investigate the bonding between the metal centers and dioxygen (Rh-O2 and Ru-O2), which was explicitly performed from examination of the metal edges due to the unreliable Cl K-edge data obtained (discussed in chapter 4). Unfortunately no oxygen K-edge data was obtained due to the difficulty and unreliability of these types of experiments. 2.4 TRANSITION METAL K-EDGE XANES Transition metal K-edge spectroscopy involves the ionization of a core 1s electron of a transition metal. This technique has been readily employed to many 3d transition metal complexes to provide electronic structure about the metal ion and geometric information of surrounding ligands62,65,81,84,85  This work was pioneered by Solomon and 33  coworkers and has provided an effective methodology for analysis of first row transition metal complexes where the K-edge energy is typically between 4-10 keV. In general, metal K-edge XAS spectra for first row transition metals can be characterized by a sharp edge feature, from the ionization of the 1s electron, followed by a series of oscillatory structure (the EXAFS). Often, small pre-edge features may be present several eV before the onset of the edge.  An example of a 3d transition metal K- edge spectrum can be seen in Figure 2.6 with an inset highlighting the pre-edge feature. The energy position of the edge, which is normally obtained from the inflection point of the edge jump, provides an estimate of Zeff on the metal center. The position of the edge is very sensitive to changes in ligand environment making metal K-edge XAS an excellent probe of the oxidation state of the metal due to changes in the surrounding ligands.62,67 The values obtained for the complexes are all relative. Therefore, reference compounds, usually in the form of pure metals, are used to make valid comparisons between complexes with different ligand environments.67,69  Figure 2.6. An example of a vanadium metal K-edge XAS spectrum for a vanadium cyclam complex. The inset shows a close-up of the pre-edge region for clarification. (Note the pre-edge region is intense due to a strong V=O bond). 34  In addition to oxidation state information gained from the edge region in the XANES spectra of first row transition metals, the pre-edge features also provide valuable information regarding the electronic structure of the complexes. For first row transition metals, a general consensus has been reached that pre-edge features in metal K-edge XAS are due to 3d ← 1s transitions.69,84,86 Such transitions are formally electric dipole forbidden for centrosymmetric molecules. However, when the centrosymmetry in a complex is distorted, 4p mixing into empty 3d orbitals can occur.84 This provides a method to allow the 3d ← 1s transition to become partially electric dipole allowed; therefore, even small amounts of 4p mixing into the 3d orbitals can have a drastic impact on the intensity of the pre-edge feature.84 Evaluation of the intensity of pre-edge features in first row transition metals has become popular as it is a sensitive method to probe the electronic structure at the metal center. The complexes investigated in this thesis work involved rhodium and ruthenium, both of which are second row transition metals. In general, there has not been much K-edge XAS performed on heaver metals due to the high energy requirement for excitation, which decreases the energy resolution significantly. However, it has been assumed that any pre-edge features present in the K-edge XAS of second row transition metals would analogously arise from 4d ← 1s transitions, although 5p ← 1s transitions are also a possibility.87,88 In the past few years, there has been an increase in investigations involving K-edge XAS of second row transition metals where 4d ← 1s transitions have been assigned.68,89-91 These studies have shown the investigation of the second row transition metal XANES can provide useful information regarding both oxidation state (by edge position), and electronic structure (pre-edge analysis) and was therefore important for the studies that will be discussed in this thesis work. 2.5 TRANSITION METAL L-EDGE XANES Transition metal L-edge XAS is a powerful tool to probe the electronic and geometric structural information of organometallic complexes. Metal L1-edge XAS refers to nd ← 2s transitions. These transitions are electric dipole forbidden making investigation of them fairly uncommon. Metal L-edge XAS normally involves the metal L2,3 edges, which arise from electric dipole allowed nd ← 2p transitions. Due to the electric dipole allowed nature of these transitions, the pre-edge features are well pronounced and both the 35  energy and intensity of the pre-edge features can provide valuable oxidation state and ligand field information about the metal center. Although similar information can be obtained from metal K-edge XAS (section 2.4), the L-edges tend to be more advantageous for several reasons. First, metal K-edges involve electric dipole forbidden transitions which often make the pre-edge features too small to extract valuable information, especially in terms of covalency. Second, the metal K-edge experiments are at much higher energy than the metal L-edge experiments, which pose a few complications, such as causing sample damage and/or photo-reduction. Additionally, the higher energy of the K-edges produces spectra with much poorer energy resolution. The development of metal L-edge XAS was in the late 1980s. This occurred significantly later then metal K-edge XAS due to the very different experimental beamline setup required for these mid-energy experiments.92  The first studies on metal L-edges were done in the early 1990s and involved 3d metals such as molybdenum, copper and manganese.92-95 As much as the technique has advanced over the years with improving optics and optimized beamline setups, the majority of the studies have concentrated on first row transition metal complexes, in particular copper and iron.58,96 There have not been many studies involving second row transition metal L-edges due to the relatively difficult nature of the analysis, often due to complicated backgrounds and/or overlapping edges.97 However, in recent years, several attempts have been made due to the potential of obtaining useful information from this spectroscopy. Successful studies have been performed on platinum L3-edges, ruthenium L2,3 edges and palladium L2,3 edges, although only the palladium study attempted to gain quantitative information of the pre-edges, the others used qualitative comparisons and oxidation state information from the edge position only.68,91,98,99 This is unfortunate, because in addition to valuable oxidation state information from edge position and ligand-field splitting information from qualitative analysis of metal L-edge XAS, this technique could be a valuable tool for metal-ligand covalency similar to ligand K-edge XAS. Using a molecular orbital description, the ground state of a metal complex can be described as a linear combination of both metal and ligand valence orbitals, as was shown previously in Equation 2.4.55 36  Metal L2,3-edge XAS involves the excitation of a metal 2p electron, which can be considered to be essentially a pure atomic metal orbital, similar to the case assumed in ligand K-edge spectroscopy. Since the transition is starting at an orbital that is thought of as pure metal, the transition must occur to an orbital that contains the metal-centered component of the ground state wavefunction.55 Therefore, the pre-edge transitions present in metal L-edge XAS serve as direct probes of the metal d manifold, and the intensity of the transitions can be translated into a measure of covalency in the partially filled or completely unfilled metal d orbitals.55 Despite the straight forward description of the basic principles behind these experiments, extracting accurate and unbiased quantitative information from metal L- edges, especially of second row transition metals, is by no means a simple task. For the work presented in this thesis (chapters 3 and 4), the research focuses on the analysis of rhodium L2,3-edge  and ruthenium L2,3-edge XAS. This work represents the first examples of extracting quantitative information from both ruthenium and rhodium L2,3- edge XAS. It would not have been possible without the development of the analysis program, Blueprint XAS, and the analysis methodology, both by Dr. Mario Jaime- Delgado, a former colleague.100,101 Details of these can be found in the references indicated, but the important aspects of them will be discussed in relevant places within the experimental investigation chapters of the thesis (Chapters 3 and 4). 2.6 EXPERIMENTAL SETUP All of the XAS data presented in this thesis was obtained at SSRL over the past three years. Two beamlines were utilized for data collection, beamline 7-3 for the high energy, metal K-edge data and beamline 4-3 for the mid/low energy, metal L-edge and ligand K- edge data. Detailed descriptions of each of these beamlines are provided in the subsequent sections. 2.6.1 Beamline 7-3 At SSRL, beamline 7-3 is a so called “hard X-ray” beamline, with energy that ranges from 4,300 eV to 37,000 eV. Beamline 7-3 is mainly dedicated for studies involving structural biology by biological EXAFS. The setup of this beamline allows investigations of both solid and frozen solution samples, making it valuable for evaluating spectra of 37  biological protein samples. For the purposes of the work presented in this thesis, beamline 7-3 was utilized for collection of all of the rhodium and ruthenium K-edge data. Beamline 7-3 is built using a 20-pole, 2 T wiggler, 0.8 mrad beam and uses a Si (220) double-crystal monochromator. The beamline has two crystal sets available for use, the Φ = 0° set and the Φ = 90°, all of our data was collected using the Φ = 0° crystal set. The beamline end station that was used is designed so that it allows transmission data to be collected simultaneously with fluorescence data.   The experimental end station consists of an Oxford-Instrument CF1208 continuous- flow liquid helium cryostat which serves as the sample chamber. The cryostat is kept at or below 10 Kelvin at all times during data acquisition. This is done in order to protect the samples from photoreduction from the high energy X-rays as well as to gain better quality data due to less sample disorder at such low temperatures.  The sample is aligned at an angle of approximately 45° to the incoming X-ray beam in order to produce a sufficient fluorescence signal.  The fluorescence data is collected using a 30- element germanium detector array. Fluorescence detection in XAS takes advantage of the secondary process involving radioactive decay of photoexcited states. For this type of detection, the assumption is made that the number of fluorescence photons is proportional to the number of X-ray photons absorbed. The fluorescence signal detected is not exactly equal to the true absorption coefficient but it is a very close approximation and is commonly used throughout the XAS field.102 Transmission is detected using three consecutive ionization chambers filled with either argon gas for very high energy X-rays, and nitrogen gas for lower energy X-rays. Argon gas was used in the ionization chambers for our experiments as both the rhodium and ruthenium K-edges are in the hard X-ray regime. In the first ionization chamber, the incident beam (I0) of the incoming X-rays is detected. The second chamber measures the beam directly after the sample absorption, providing a sample signal (I1), and the third ionization chamber is placed after a reference foil, measuring the absorbance of the reference (I2). A schematic diagram showing the setup of beamline 7-3 is illustrated in Figure 2.7. 38   Figure 2.7. A schematic illustration of high energy beamline 7-3 at SSRL. 2.6.2 Beamline 4-3 At SSRL, beamline 4-3 is a brand new soft energy beamline used for investigations of biological, material and environmental samples. The beamline energy ranges from 2,400 eV to 6,000 eV, making it useful for both sulfur and chlorine K-edges as well as many metal L-edges. All of the rhodium and ruthenium L-edges as well as the chlorine K-edges that will be presented in this thesis were collected on this beamline. Beamline 4-3 is built using a 20-pole, 2 T wiggler, 0.75 mrad beam and uses a Si (111) double-crystal monochromator. The beamline has two crystal sets available for use, the Φ = 0° set and the Φ = 90°, all of our data was collected using the Φ = 0° crystal set. The end station of beamline 4-3 is equipped with a simple sample box that is filled with helium gas.  The sample is placed inside the sample box at a 45° angle to the incident beam in order to maximize the fluorescence signal.  Detection of the sample signal is performed using a Lytle detector that uses nitrogen gas. The Lytle detector is physically attached to the sample chamber but separated from the gas in the sample box by a thin polypropylene window.  The incident X-ray beam is collected using an 39  ionization chamber, similar to the ones on beamline 7-3; however, because of the lower incoming photons, the chamber is filled with nitrogen gas rather than argon. Unlike beamline 7-3, reference signal cannot be collected simultaneously with the sample signal. This is because at these lower energies, not enough photons can penetrate through the sample to provide adequate signal for the reference. Therefore, external calibration must be relied on in these experiments. It has become practice that external calibration on this beamline can be achieved using different, well known reference standards. The standard is scanned before and after each sample to ensure accurate calibration will be possible. This was the procedure used for the ruthenium L- edges as will be described in chapter 4, but the rhodium L-edges provided a unique opportunity where an internal calibration was achievable as will be discussed in chapter 3. 2.7 Density Functional Theory Calculations Before any descriptions of the DFT calculations run, or any outcomes of the calculations will be discussed, a short overview of the foundation of DFT will be provided. In particular the basics of the theoretical approach, along with important parameters and considerations will be briefly outlined. 2.7.1 Basics of DFT   Over the past two decades, there has been an increase in the use of theoretical DFT calculations to aid in the explanation of experimental data in many different chemistry disciplines. The increase in popularity is largely due to the computational expedience of the technique making it suitable for large and real-life molecules as opposed to other computational methods.103 Additionally, in most circumstances, information derived from DFT is in better agreement with experimental data than results obtained from other methods, especially in the case of transition metals.103 Due to the popularity of this innovative theoretical approach, Walter Kohn (for developing DFT) and John Pople (for his outstanding developments in computational chemistry) were awarded the Nobel prize for chemistry in 1998.104 DFT, as it is currently known and utilized, is based on the theorems developed by Hohenberg and Kohn in the early 1960s.105 The main idea in their 1964 paper, which is 40  the foundation of DFT, was that the exact ground state energy of a molecular system can be determined by its total electron density ρ(r).105  An energy expressed in terms of density can be written as shown in Equation 2.6. The second Hohenberg-Kohm theorem states that for an initially guessed density, the electron correlation functional of the system energy, will be greater than or equal to the electron correlation functional with the exact density (E[ρ]) of the system.104 Therefore, by finding the minimum E[ρ] over the entire range of densities explored, the exact ground state density and thus energy and all other properties of the molecular system can be determined.104 E[ρ] = VNN + VeN[ρ] + J[ρ] + T[ρ] + EʹXC[ρ] Where,  E[ρ] is the electron correlation functional in terms of density VNN is the nuclear-nuclear term VeN[ρ] is the nuclear-electronic term J[ρ] is the inter-electronic term T[ρ] is the unknown kinetic energy functional EʹXC[ρ] is the unknown exchange-correlation functional Equation 2.6. Electron correlation functional in terms of density. 104  In order to deal with solving this equation, with multiple unknown values, modern DFT programs are based on Kohn-Sham (KS) theory. The formation of this theory is detailed in a 1965 article.106 Kohn and Sham derived a set of one-electron equations from which the exact electron density and thus total energy of a system could be solved in principle.103 Thus, current DFT programs rely on the assumption of non-interacting electrons, and make an initial guess of the electron density utilized to calculate the total electron density of a molecule.  The calculated density is only an approximation of the exact density; consequently, an iterative procedure is then performed until the solution reaches self consistency.  In the KS formation, an exchange-correlation functional is required. Since this term is unknown, the need for user input to decide on which functional should be used is 41  compulsory. The local density approximation (LDA), which assumes homogeneity of electron distribution, was one of the first functionals used in DFT; however, it hasn‟t been overly successful in chemistry.104,107 In general, LDA has been found to drastically overestimate correlation energies.103 To improve approximations and utility in chemistry, new functionals were designed which account for the inhomogeneity in electronic distribution. In particular, the development of the generalized gradient approximation (GGA) by both Becke and Perdew have led to wide utilization in the chemical community.103,108,109 Despite the vast improvements, there is not currently a functional that has found widespread use in all disciplines of chemistry, although it appears that high accuracy is usually achieved within the GGA framework.104 One of the most significant improvements to DFT was in 1993 when Becke introduced using hybrid functionals that incorporate a fraction of the non-local Hartree Fock exchange.110,111 The most popular hybrid functional used is B3LYP, while PBE0 and TPSSh are two alternate hybrid functionals that have been found useful for chemistry.104   In addition to the choice of functional used in KS based DFT, the choice of the basis set utilized is also important. Due to the complicated shape of KS orbitals, they need to be expanded by a set of pre-fixed basis functions.104 There are several different types of basis functions that can be used in DFT calculations, but the two most common ones are Gaussian-type orbitals (GTOs) and Slater-type orbitals (STOs). STOs allow the construction of high-quality basis sets with a relatively small number of functions because they possess the required cusp behavior and long-range decay to more accurately expand the MOs, in comparison with GTOs.112 GTOs would typically require a factor of three more functions for the same level of basis set quality.112 The type of orbital utilized is dictated by the DFT calculation package used and results from either type are well accepted in the literature. For the DFT computational programs utilized for studies of this thesis work, STOs were employed. The number of STOs utilized to model each orbital dictates the level of DFT in the calculations. The lowest accuracy would be achieved using 1 STO for each orbital, known as a single-zeta basis set. The more STOs used, the more accurate the calculation. However, there is a large increase in the computational time and effort required for the increased accuracy. Currently, the major standard for basis sets in 42  calculations is that at least triple-zeta quality with at least one set of polarization functions should be utilized for acceptable results.104  This basis set employs 4 STOs in total; 3 STOs for each Slater orbital (triple-zeta TZ), which allows for the orbital to change its size and a fourth STO for the polarization (P), which allows for the orbital to change its shape. The combination of TZP produces realistic orbital shapes that more accurately account for electron density.  The exact basis sets utilized for all calculations performed in this thesis work will be designated when the individual calculations are discussed; however, for the most part, a triple-zeta with polarization (TZP) basis set was employed. The last technical aspect of DFT that was important to all calculations carried out involves the frozen core approximation. The frozen core approximation is often applied in large systems, especially for heavier transition metals where deep-core atomic orbitals do not change very much, to decrease the computational time required for the calculation by reducing the size of the variational basis set.112 This approximation uses frozen core orbitals that are acquired from extremely accurate single-atom calculations with large STO basis sets.112 Thus, for larger atoms, such as rhodium, the frozen core approximation can be applied by using a function (contained within the DFT software package) that represents the core electrons up to a specific shell. Essentially these electrons are frozen, and treated as part of the larger rhodium nucleus. In this thesis work, the approximation was used for the transition metal centers (rhodium and ruthenium) only. All DFT calculations performed for this thesis work, were completed utilizing two different software packages. The Amsterdam Density Functional (ADF) package (version 2009.01), was used for some of the first calculations performed.112 Later calculations were acquired using the ORCA software (version 2.6), developed by Frank Neese.113 Both DFT packages employ the KS self consistent field (SCF) methodology, use STOs and contain many of the same exchange correlation functionals, which allowed essentially identical calculations to be performed in both programs. The types of DFT calculations executed will be discussed in the next section, and all of the exact parameters used in the calculations will be provided when each calculation is discussed. 43  2.7.2 Relevant Types of DFT Calculations There are numerous forms of DFT calculations available to assess different properties of the molecule under investigation. In this thesis work, four main types of DFT calculations were utilized: geometry optimizations, single point, relaxed potential energy surface, and frequency computations. Generally, the first step for any DFT investigation is performing a geometry optimization of the molecule. In geometry optimizations, the first derivatives of the energy with respect to nuclear displacements are calculated analytically.112 This type of calculation provides the solution of what DFT predicts to be the lowest energy structure of a single molecule under investigation. When crystal structural data was available, it was utilized as the starting point for the input of the geometry calculations performed in this thesis work. The final geometry predicted by DFT was in good agreement with the crystal structures where applicable. Since the same level of DFT was applied for all compounds, it was assumed the structures predicted for molecules where no crystal structures were available would be similarly accurate. In general, it has been shown that the accuracy of DFT optimized structures is normally in very good agreement when compared with crystal structural data, even for structures containing 3d, 4d and 5d transition metals.114 The second type of computation performed was a single point energy calculation. These are executed after the final structure from the geometry optimization has been obtained. The final geometry optimized structure is utilized as input, and from the single point DFT calculations, the energy and electron occupancy of all of the different molecular orbitals is predicted. Contributions from the atomic orbitals that make up each molecular orbital are also provided. Additionally, estimated atomic charges for each atom, based on Mulliken Population Analysis, Voronoi and Hirshfeld analyses,  are also obtained.115,116 The results from these single point calculations are often utilized to construct valence MO diagrams for the molecules investigated to gain insight into the electronic structure predicted by DFT.  In addition, pictorial representations of the appearance of each MO can be obtained. Relaxed potential energy surface calculations are a combination of the first two types of DFT computations discussed. In these types of computations, usually one or more nuclear coordinate is varied between user-specified initial and final values. A geometry 44  optimization, followed by a single point calculation is performed at each stage corresponding to a specific value. Therefore, the overall output is comprised of several geometry optimization and single point energy computations. These types of calculations are generally performed to examine a specific reaction pathway, or as in the case with this thesis, to observe the effect on the entire molecule (both geometric and electronic structure) by changing a specific nuclear coordinate. Because the output file is made up of multiple geometry optimizations and single point energy calculations, the ability to monitor the exact point where significant changes in either the geometry or electronic structure (or both) occur is easily identifiable. The final type of DFT computation performed in this thesis work was frequency calculations. The prediction of harmonic vibrational frequencies has been extensively investigated, and when utilizing GGA functionals in DFT, their predictions have been in reasonable agreement with experimental observations.117 This makes it useful for helping in the assignment of complex experimental spectra. Also, from these calculations, DFT quite successfully predicts the IR intensities for the molecule under investigation. In this thesis, these calculations were useful for predicting the energy at which the O-O stretching frequency would occur for various molecules. Additionally, frequency calculations can prove that your geometry optimization is at a minimum by showing all positive frequencies. The next chapter of this thesis introduces a series of rhodium-dioxygen complexes that were studied using XAS. The principles of the technique covered in the current chapter are of key importance to the understanding and analysis of the data that will be discussed in both chapters 3 and 4. Chapter 3 will describe in detail how the raw XAS data was processed, including scan averaging, data calibration, and background subtraction. In particular the new methodology that was implemented for the analysis will be discussed in detail. The experimental findings will be thoroughly discussed along with support from the DFT calculations utilized as an aid to corroborate the experimental findings. 45  Chapter 3 : XAS INVESTIGATIONS OF RHODIUM- DIOXYGEN COMPLEXES 3.1 INTRODUCTION As discussed in section 1.2.2, in general, it has been found that the binding of molecular oxygen to a transition metal center occurs in conjunction with intramolecular electron transfer. This electron transfer includes the formation of either a transition metal-superoxo species (with the transfer of a single electron from the metal to the dioxygen ligand), or a transition metal-peroxo species (resulting from a two-electron transfer to dioxygen).8,18 These findings date far back into the literature and form the current paradigm for transition metal-dioxygen complexes. However, there have been examples over the years where either the O-O bond lengths or O-O stretching frequencies have not fallen into the allotted ranges indicative of either superoxo or peroxo complexes. For instance, in 1977, the report of the first four coordinate Rh-O2 side-on complex was described.118 The IR O-O stretching frequency was reported to be 990 cm-1, which is outside the range for either peroxo or superoxo complexes shown previously in Table 1.1. At that time, the investigators commented that the high frequency was indicative of a fairly strong O-O bond.118 Similarly in 1977, van Gaal and van den Bekermon described the synthesis of another Rh-O2 complex where the IR O- O stretch was reported to be 993 cm-1 also falling outside the allotted ranges for either peroxo or superoxo complexes.119 After that time, most transition metal-dioxygen complexes in literature astonishingly fell into the ranges provided in Table 1.1 so the structures reported in 1977 were thought of as anomalies and the bonding was not explored further. More recently, there have been reports of a small number of new complexes that could be similar to the two Rh-O2 structures from 1977 discussed above. In 2006, Milstein and coworkers reported the synthesis of a new Rh-O2 pincer complex, by bubbling oxygen through a benzene solution of a pincer-based dinitrogen complex as shown in Scheme 3.1.120  From the crystal structure data, they reported an O-O bond distance of 1.365 Å, which was the shortest such distance for Rh complexes in the 46  literature at the time.120 From the short bond distance, in conjunction with the discovery of a high coupling constant in a 31P{1H} nuclear magnetic resonance (NMR) spectrum, they concluded that their data was consistent with dioxygen ligation with no formal oxidation state change at the metal center.120  Despite this discovery, the authors did not probe into the implications this could have for the types of bonding exhibited by dioxygen when bound to transition metals.  Scheme 3.1. Synthesis, by Milstein and coworkers, of Rh-O2 pincer complex with short O-O bond length. In 2008, during the search for a new hydrogenation catalyst, Cathleen Crudden‟s research group, at Queen‟s University, synthesized an unusual Rh-O2 complex with unique properties. The synthetic route used by the Crudden group is shown in Scheme 3.2.  They reported the shortest O-O bond distance (1.315 Å), for a Rh-O2 complex known to date.1 This value is lower than what was reported by Milstein and coworkers in 2006, and also falls into the region between both rhodium peroxo complexes (1.4 – 1.5 Å), as well as what would be expected for a rhodium superoxo complex (1.2 – 1.3 Å) although none have been isolated thus far.18,121 However, as was alluded to in section 1.2.2, obtaining accurate O-O bond lengths in transition metal-dioxygen complexes is difficult because of positional disorder. This is particularly prominent in cases of symmetrical square planar complexes because of increased positional disorder of the ligands. Therefore, other experimental tools need to be used in conjunction with the X- ray crystallography to support the unusual bond length established for the complex. 47   Scheme 3.2. An example of the Crudden group synthesis of novel Rh-O2 complex. When the O-O stretching frequency could not be located in the IR spectrum, rR spectroscopy was utilized in collaboration with the Kennepohl group. Raman excitation at 568 nm provided an enhanced vibrational band at 1010 cm-1, which shifted in the 18O2 isotopomer. 1 This allowed them to assign the 1010 cm-1 band to the O-O stretch, making it significantly higher than what is expected for rhodium peroxo complexes (800- 930 cm-1). Based on the very short O-O bond distance and high stretching frequency, the data suggested the first possible synthesis and isolation of a rhodium(II)-superoxo (Rh(II)-(O2 -)) species.1 However, due to the square planar nature of the complex as observed from the X-ray structure, they proceeded to further investigate the electronics of the complex by implementing another experimental tool, Rh L-edge XAS, along with theoretical support from DFT calculations. Rh L-edge XAS spectra of the Rh-O2 complex, shown in Scheme 3.2, along with Rh(I) and Rh(III) reference compounds, were obtained at SSRL. Figure 3.1 shows the Rh L3-edge XAS spectra obtained for these complexes. The most noticeable difference between the two spectra is the discrepancy in the intensity of the main feature. Rh L3- edge XAS spectra are very sensitive to the electronic nature of the complexes, and as described in section 2.5, the technique probes excitations of core electrons (4d ← 2p3/2 transitions). The difference in intensity for the two reference complexes can be explained by use of a simple ligand field diagram of d orbitals for both a Rh(I) square planar complex and a Rh(III) octahedral complex, as exhibited in Figure 3.2. The Rh(III) d6 octahedral complex would have four holes available for excitation of the core electron; whereas, the Rh(I) d8 square planar complex would only have two holes. This explanation addresses the issue of the large increase in intensity between the two reference complexes and nicely illustrates the utility of assigning an oxidation state to the rhodium center using L3-edge XAS. 48   Figure 3.1. Rh L-edge XAS of RhCl3 (Rh(III) reference) and Rh(IPr)(OAc)(CO)2 (Rh(I) reference) compounds (adapted from reference 1, where IPr is 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene).   Figure 3.2. Simplified ligand field picture of d orbital splittings for Rh(III) (octahedral) and Rh(I) (square planar) complexes. After demonstrating the ability of Rh L3-edge XAS to distinguish between rhodium complexes of different oxidation state, the spectra of the newly synthesized Rh-O2 complex can be examined.  The Rh L3-edge XAS of the Rh-O2 complex, in comparison with both the Rh(I) and Rh(III) references is shown in Figure 3.3. The inset included at the top right hand corner of the figure is showing the first derivative spectrum of the 49  complex in the pre-edge region. There are two important factors to take into account for the spectrum of the Rh-O2 complex. First, it can easily be seen that the intensity of the complex is close to that of the Rh(I) reference compound, strongly suggesting the presence of a Rh(I) metal center in the Rh-O2 complex. Secondly, the complex exhibits a shoulder, at approximately 3009 eV, roughly 2 eV lower than the main feature. This shoulder peak can be more easily identified in the first derivative spectrum of the complex (see Figure 3.3 inset), and clearly indicates the presence of a second feature in the spectrum. A Rh(I) complex should only exhibit one excitation, as there is only one empty Rh d orbital (Rh 4dζ*(x 2-y2) (as can be seen from the simplified d orbital diagram in Figure 3.2). However, the L3-edge XAS spectrum clearly reveals the presence of a second feature, indicating the presence of a second transition.  Figure 3.3. Rh L3-edge XAS of Rh-O2 complex (black) compared to the Rh(I) (red) and Rh(III) (blue) references (adapted from reference 1). From the transition intensity rules for L-edge XAS, described in section 2.5, a transition can only occur if the rhodium core electron is being excited to an orbital that contains Rh 4d character. The presence of a second transition implies that a low-lying empty orbital containing Rh 4d character is present (in addition to the Rh 4dζ*(x 2-y2)). DFT calculations were utilized to gain insight into the identity of the orbital causing the pre-edge transition in the L3-edge spectra of the complex. 50  DFT calculations on the N-methyl derivative of the Rh-O2 complex, using the Gaussian software package and MPWPW91/genecp method, were used to create a valence MO diagram of the complex. The calculated results were consistent with a square planar, Rh(I) d8 metal center, coordinated to a dioxygen ligand in a singlet rather than triplet state.1 The bonding description from the DFT calculation was consistent with all of the experimental data discussed in the paper, and provided a rationale for the low energy shoulder seen in the L3-edge XAS spectrum of the complex. 1 The valence MO diagram from the DFT calculation indicated that there would be an orbital, approximately 1.7 eV lower in energy than the Rh 4dζ*(x 2-y2), made up of π- backbonding from a full rhodium d-orbital into an empty π* orbital on the dioxygen ligand (which they termed the RhO2π* orbital). 1 This orbital, contains Rh 4d character, as required by the electric dipole selection rule for Rh L-edge XAS, and falls almost 2 eV lower in energy from the main transition, in good agreement with the experimental spectrum. Consequently, this initial paper yielded a fully characterized transition metal- dioxygen complex exhibiting dioxygen bound to the metal center without concomitant oxidation of the metal and reduction of the ligand. This is therefore the first solid evidence for a heretofore unidentified bonding motif, side-on singlet dioxygen, bound to a transition metal in addition to the already well precedented cases of transition metal peroxo and superoxo complexes. The discovery and analysis of the above complex provided the basis for all of the experimental work to be presented and discussed in the remainder of this thesis. The analysis revealed not only side-on singlet dioxygen bound to a transition metal, but it provided evidence for dioxygen undergoing π-backbonding with a transition metal. This is an important factor because as discussed in section 1.2.4, the chemical community is in need of an experimental method whereby π-backbonding in transition metal complexes can be probed directly through experimentation. The unveiling of the availability of Rh L3-edge XAS to observe a direct transition to a RhO2π* orbital (made from π-backbonding of a filled Rh d orbital into an empty π* orbital on the O2) provided the inspiration that this could be a useful experimental tool that would be capable of directly probing π-backbonding in transition metal complexes, as that was not directly expressed in the paper. 51  Therefore, this work focuses on utilizing the technique to develop a method whereby quantitative π-backbonding data can be extracted from Rh L2,3-edge XAS. Therefore, the remainder of this chapter will include the introduction, discussion and analysis of different Rh-O2 complexes that were studied using Rh L2,3-edge and Rh K-edge XAS in conjunction with theoretical DFT calculations in order to identify complexes that also exhibit side-on singlet dioxygen bound to a rhodium center. A brief description of how the experimental data was collected on both the high and low energy beamlines (beamline descriptions in section 2.6), will be provided. Data processing, including calibration, background subtraction and normalization will be described. For the low energy L-edge data, this description will be in depth as a new methodology needed to be implemented in order to facilitate the fitting of the complex Rh L2,3-edge XAS spectra. The quantification of π-backbonding will be thoroughly addressed from the analyses. 3.1.1 Rhodium Complexes Examined One of the main goals of this thesis is to provide evidence for an experimental method, in this case Rh L2,3-edge XAS, capable of directly probing π-backbonding interactions in transition metal complexes. The major goal of the research herein was demonstrating a new procedure for quantifying second row transition metal L-edge data. In particular, we demonstrate a method whereby we can extract quantitative π- backbonding information from a set of Rh-O2 complexes.  In the original collaborative work, a new dioxygen bonding motif, side-on singlet dioxygen, was found with a rhodium metal center. We would like to further study this compound, along with similar complexes to evaluate the bonding exhibited by the dioxygen in the different systems. A set of five rhodium complexes, presented in Figure 3.4, were examined for this portion of the thesis work. 52   Figure 3.4. Rhodium complexes examined by Rh L2,3-edge and Rh K-edge XAS. The five complexes shown in Figure 3.4 were provided by Dr. Cathleen Crudden and coworkers at Queen‟s University. Complex 1 is the same compound that was reported and investigated in earlier work, where the side-on singlet dioxygen bonding was identified.1 We choose to study it again for a few different reasons. First, the original data was collected on a different beamline, and for our quantitative study we wanted the data to be from the same beamline for consistency. Second, the original data was only collected at the Rh L3-edge and in order to apply our new methodology to fit the experimental spectrum, we require the entire Rh L2,3-edge XAS spectrum to reduce the amount of user bias in the fitting procedure. The synthesis, X-ray crystal structure, and O-O bond length (1.267 Å/1.271 Å) of 2, were also reported in the 2008 article and because of the short O-O bond length and square planar structure, it was assumed to have the same coordination as 1.1 Therefore including it in our study was useful to provide experimental support for that assumption made in 2008, but more importantly, to provide us with an additional complex where we can test our method for quantifying π-backbonding. Complex 3 was newly synthesized in the Crudden group. Its synthesis, X-ray crystal structure, IR stretching frequency, and XAS spectra are described in a collaborative paper, between our group and the Crudden group, which will shortly be submitted for publication.122 Examining the XAS spectrum of this complex was important to assess the differences in the dioxygen coordination when the complex was 53  made cationic by adding an additional ligand. Complexes 4 and 5 serve as Rh(III) and Rh(I) reference compounds, respectively. Rhodium reference compounds are very important to this quantitative study for reasons described in section 3.1 and shown in Figure 3.2. When analyzing Rh L2,3-edge data, we will expect the intensity of a Rh(I) reference complex to be essentially half of a Rh(III) reference complex. Applying our new data fitting methodology for the reference complexes will be useful to demonstrate the validity of our method and it will serve as an easy comparison to decipher the oxidation states of 1, 2, and 3. 3.2 EXPERIMENTAL This section will describe how all of the high and low XAS data was collected on the various beamlines at SSRL. 3.2.1 Collection of Rhodium XAS Data The collection of all high energy, Rh K-edge data was performed on beamline 7-3 at SSRL (previously described in section 2.6.1). The samples were prepared for the beamline by gently mixing approximately 5 mg of complex with approximately 35 mg of Boron Nitride in a mortar and pestle until a homogeneous mixture was obtained. This mixture was spread onto Katpon tape and sealed into a typical XAS cell. The samples were prepared both in a glove box, under inert nitrogen atmosphere, as well as on the bench top to ensure that exposure to air was not causing a change in the complexes. For all complexes examined, this was never a problem and therefore all of the rhodium complexes were found to be non-air sensitive. Data was collected using a Si (220) phi= 0° double crystal monochromator, and although the signal was detected both by transmission and fluorescence (as described in section 2.6.1), the use of a 30 element germanium detector array provided high quality data and thus the fluorescence data was utilized for all analysis.  A rhodium metal foil was employed as a reference for energy calibration for all Rh K-edge data. In order to ensure high quality data was obtained, at least eight successive scans of each sample were acquired on at least two separate samples. These high repetition scans are important for two reasons; first to ensure that exposure to the X-ray beam is not causing decomposition or photo-reduction of the complexes, and second, to provide 54  good signal to noise data by means of averaging successive scans. XAS data were collected in the energy range from 22,890 – 24,140 eV using XAS Collect software. The collection of all low energy, Rh L2,3-edge data was completed on beamline 4-3 at SSRL (previously described in section 2.6.2). The samples were prepared by gently mixing an approximately 75:25% volume ratio of complex to Boron Nitride in a mortar and pestle until a homogeneous mixture was prepared. The mixture was then lightly spread onto a piece of sulfur and chlorine free Kapton tape. Data was collected using a fully tuned Si (111) double crystal monochromator and the signal was detected with a fluorescence (Lytle) detector filled with nitrogen gas (as previously mentioned in section 2.6.2). Also, discussed earlier, the low energy beamline does not allow for transmission spectra to be recorded because of the lack of signal strength at these lower energies. Therefore under normal circumstances, references for energy calibration are run before and after each sample to ensure the beam remains stable. However, this was not required for the rhodium complexes because the nitrogen gas in the Lytle detector provides an internal standard due to an impurity of argon gas. The Ar K-edge excitation occurs at an energy very close to the Rh L2-edge, and was therefore used as an internal calibration for all of the rhodium complexes examined. Samples were prepared both in an inert nitrogen atmosphere glove box, as well as on the bench top to ensure no sample degradation was occurring in air. At least three successive scans of each sample were run to ensure reproducibility and to produce higher quality spectra from scan averaging. Only three scans were run at the L-edges because of the superior high resolution spectra obtained due to the low energy X-rays used.  All XAS data were collected in the energy range from 2905 - 3250 eV using XAS Collect software. 3.3 PROCESSING OF RHODIUM XAS DATA In order to provide corroborative data for the non-routine Rh L-edge XAS analysis, Rh K-edge data was employed. All processing of the Rh K-edge XAS data was performed using SIXPACK, a well documented software package developed by Sam Webb.123 The first step of the K-edge analysis involved comparing all data for each sample and averaging the identical scans together. Secondly, energy calibration was performed by utilizing the internal reference spectrum of a rhodium metal foil. The 55  lowest inflection point of the Rh K-edge foil spectrum was set as 23,220 eV. This was followed by background subtraction and normalization, which was carried out by employing a linear pre-edge function and a quadratic post-edge function. This form of analysis is fairly straight forward for transition metal complexes and due to the fact that the spectra are dominated by a single, very intense edge jump, the overall user bias brought into the analysis is minimal. Unlike the Rh K-edge analysis, the analysis of Rh L2,3-edge XAS is not a routine procedure and has not been previously developed in the literature. In addition to the difficulties with simple data processing to obtain useful information from the spectra in general, the fact that our goal is to gain quantitative information further complicates the analysis of these spectra. Thus, the employment of a new program for XAS analysis was required. Blueprint XAS was developed in 2009/2010 by a former colleague, Dr. Mario Delgado.100,101 In addition to the new data processing program, a new methodology, also developed by Dr. Delgado, was essential for the completion of the analyses. Therefore, the next section will provide an overview of the processing steps and methodology needed to perform the untraditional and more complex analysis of the Rh L2,3-edge XAS spectra. The methodology was part of a collaborative work between myself and Dr. Delgado, and together we wrote a manuscript, that will soon be submitted to Inorganic Chemistry, based on applying his methodology to the Rh-O2 complexes explored in this chapter.124 3.3.1 Rhodium L2,3-Edge XAS Data Processing Steps All of the Rh L-edge data that will be discussed in this work were processed using Blueprint XAS, a Matlab-based graphical interface, for analysis and fitting multi-edge spectra. This program allows for data from various synchrotron facilities to easily be imported and the first data processing steps, including data re-sampling, scan averaging and calibration, to be executed without much difficulty. For all of the Rh L- edge data, this program was used for these initial steps. Calibration, as mentioned earlier, was performed internally by using the argon impurity present in the nitrogen gas inside the fluorescence detector. Although the Ar K-edge occurs very close to the edge jump of the Rh L2-edge, which works well for calibration, the closeness in energy of the two features complicates the quantitative fit because more fitting parameters are 56  required for the evaluation function that will be discussed below. For calibration purposes, the argon K-edge, which appears as a negative dip in the Rh L2-edge, was set at 3205.9 eV for all complexes. Besides very complex backgrounds that tend to change in steepness after every edge jump and are typical in second row transition metal L-edge data, processing of this data is further complicated by common analysis programs because too much lenience is given to the user. This can result in the final fitted data being quite biased towards the users‟ input. Like most analysis programs, Blueprint XAS allows the user to define a physical model that will be utilized for fitting the experimental data; however, the uniqueness of the methodology behind Blueprint XAS is three fold. First, the program still requires that the user defines a physical model, an evaluation function, which will be used to fit the data. However, the background is included in this function in order to minimize errors associated with fitting the background first (or subtracting the background first) as has been most commonly done in XAS analysis.101   Second, the methodology is founded by utilizing a Monte Carlo-based method to generate starting points for multiple independent fits of experimental data, which should lead to much less user bias in the analysis.101 Finally, the program is designed to allow a large number of fits to be acquired, which provides quantitative estimations of error that is not commonly practiced when fitting XAS data due to the tedious nature of the more common fitting practices .101 Another important factor that has prevented second row transition metal L-edge data from being thoroughly examined involves the double edge (both L3 and L2 edges) being split over a fairly large energy range. The dominant interaction of the strong 2p spin- orbit coupling in the excited state of the rhodium causes the large energy splitting of the L3 and L2 edges. The background tends to change after each edge jump, meaning that the function modeling the background must be adjusted after each absorption edge, which is quite a challenge. However, Blueprint XAS implements the use of a switch-like background function, which has parameters that are linked to at least one of the edge parameters, helping to reduce the number of parameters needed to create the overall evaluation function.101 Also, in the past, the intensity ratio between the L2 and L3 edges has often been considered statistically 1:2; however, it has been shown that this value 57  can vary significantly from one compound to another.97,125  Thus, the evaluation function allows for a parameter (B) to be used to address the branching ratio of the two L-edges, and in turn B is permitted to float in the fitting procedure and is used to relate the ratios of the intensities of the features in both edges. Another complication that arises from simultaneous fitting of multi-edge spectra involves the large number of parameters required to account for all of the transitions and features observed in the experimental data. The methodology is based on the recognition that the large splitting of the 2p spin orbit coupling makes the Rh L2,3-edges under almost jj-coupling conditions. This is significant because it establishes that the dominant interaction in the spectra is the 2p spin orbit coupling. Thus all other interactions, ligand-field, bonding and d-d inter-electronic interactions, should be considered as perturbations of the system thereby making the features in the Rh L2- edge duplicates of the features in the Rh L3-edge. This is why the branching ratio can be used to relate the intensities of the features in the two different edges, and parameter Δ, which is roughly equivalent to 3/2 of the spin orbital coupling, can be used to shift each equivalent feature in the L3-edge to the L2-edge. 65 Additionally, the methodology recognizes that for 4d transition metals, the broadening of the two edges should be approximately the same because Coster-Kroning processes are not possible.126 Therefore, the parameters for width and Gaussian-Lorentzian ratio (for pseudo-Voigt function shape) for all rhodium peak and edge parameters will be approximately the same at both the L3 and L2 edges (for peaks occurring before the edge). Although this approximation should be less accurate when using fluorescence detection, it appears to hold true for these complexes, as evidenced by our fits. Also, because the overall goal of the research was to gain quantitative π-backbonding information, internal normalization was implemented within the fitting procedure. In each case, the intensity of the main peak was represented by the product of the total intensity of the two edge jumps and the intensity of the peak itself.  Details showing how the parameters are used for the model to fit the experimental Rh L2,3-edge XAS spectrum of 1 is shown in Equation 3.1. 58   Equation 3.1. Model used to fit experimental Rh L2,3-edge XAS spectrum of 1. Using this methodology, the multi-edge XAS spectra of various Rh-O2 complexes were successfully fit. An example of the model function used to fit the Rh L2,3-edge spectrum of 1 is shown in Equation 3.1, along with a description explaining the identity of each parameter.  Figure 3.5 also shows a pictorial representation of how each component of the evaluation function makes up an important part of the fitting of the experimental spectrum. In this figure, the total fitted spectrum (represented as F), is shown directly on top of the experimental data to demonstrate the closeness of the fitted spectrum to the experimental spectrum. Additionally, the background function (presented as b in the figure), is a linear combination of functions that fit the different quasi-linear background regions (3 in this case; 1 before the L3-edge, 1 between the 2 rhodium edges, and 1after the L2-edge). The two rhodium edges were fit using step-like functions, shown as (eL3 + eL2) and the argon edge was accounted for by subtracting a negative step-like function (eAr). 59   Figure 3.5. Generic model used to quantitatively fit experimental Rh L2,3-edge XAS spectra of complexes 1-5. BlueprintXAS and our fitting methodology was utilized because we chose to fit the entire Rh L2,3-edge XAS spectrum; however, it would be possible to fit only the L3-edge and thus use only a single background correction. There are a few problems with performing the analysis in this less sophisticated manner.  If only the L3-edge is fit, removing the background as a single correction can introduce error into the fit right from the beginning, because the background tends to change after edge. More importantly, if only the L3-edge is fit, we arrive back at the original problem that is common in XAS data fitting, which is the introduction of user bias into the fitting procedure. The entire 60  premise of using our fitting methodology is to eliminate as much user bias into the analysis as possible. If only the L3-edge of the spectrum is fit, the positioning of the L3- edge in particular, is a lot more user dependent. One of the strengths of our methodology lies in the use of the Δ term. Connecting all of the features, in particular the L2,3-edges, by this term is important for the validity of fitting output. This methodology represents a cautious approach to fitting that seeks to provide bias-free fits of the data with representative error bars. Other approaches to fitting these data yield lower systematic errors in the fits, but with significant user bias. Although the model function shown in Equation 3.1 was specific for 1, similar functions were utilized for all of the complexes examined. The major difference in the fits of the complexes involved the number of parameters used, which directly resulted due to the presence or absence of the low energy shoulder feature.  Also, because the low energy feature present in the spectrum of complex 1 was so close in energy to the more intense feature, a few extra variables were used in this fitting model. For instance, the use of the variable „c‟ was necessary for all complexes that had two pre-edge peaks very close together (this was the case for 1 and 2). Variable “c” was used to act as a fraction of the total pre-edge intensity (I1). Also, because the first and second feature of complexes 1 and 2 are so close together, the energy of the very first feature (shoulder) was determined relative to the position of the main feature (hence E1 + E2 in the model). Thus, parameter E2 provided us with an energy position for the main feature, and E1 was a number relative to that energy in order to ensure positional swapping did not occur during the fitting procedure. Another important factor to discuss for all of the rhodium L-edge fits involves the shape and width of the pre-edge peaks. All rhodium pre-edge peaks (V1, V1ʹ, V2, V2ʹ for complexes 1 and 2, and just V1 and V1ʹ for complexes without the low energy shoulder) were fit using the same Gaussian-Lorentian shape and peak-width as demonstrated in Equation 3.1. However, for all five rhodium complexes, there was also a smaller feature present just after the main feature, at an energy higher than the edge jump. Because this feature is after the rhodium edge in all complexes, its peak width should be larger. Thus, as demonstrated in Equation 3.1, it is forced to have a width at least as wide as 61  the pre-edge features by making the total width equal to the width of the pre-edge features (W2) plus a new floating width variable (W3, which may not be less than zero). Overall, this holistic fit model, where the background removal is incorporated into the fitting procedure, should ensure that the intensities of other spectral features are not being unevenly and inappropriately removed during analysis. A table of parameters used for fitting each complex (1-5), is included in Appendix A. In addition, Appendix A also contains graphical representations showing the overall fit compared to the actual experimental data for complexes 1-5. Blueprint XAS also has a post-fitting tool which allows for normalization and background subtraction of the data. These steps are performed after the fitting procedure for the purposes of graphically representing the experimental data. Thus, all of the analyzed spectra that will be shown throughout the rest of this chapter have been processed in this manner for visual representation and comparison only. In order to obtain good quality quantitative fits, it was imperative that the entire spectrum was fit, including both L3 and L2 edges as well as the negative argon edge as shown in Figure 3.5. However, the spectra that will be presented for the remainder of this chapter will display the L3-edge only for clarity purposes. 3.4 DATA ANALYSIS AND DISCUSSION The XAS spectra of complexes 1 – 5 were all processed in Blueprint XAS, utilizing the new methodology described above. The normalized and background subtracted Rh L3-edge XAS data for the two reference complexes, 4 and 5 is shown in Figure 3.6. From this figure it is apparent that 4, the Rh(III) reference, is more intense than 5, the Rh(I) reference, similar to what was shown in Figure 3.1, from the Rh L3-edge data of reference compounds studied in the 2008 article. As discussed previously, a Rh(III) complex should be more intense than a Rh(I) complex because there are two empty 4d orbitals to which the core electron could be excited, rather than one as is the case for a Rh(I) species. The availability of an extra empty 4d orbital provides two extra “holes” for excitation of the electron and thus the intensity of a Rh(III) complex should be almost double that of a Rh(I) complex. 62   Figure 3.6. Normalized and background subtracted Rh L3-edge XAS spectra of 4 (Rh (I) reference, Rh(IPr)2H2Cl) and 5 (Rh(III) reference, Rh(IPr)(OAC)(CO)2). From the fully fitted spectra of 4 and 5, quantitative information was extracted to aid in assessing the differences in oxidation state. Important values, with their associated errors, from the fully fitted Rh L2,3-edge XAS spectra are presented in Table 3.1. From these values it is evident that the fitted intensity of 4 is sufficiently larger than 5 with normalized intensities of 5.45 eV and 3.27 eV respectively. These values indicate that 4, the Rh(III) reference is roughly 1.7 times as intense as 5, the Rh(I) reference, which is to be expected when considering bonding and ligand field interactions. This demonstrates the effectiveness of our new fitting methodology and provides evidence that we should obtain reliable results for the more intriguing Rh-O2 complexes. The rest of the quantitative data obtained from fully fitting the XAS spectra of complexes 1 – 5 63  will be discussed later in the chapter (section 3.4.2), when the implications for quantifying π-backbonding in transition metal complexes is addressed. Complex  Peak Position (eV) Normalized Pre-edge Intensity (eV) 4  3008.3 ± 0.0 eV 5.45 ± 0.60 5  3008.3 ± 0.0 eV 3.27 ± 0.38  Table 3.1. Quantitative parameters extracted from fully fitted spectra of 4 and 5. Complexes, 1, 2, and 3 were investigated in a manner similar to the two reference compounds. The initial fitting parameters used were similar for all complexes with major differences coming from the extra shoulder pre-edge feature present in a few of the complexes (see Appendix A for details). Figure 3.7 shows a comparison between 1, 4, and 5. The first detail to recognize is the  of the main feature in 1 is much lower than 4 but still somewhat higher than 5. Visually, this could be misleading as a rhodium complex with a peak intensity between a Rh(I) and Rh(III) could indicate the presence of a Rh(II) species. However, quantitatively, from our fitting procedure, we obtain intensities for 1 of 5.07 ± 0.46 eV, 4 of 5.45 ± 0.46 eV and 5 of 3.27 ± 0.38 eV. This shows that the intensity of 1 and 4 are very close together indicating they should both contain a rhodium center with the same oxidation state. However, this is not the case because the quantity of the intensity reported for 1 is the total intensity, which includes two transitions, both the main feature as well as the shoulder peak discussed below. 64   Figure 3.7. Normalized and background subtracted Rh L3-edge XAS spectra of 1, 4 and 5. A small shoulder can be seen in the XAS spectrum of 1 that is not present in either 4 or 5. The presence of a shoulder peak is intriguing, because as mentioned previously, whether 1 has a Rh(I) or Rh(III) metal center, only one transition is expected in the Rh L2,3-edge spectrum. The detection of an additional spectral feature provided strong evidence for an interesting electronic structural description of 1. It was the discovery of this extra transition, in 2008, that prompted the use of theoretical DFT calculations to explore possible explanations for the experimental findings. Results from the DFT provided a well supported rational for the suggested electronic description of the complex as a Rh(I)-1O2. Figure 3.8 shows a comparison of 1 and 2 with both reference complexes. It is evident that the spectra for 1 and 2 are similar, their spectra almost completely overlap. 65  Additionally, the extra shoulder feature is also present in the spectrum of 2. The similarity between the complexes should not be overly surprising as they both appeared to have a square planar geometry according to their crystal structures and the only difference between them is an increase in bulk on the NHC ligand of complex 2. Therefore, 2 is another example of a Rh(I) complex with an side-on singlet dioxygen bound ligand.  Figure 3.8. Normalized and background subtracted Rh L3-edge XAS spectra of 1, 2, 4 and 5. The last complex in the series to be investigated is the cationic species 3. Figure 3.9 displays the Rh L3-edge XAS spectra of 3, 4, and 5 for comparison. By examining the spectrum of 3, two details are apparent. First, and most evident, the spectrum of 3 is almost completely overlapping the spectrum of 4, our reference Rh(III) complex – suggesting that 3 has a similar valence electronic configuration (i.e. it contains a Rh(III) 66  center). In addition to the almost direct overlap with the reference complex, the spectrum of 3 also exhibits no low energy shoulder feature. Since we have assigned this low energy shoulder as the π-backbonding peak between dioxygen and the rhodium metal center, it can be inferred that 3 does not exhibit this unique side-on singlet dioxygen bonding motif found for 1 and 2. The lack of the shoulder supports the idea that 3 has a Rh(III) center as peroxo species are not capable of π-backbonding as there is no π system. Therefore, our experimental results strongly support the description of complex 3 having dioxygen bound in the commonly found peroxide form.  Figure 3.9. Normalized and background subtracted Rh L3-edge XAS spectra of 3, 4 and 5. To further explore the electronic structure, Rh K-edge XAS was examined. The basic principles of transition metal K-edge XAS were explained in section 2.4. By contrast to that observed for Rh L-edge spectra, Rh K-edges have a poor energy resolution due to 67  the high energies required for the 1s excitation. Despite this, some very useful electronic information may be obtained. The inflection point, obtained by the first derivative of the K-edge spectrum, provides a direct measure of the ionization energy of a core 1s electron.  Therefore, for similar systems, the energy position of the inflection point at the rhodium metal K-edge is indicative of the charge on the rhodium center. The Rh K-edge XAS spectra for complexes 1, 2, and 3, with their corresponding edge inflection points, are shown in Figure 3.10. The spectra of 1 and 2 are very similar to one another, and have absorption edges at significantly lower energy than 3.An increase in the edge inflection point of approximately 2 - 3 eV is apparent for complexes 1 and 2 when compared with 3. Since a difference of approximately 1 - 2 eV normally corresponds to an increase in oxidation state of one, the K-edge data agrees with the electronic description that complexes 1 and 2 are Rh(I) species bound to side-on singlet dioxygen, while 3 is an example of a typical Rh(III) peroxo complex.  Figure 3.10. Normalized and background subtracted Rh K-edge XAS spectra for 1, 2 and 3.  68  Although all of the above XAS data provides strong support that 1 and 2 have a very different electronic description than 3, more insight into the differences is desired. Since the DFT calculations on the N-Methyl derivative of 1, examined in the original 2008 communication, provided a rationale for the low energy shoulder seen in the Rh L3-edge XAS as well as accounted for other unusual physical properties attained from experimental investigations of 1, we wanted to further pursue theoretical investigations. In particular, since the new XAS data described above supports that both 1 and 2 exhibit a new side-on singlet dioxygen coordination and 3 is another example of a typical transition metal-peroxo species, additional insight into the cause for the differences of the electronic descriptions of the three Rh-O2 complexes is desired. Therefore, the next section presents results from DFT calculations performed to support the XAS data as well as to provide possible insight into the cause of the differences in electronic structure between the fairly similar Rh-O2 complexes. 3.4.1 Results and Discussion of DFT Calculations Complete geometry optimizations of the unmodified, full structures of complexes 1, 2, and 3+ (the counter ion BF4 - was not included) were all performed using the ADF software package. The starting geometry input for these three complexes was extracted from the crystallographic data in the Crystallographic Information File (CIF) provided by our collaborators from the Crudden group at Queen‟s University. The final DFT geometry optimized structures were in good agreement with the X-ray crystallographic data, tables showing a comparison of important bond distances and angles are provided in Appendix C. Despite having the crystallographic data, performing geometry optimizations is good practice to ensure that DFT is in agreement before going on to more advanced calculations. All final geometry optimizations for the three complexes were conducted utilizing the Vosko-Wilk-Nusair LDA in conjunction with the GGA containing exchange and gradient corrections by Becke and Perdew (BP86 functional in ADF), along with a TZP basis function and an integration accuracy of 6.0.107-109,112  The only frozen core approximation that was used was for the rhodium center; it was frozen using the TZP 3d core orbitals (i.e. all orbitals from n=1 to n=3 were frozen) built into ADF. 69  The finalized structures from the geometry optimizations were utilized for the single point energy calculations. These were all performed in ADF, and the exact same settings (including basis sets, functionals, integration accuracy and frozen core) were used in the single point calculations. The results from these DFT calculations were used to construct valence MO diagrams for complexes 1, 2 and 3+. The resulting valence MO diagrams, including images of the important MOs are illustrated in Figure 3.11, Figure 3.12 and Figure 3.13 respectively. The first important thing to note is that the diagram shown in Figure 3.11 for 1 is very similar to what Praetorius et al.  reported for the N- methyl derivative of 1 in their 2008 paper, indicating that the modification to the large NHC ligand didn‟t alter the overall DFT predicted results.1 The realization of this finding is important because for some of the more time-consuming frequency calculations upcoming in this thesis, the N-methyl derivative of various complexes was utilized in order to achieve reasonable computation times.  Figure 3.11. A valence MO diagram of 1 constructed from single point DFT calculation (Hydrogens left out for clarity and contour of 0.05 used to represent orbitals). 70   Figure 3.12. A valence MO diagram of 2 constructed from single point DFT calculation (Hydrogens left out for clarity and contour of 0.05 used to represent orbitals). 71   Figure 3.13. A valence MO diagram of 3 +  constructed from single point DFT calculation (Hydrogens left out for clarity and contour of 0.05 used to represent orbitals). From these valence MO diagrams, it is quite apparent that DFT predicts that the electronic structure for complexes 1 and 2 is very similar. First of all, the HOMO-LUMO gap is predicted to be 0.768 eV for 1 and 0.796 eV for 2. Within the accuracy of DFT, these values are identical. Secondly, the distance between the LUMO and the Rh 4dζ*(x 2-y2) orbital is 1.454 eV for 1 and 1.452 eV for 2; again, these values are essentially the same. More importantly however, is the fractional composition of the MOs predicted from these calculations. Of particular interest is the LUMO. Praetorius et al  reported that the LUMO of the simplified N-methyl derivative of 1 corresponded to an orbital comprised of π-backbonding from a full rhodium d-orbital into an empty π* orbital on the dioxygen ligand (which was termed the RhO2π* orbital). 1 In our calculations for the complete structures of both 1 and 2, we observe a similar result. In particular, the compositional breakdown of the MO in question, from the single point calculations, indicates this orbital consists of more oxygen p character (53%O p) than rhodium d character (40% Rh d), indicating that it is in fact a ligand based orbital. Additionally, the 72  bonding version of this orbital is inverted such that the bonding/antibonding pair can be relatively easily ascribed to a metal-based filled orbital and a ligand-based empty orbital.  These calculations provide a rationale for the appearance of the shoulder peak in both Rh L2,3-edge XAS spectra of complexes 1 and 2 as observed in Figure 3.8 and as previously postulated in the article by Praetorius et al. Both the experimental XAS data and these theoretical calculations strongly support the proposal that the LUMO in both complexes is indeed ligand-based and representative of the RhO2π* orbital. This description is indicative of the side-on singlet dioxygen bonding, proposed by Praetorius et al, whereby the dioxygen has successfully bound to the rhodium center without formal oxidation of the rhodium metal center or formal reduction of the dioxygen ligand. In other words, both 1 and 2 are examples of Rh(I)-1O2 complexes. The valence MO diagram of 3+, as seen in Figure 3.13, is significantly different than those for complexes 1 and 2. The two most evident differences as can be seen from the figures include a much larger HOMO-LUMO gap (1.517 eV) and a significantly smaller gap between the LUMO and the Rh 4dζ*(x 2-y2) orbital (LUMO+1) (0.189 eV). Although the actual HOMO-LUMO gap isn`t probed by our experiments, the increase in the gap causes the LUMO to be much closer in energy to the empty Rh 4dζ*(x 2-y2) orbital. Therefore, no shoulder peak would be expected in the Rh L2,3-edge XAS spectrum of 3, and, as indicated in Figure 3.9, only one main feature was seen with no lower energy feature.  More importantly, the predicted composition of the LUMO for this complex is much different than for both 1 and 2. As opposed to 1 and 2, where the LUMO was predominately ligand-based, the LUMO of 3+ is dominated by Rh 4d character (50% Rh d, 40% O p). This finding indicates that unlike complexes 1 and 2, which exhibit side-on singlet dioxygen bound to rhodium and significant π-backbonding, complex 3 is another example of the well known transition metal-peroxo bonding complexes. Thus, all DFT calculations are in good agreement with both Rh L2,3-edge and Rh K-edge XAS experimental data which indicate that 1 and 2 are examples of Rh(I)-1O2; whereas 3 is a typical Rh(III)-peroxo complex. To provide further evidence of the differences in the electronic structure of complexes 1 and 2 compared to 3, several atomic charges from the single point energy calculations were examined. The Hirshfeld and Voronoi charges on the rhodium metal 73  center, calculated from the single point energy DFT calculations of each complex, are tabulated in Table 3.2. From these data it is clear that DFT predicts that from both types of charge calculations, the rhodium center in complex 3+ is more positively charged than either of the rhodium centers in 1 or 2. This, in combination with the large increase in the HOMO-LUMO gap, and composition of the LUMO in particular, provide theoretical calculated support for the experimental data which points to a significant difference in electronic description for 3 compared to both 1 and 2. Complex Hirshfeld Charge on Rhodium Voronoi Charge on Rhodium  1  0.2  1.23 2 0.21 1.25 3 + 0.28 1.62 Table 3.2. Hirshfeld and Voronoi charges for the Rhodium metal centers extracted from single point DFT calculations. From the above theoretical calculations in conjunction with the experimental Rh L2,3 and K-edge data, complexes 1 and 2 should be considered to be square planar, Rh(I) metal centers with π-backbonding between the dioxygen and the metal center, whereas 3 is not square planar and has a Rh(III) metal center bound to a reduced dioxygen ligand. From these findings, it appears that we may have two limiting cases, the first novel case where there is no formal oxidation at the metal center when dioxygen is bound and the second, very well-known case, where the metal is bound to a peroxo- type ligand. A pictorial representation of these two limiting situations is illustrated in Figure 3.14. Since both experiment and theory support these two distinct cases, our scientific curiosity desires to probe the region between these two limiting electronic states. 74   Figure 3.14. A proposal of the two limiting cases of rhodium dioxygen bonding exhibited.  Exploring the region between the two electronic descriptions shown in Figure 3.14 may have important implications for controlling π-backbonding. Altering the ligand trans to the dioxygen may have significant impacts on the π-backbonding. We sought to explore this possibility through a combination of crystallography, vibrational spectroscopy, XAS and computational studies. Unfortunately, synthetic challenges have limited our ability to perform the experimental aspects of these studies, so only computational exploration on hypothetical complexes was investigated. As a first step towards understanding the factors that control the electronic structure, the influence of the ligand trans to the dioxygen was investigated through computational studies. Thus, a relaxed potential energy surface DFT calculation was employed to assess the effect of altering the trans ligand. As these calculations are large, computationally demanding and time-consuming, the calculation was performed on the N-methyl derivative of 1 shown in Figure 3.15. To perform this calculation, the first step included optimizing the truncated structure.  Figure 3.15. An example of the N-Methyl derivative of 1 used for linear transit DFT calculations. 75  For the relaxed potential energy surface calculation, the Rh-Cl bond was the variable that was systematically changed to model the effect of having a strong π-donor trans to the dioxygen. An illustration of the major compositional changes in the LUMO as a function of Rh-Cl bond length is displayed in Figure 3.16. From this figure, it is apparent that, not surprisingly, the amount of Cl p character significantly increases as the Rh-Cl bond length is shortened. However, the more unexpected finding was the Rh d character was significantly decreased with shortening of the Rh-Cl bond; whereas the O p character remained essentially unchanged. Additionally, the O-O bond length remains essentially unchanged for the entire range of Rh-Cl bond lengths tested. Thus, the RhO2 π* orbital remains ligand-based throughout a wide range of Rh-Cl bond distances. This finding suggests that even relatively significant changes in the ligand trans to dioxygen only have minimal affects on the overall rhodium-dioxygen bonding description. The other important factor is the drastic decrease in Rh d character present in the LUMO due to increasing Cl p character as the Rh-Cl bond was compressed. The fact that the overall bonding description remains unchanged, despite significantly less π-backbonding, implies that the π-backbonding cannot be the sole contributor causing the stabilization of the Rh(I)-1O2 species. 76   Figure 3.16. The percentage of Rh d, Cl p, and O p character present in the LUMO as the Rh-Cl bond is shortened in a linear transit DFT calculation.  In attempts to further explore the effect of altering the ligand trans to dioxygen, a series of in silico models, with varying trans ligands were investigated. The investigation of the modified structures was initially started by Oscar Hernándes Fajardo, an undergraduate student working under my supervision for a few months. All of these calculations were completed using ORCA, and the structures investigated are shown in Figure 3.17. The same basis set and functional used in the previous ADF studies were also employed for these calculations. A list of the extracted O-O bond distances from the calculation of each complex is provided in Table 3.3, along with the estimated O-O stretching frequencies from the frequency calculations. The most evident observation from this data is that DFT predicts that the O-O bond distance will remain lower than what is expected for rhodium peroxo complexes, and O-O stretching frequencies higher than what is expected for peroxo complexes in all cases. 77   Figure 3.17. A list of N-methyl-derivatives of 1 where the ligand trans to the dioxygen was altered for DFT calculations.  Complex  O-O Bond Distance (Å) O-O Stretching Frequency (cm -1 )  RhO2(NHC)2CO  1.359  n/a* RhO2(NHC)2I  1.380 1007.89 RhO2(NHC)2Cl  1.382 1009.62 RhO2(NHC)2Br  1.383 1008.92 RhO2(NHC)2CH3S  1.388 992.03 RhO2(NHC)2CH3O  1.392 989.86 *(RhO2(NHC)2CO O-O stretching frequency n/a because it did not converge. Table 3.3. Tabulated O-O bond distances and O-O stretching frequencies from DFT calculations. A graph indicating the DFT predicted O-O bond lengths in conjunction with the amount of Rh 4d present in the RhO2π* orbital for each of the modified structures is shown in Figure 3.18. From these results, it is clear that DFT not only predicts all of the structures to have an O-O bond length shorter than expected for peroxo complexes, but also that all of the compounds will have less than 50% Rh character in this RhO2π* orbital. Therefore, in all of the structures investigated, the only structure DFT predicts will not be an example of the novel Rh(I)-1O2 is the cationic complex 3. It should be noted that DFT predicts a large amount of backdonation from the rhodium into the O2π* orbital, which is perhaps misleading as to why there is no formal oxidation at the rhodium center and this is not better described as a Rh(II)-superoxide complex. The overall explanation, from both the spectroscopy and the DFT combined, is a formal description of a Rh(I)-1O2 that is highly covalent in character and has lots of charge 78  delocalization. To test this, recent theoretical work in our group shows the presence of a high energy broken symmetry excited state which is too high in energy to be considered.  From the current DFT results in Figure 3.18, there may be a trend showing that when ligands capable of π-backbonding (CO) are trans to the dioxygen the amount of π-backbonding in lessened; however, the overall formal bonding description remains unchanged. Having synthetic versions of some of these complexes would be valuable in order to provide experimental data to compare with the DFT predictions.  Figure 3.18. Graphical DFT results showing the affect of the trans ligand on RhO2 backbonding. One final, and perhaps most interesting, structure studied by DFT utilizing ORCA, was a mono-nitrile analog of 3 ([RhO2(N-methyl-NHC)2(MeCN)] +). The study of this model was of particular importance to determine whether the overall charge of the complex was causing any significant changes in the π-backbonding or alterations in the type of RhO2 bonding scheme formulated. The calculated structure of this modified complex was similar to the neutral mono-chloro complex, including a similar O-O bond distance of 1.373 Å. Additionally, the overall bonding description of the calculated 79  complex is essentially identical to the mono-chloro complex, indicating that the charge of the complex doesn‟t seem to be affecting the overall bonding description. This is in complete opposition to what is seen both experimentally and computationally for the bis-nitrile complex (3+). The cationic complex 3+ exhibited substantially different bonding at the rhodium metal center. From the experimental data, the O-O bond distance was much longer (1.428 Å), and the Rh L2,3-edge and K-edge data was indicative of a Rh(III)-peroxo species. Computationally, the DFT results are in good agreement with this experimental data, predicting a longer O-O bond distance (1.43 Å) and a LUMO which is dominated by metal character rather than oxygen character. Therefore, the inclusion of an extra ligand, rather than altering the trans ligand may have more to do with the stabilization of the Rh(I)-1O2 species. Having additional complexes of this nature to study experimentally would be of great use to substantiate this potential rationale. 3.4.2 Quantifying π-Backbonding One of the overall goals of this thesis work was to develop a method whereby quantitative backbonding information for second row transition metal complexes could be extracted directly from experimental data. By using the methodology described in section 3.2.1, this was accomplished for complexes 1 and 2 through completely fitting their experimental Rh L2,3-edge XAS spectra. These quantitative investigations of the π- backbonding exhibited between the rhodium metal center and the dioxygen ligand may provide significant insight into the strength of this interaction. To assess the bonding, the Rh L2,3-edge XAS spectra of all five rhodium complexes were completely fit utilizing the methodology vide supra in order for appropriate comparisons to be completed. Table 3.4 displays important parameters extracted from the completed fits of the complexes. As shown earlier, in Table 3.1, the quantitative values for 4 and 5 validate the reliability of the fitting to correctly assess the oxidation states of the two reference complexes. To that effect, the total intensity of the main feature of 3 (4.99 ±1.09 eV) is close to that of 4 (5.45 ± 0.60 eV) the Rh(III) reference, again providing strong evidence that this cationic species underwent rhodium oxidation when dioxygen coordinated forming a typical rhodium-peroxo complex in agreement with all other experimental and theoretical data. On the other hand, the interpretation of 80  the data presented in Table 3.4 for 1 and 2 requires more consideration and explanation. Parameter/Complex 1 2 3 4 5  Total Normalized Intensity (eV)  5.07 ± 0.46  5.08 ± 0.52  4.99 ± 1.09  5.45 ± 0.60  3.27 ± 0.38 Fractional Intensity of Shoulder (relative to total) 0.43 ± 0.09 0.54 ± 0.13 - - - Main Peak Position (eV) 3008.2 ± 0.1 3008.3 ± 0.1 3008.2 ± 0.0 3008.3 ± 0.0 3008.3 ± 0.0 Relative Shoulder Peak Position (eV) -1.41 ± 0.10 -1.49 ± 0.16 - - -  Table 3.4. Relevant fit parameters from fits of complexes 1-5. The total normalized intensities of 1 and 2 are in line with that reported for the Rh(III) reference complex 4, as discussed previously in section 3.4. This should not be the case if 1 and 2 are Rh(I) complexes as assigned. However, the total normalized intensity includes both the main feature at 3008.2/3008.3 eV as well as the shoulder peak at -1.41 and -1.49 eV lower in energy than the main feature. As can be seen from the table, the shoulder peak accounts for almost 50% of the total normalized intensity. Therefore, this puts the calculated intensity of the major feature for 1 (2.89 eV) and 2 (2.34 eV) approximately equivalent to what is reported for 5 (3.27 eV), the Rh(I) reference. This substantial contribution to the total normalized intensity from the shoulder peak indicates the presence of a large Rh 4d charge back-donation into the dioxygen π* orbitals. Thus, the quantitative values reported in Table 3.4 for the intensities of the shoulder feature in 1 and 2 represent the quantitative amount of π-backbonding present in each complex. It is important to mention that although we have obtained these quantitative values for the intensity of the π-backbonding in the complexes, we cannot report, at this stage, an accurate percentage for π-backbonding. In order to report an absolute value, we require a value for the pure transition dipole integral of a Rh 4d ← 2p transition. If we assume that our DFT calculations provide a reasonable estimate for the 4d orbital contribution to the 4dζ* orbitals in all complexes, we can use this value as an initial approximation of the conversion factor to determine quantitative values for 81  covalent bonding. This provides an estimate of the transition dipole integral (I (Rh4d ← Rh2p as discussed in Equation 2.5) of 0.09, which corresponds to 24% π-backbonding and 30% π-backbonding in complexes 1 and 2 respectively. When new complexes with differing trans ligands are obtained, similar experimental studies will be performed to observe how the ligand alteration affects the quantity of π- backbonding present. Additionally, this technique could be useful for assessing π- backbonding in additional second row transition metal complexes, such as palladium or molybdenum, with other π-backbonding ligands such as alkenes, carbonyls, and nitrosyls. The next chapter will focus on a similar experimental study on novel ruthenium dioxygen complexes. 82  Chapter 4 : XAS INVESTIGATIONS OF RUTHENIUM DIOXYGEN COMPLEXES  4.1 INTRODUCTION The previous chapter provided a detailed explanation of how the new XAS fitting methodology could be used to extract quantitative π-backbonding information from second row transition metal systems directly from experimental data rather than relying on calculations and simulations. Even though chapter 3 provided strong evidence, both from experimental data and theoretical calculations, for the ability of Rh L2,3-edge XAS to serve as an experimental means by which to accomplish this directly, our goal is to demonstrate the utility of the methodology for additional second row transition metals as well as vastly different complexes. Therefore, chapter four involves the investigation of a set of ruthenium complexes. This will serve to further emphasize the utility of the technique and methodology and to provide important bonding information for newly synthesized compounds. Shortly after the 2008 publication of complex 1, Dr. Michael Whittlesey, from the University of Bath, contacted our group regarding the potential to study new ruthenium dioxygen complexes with unusual physical properties. The experimental synthesis and several unique physical properties of one of the ruthenium complexes were reported in a 2009 communication. 127  The structure of this species is provided in Figure 4.1 and the route employed for its formation is given in Scheme 4.1.  In addition to the synthesis, they also reported that the coordination of dioxygen was simple and reversible and upon dioxygen coordination, utilizing 1H NMR, they found a very positive hydride chemical shift from δ - 41.2 ppm with no dioxygen bound, to δ + 4.8 ppm with dioxygen bound.127 From their X-ray crystal structure, they reported an O-O bond length of 1.354 Å and what appeared to be an ɳ2-dioxygen complex. Additionally, from IR spectroscopy, they reported an O-O stretch of 1047 cm-1. In addition to the experimental data extracted, they performed a DFT geometry optimization of the structure which predicted very similar structural parameters supporting their conclusion of an ɳ2- dioxygen complex rather than an ɳ2-OOH complex. Overall, from the work reported in 83  2009, their experiments were able to support the discovery of a novel ɳ2-dioxygen hydride complex which exhibited unusual chemical and spectroscopic properties. To assess the bonding between the ruthenium center and the dioxygen ligand, further spectroscopic studies were essential and provided the motivating force for the work described herein.  Figure 4.1. Ruthenium-dioxygen complex reported by Whittlesey group in 2009.  Scheme 4.1. The synthetic route utilized by the Whittlesey group for the synthesis of this ruthenium- dioxygen complex. This chapter will continue by first introducing the different ruthenium complexes studied. Subsequently, a brief description of how the experimental data was collected on both the high and low energy beamlines (beamline descriptions in section 2.6), will be discussed. Data processing, including calibration, background subtraction and normalization, which were different from the rhodium cases presented in chapter 3 will be briefly mentioned. Specifically, a few alterations to the fitting for the low energy L- edge data were required, due to the presence of the close lying chlorine K-edge, and will be discussed.  Detailed analysis and discussion of the processed XAS data, in combination with theoretical DFT calculations to provide support for the experimental conclusions will be presented, including quantification of any potential π-backbonding. 84  The chapter will conclude with a brief comparison of the π-backbonding interactions found in different complexes. 4.1.1 Ruthenium Complexes Investigated A series of newly synthesized ruthenium complexes was provided to our laboratory for spectroscopic investigation from Dr. Whittlesey‟s research group at the University of Bath. The various ruthenium complexes investigated are displayed in Figure 4.2. Complex 8-O2 is the same one reported in the 2009 paper mentioned previously. To determine whether the unusual properties reported in the paper correspond to a ruthenium dioxygen complex exhibiting a similar side-on singlet bonding motif as seen for some of the rhodium complexes in chapter 3, ruthenium XAS studies were employed. The first four complexes presented in Figure 4.2 involve tridentate phosphorus based pincer ligands with two phosphines and a central oxygen linker group rather than NHC ligands. The use of these types of ligands has become increasing popular because of their ability to stabilize less common metal oxidation states as well as produce transition metal complexes capable of either activating inert bonds or performing novel catalytic transformations.59,128-132 The synthesis of these four complexes was described in a 2010 article, where the crystallographic data of the two dioxygen complexes appears to indicate a peroxide bound dioxygen, with O-O bond lengths of 1.453 Å and 1.436 Å for 6-O2 and 7-O2 respectively. 128 Gaining information from ruthenium XAS data of these complexes should provide further support for this, which is especially important considering the O-O stretching frequencies could not be located in the IR spectra.128 Therefore, both Ru L2,3-edge and Ru K-edge XAS was utilized to assess the electronic description of the bonding between the ruthenium metal and dioxygen. Additionally, the extraction of quantitative π-backbonding information may be acquired where applicable. 85   Figure 4.2. Representation of the six ruthenium complexes that were studied in this thesis. The structures shown in Figure 4.2 will be divided into two categories and discussed separately. The first category will involve the four complexes on the left of the figure, both the oxygenated and chlorinated versions of 6 and 7 and the second group will include 8-CO and 8-O2. The reason for this differentiation is that the two sets of complexes have different ligand environments about the ruthenium metal center and therefore a direct comparison between them would not be suitable. Thus, for comparative purposes, to be presented later in this chapter, the first four complexes will be discussed and compared separately from the latter two. 4.2 EXPERIMENTAL This section will provide the basic details on how all of the Ru K-edge and L2,3-edge data was collected at SSRL.  Additionally, this section highlights the small differences in processing needed for the Ru L-edge data compared to the treatment of the Rh L-edge data. 4.2.1 Data Collection and Processing The collection of all high energy, Ru K-edge data was performed on beamline 7-3 at SSRL (previously described in section 2.6.1). The samples were prepared for the beamline by gently mixing approximately 5 mg of complex with approximately 35 mg of 86  Boron Nitride in a mortar and pestle until a homogeneous mixture was obtained. The powder was then pressed into an XAS cell on Kapton tape. All samples were prepared in a glove box, under inert nitrogen atmosphere, as the collaborators informed us of their sensitivity to air and moisture. Data was collected using a Si (220) phi= 0° double crystal monochromator, and although the signal was detected both by transmission and fluorescence (as described in section 2.6.1), similar to the Rh K-edge data, the fluorescence data was utilized for all analysis.  To ensure high quality data was obtained, at least eight successive scans of each sample were acquired on at least two separate samples for the same reasons discussed for the Rh K-edge data in chapter 3. XAS data were collected in the energy range from 21,000 – 22,912 eV using XAS Collect software. All processing of the Ru K-edge XAS data was performed using SIXPACK, similar to the Rh K-edge data. The steps of the analysis were the same as previously described for the Rh K-edge data. A ruthenium reference foil was used as an internal reference for energy calibration of the Ru K-edge data. The lowest inflection point of the Ru K-edge foil spectra was set as 21,117 eV. This was followed by background subtraction and normalization which was carried out by employing a linear pre-edge function and a quadratic post-edge function as was done for the Rh K-edge data. The collection of all low energy, Ru L2,3-edge data (in conjunction with Cl K-edge data) was completed on beamline 4-3 at SSRL (previously described in section 2.6.2). The samples were prepared by gently mixing an approximately 25:75% volume ratio of complex to Boron Nitride in a mortar and pestle until a homogeneous mixture was prepared. The mixture was then lightly spread on a piece of sulfur and chlorine free Kapton tape. All samples were prepared in a glove box under an inert nitrogen atmosphere. Data was collected using a fully tuned Si (111) double crystal monochromator and the signal was detected with a fluorescence (Lytle) detector filled with nitrogen gas (as previously mentioned in section 2.6.2). At least three successive scans of each sample were run for the same reasons discussed for the Rh L-edges in chapter 3. All XAS data were collected in the energy range from 2710 - 3152 eV using XAS Collect software. 87  For data processing, the steps performed were essentially identical to those described for the low energy Rh L2,3-edge XAS data. The biggest difference involved how the energy calibration was performed. For ruthenium complexes, an external reference was needed as opposed to the internal argon signal that was used for the rhodium complexes in chapter 3. For external calibration, a well established reference complex Cs2CuCl4 was run before and after each ruthenium complex to ensure the beam remained stable. The intense pre-edge feature, shown in Figure 4.3, for the Cl K- edge XAS of this compound should be at 2820.2 eV, and any variations in this value were linearly shifted as applicable for the compounds under investigation.  Figure 4.3. The Cl K-edge XAS spectrum of Cs2CuCl4, used as a reference compound to calibrate all ruthenium data. The intense pre-edge feature used for calibration purposes is highlighted. Similar to the Rh L-edge analysis, the analysis of the Ru L-edges has not been well established in the literature. To analyze these spectra, the employment of Dr. Delgado‟s Blueprint XAS analysis program, in conjunction with the novel fitting methodology described in section 3.3.1 was once again required. The fitting parameters and an 88  example of the fits for each complex are all included in Appendix B. The basic fitting procedure utilized for all ruthenium complexes is similar to what was shown for the rhodium complexes, with one major difficulty arising from the closeness in energy of the Cl K-edge to the Ru L3-edge. Although the Ru L-edges are similar to the Rh L-edges, the presence of the Cl K- edge in close proximity to the beginning of the Ru L3-edge complicates the fitting the analysis of these complexes. An example of this is presented in Figure 4.4. Thus, in order to fit these spectra, the use of additional peaks and an extra edge to fit the Cl K- edge spectra was required. These extra features provide further evidence for the necessity of the methodology discussed in section 3.3.1, as the number of parameters required to accurately fit the spectra, even with our methodology is significantly large.  To demonstrate the increased complexity of the ruthenium spectra, the model used to fit the spectrum of 7-Cl, is provided in Figure 4.5 and the corresponding parameters are given in Equation 4.1.  This demonstrates an example of the generic type of model used to simultaneously fit all of the Ru L2,3-edge and Cl K-edge XAS data. This is similar to the generic model provided earlier in Figure 3.5 and explained in Equation 3.1, only there are significantly more features present in the current example. Therefore, the fitting of the experimental spectra for these complexes, which will be shown in the following sections of this thesis, represent the first examples of utilizing this methodology for extracting quantitative information from such complicated experimental XAS data. The full lists of extracted parameters from all fits are provided in Appendix B. 89   Figure 4.4. An example of a Ru L2,3-edge spectrum with the Cl K-edge highlighted to show its close proximity to the Ru L3-edge.  90   Figure 4.5. The generic model used to simultaneously fit Ru L2,3-edge and Cl K-edge XAS data. 91   Equation 4.1. General equation used to fit Ru L-edge XAS data. 4.3 DATA ANALYSIS AND DISCUSSION As previously mentioned, the analysis and discussion of the results from the experimental investigation of the complexes shown in Figure 4.2  will be presented in two subsections. The first will include complexes 6-Cl, 6-O2, 7-Cl, and 7-O2 and, due to the difference in ligand field environments, the second subsection will involve complexes 8-CO and 8-O2. 4.3.1 Discussion of Complexes 6-Cl, 6-O2, 7-Cl and 7-O2 As stated in section 4.2, all low energy experimental data that will be shown was processed and analyzed using Blueprint XAS software in conjunction with the methodology described previously in section 3.3.1. The fully processed and normalized low energy Ru L3-edge spectra of 6-Cl compared with 6-O2 and 7-Cl compared with 7- 92  O2 are shown in Figure 4.6 and Figure 4.7 respectively. From these spectra there are several notable differences including the number of spectral features, energy shifts of the main feature, intensity differences, and the broadness of the peaks. First, both chloride complexes have an extra feature after the edge jump. The extra high energy peak must involve transitions involving antibonding chlorine-based MOs as the only difference between the chloride complexes and their dioxygen counterparts is the substitution of the dioxygen for the chlorine ligand.  Figure 4.6. A comparison of the normalized Ru L3-edge XAS spectra of 6-Cl and 6-O2. 93   Figure 4.7. A comparison of the normalized Ru L3-edge XAS spectra for 7-Cl and 7-O2. Another major distinction between the chloride and dioxygen complexes is the energy of the main feature, a departure from what was observed with the Rh complexes discussed in chapter 3. For these ruthenium complexes, all ligands were kept constant with the exception of the substitution of a chloride for a dioxygen. Therefore, for complexes with similar ligand fields, edge shifts to higher energy generally correspond to an increased oxidation state (generally +1 eV corresponds to +1 increase in oxidation state).91  Although Cl- is certainly a better donor than dioxygen and thus more likely to influence the d orbital splitting, the NHCs in these complexes strengthen the ligand field, strongly influencing the overall bonding. Since they remain unchanged in the complexes, the affect of the chlorine should not be drastically different than the dioxygen. Therefore, since there is an apparent shift to higher energy when the chloride is changed to a dioxygen for both complexes 6 and 7, the data suggests oxidation at the ruthenium center occurs upon coordination of the dioxygen. 94  An additional factor that needs consideration is the large increase in intensity of the main feature for both dioxygen complexes when compared to their chloride counterparts. Table 4.1 lists important parameters extracted from the complete fits of the Cl K-edge/Ru L2,3-edge XAS spectra of the ruthenium complexes. Complete tabulations of extracted parameters for all ruthenium complexes are provided in Appendix B. From Table 4.1, it is apparent that the intensity of the major feature for both dioxygen complexes is significantly larger than that for either of the chloride species. In the case of the rhodium complexes (from chapter 3), an increase in intensity corresponded to an increase in oxidation state at the rhodium center. However, in the case of ruthenium complexes, the situation is not as straightforward. Parameter/Complex  6-Cl 6-O2 7-Cl 7-O2  Intensity of Main Peak (eV)  3.75 ± 0.87  7.95 ± 0.98  4.3 ± 0.92  6.0 ± 0.4 Main Peak Position (eV)  2841.4 ± 0.1 2842.5 ± 0.1 2841.2 ± 0.1 2841.9 ± 0.0 Ruthenium Edge Position (eV)  2845.0 ± 2.6 2846.5 ± 2.6 2844.9 ± 2.3 2844.5 ±1.4  Table 4.1. Important parameters extracted from fitted Ru L-edge spectra. For rhodium complexes, the analysis was more straight forward because there was a major geometric change from pseudo-octahedral complexes (Rh(III)) to square planar complexes (Rh(I)). However, the lowest oxidation state for the ruthenium complexes investigated is Ru(II). Ruthenium(II) complexes have a d6 electron configuration, and if they are octahedral or pseudo octahedral, the t2g orbitals are completely filled leaving the two eg orbitals empty. Thus, one major transition is expected in the Ru L2,3-edge XAS spectrum corresponding to an excitation from the Ru 2p orbital to the eg set within the Ru d orbital manifold. Due to the fact that chlorine ligands are good donors, one could imagine mixing of the chlorine with the Ru eg orbitals, which would account for the two distinct transitions observed in the spectra of 6-Cl and 7-Cl. However, the splitting of these two features in the spectra is almost 5 eV, which is inconsistent with a splitting of the eg orbitals, and this higher energy transition is more appropriately due to some charge transfer. When the ruthenium center is oxidized by one or two electrons (creating Ru (III) d5 or Ru(IV) d4 complexes),  the t2g orbitals are no longer completely filled, and lower energy, 95  shoulder transitions would be expected in the Ru L2,3-edge spectra. However, significant geometric changes are also plausible causing further complications. To try to demonstrate these complexities, Figure 4.8, shows putative ligand field splittings for a pseudo octahedral (d6) complex and its square pyramidal d5 and d4 counterparts.  Figure 4.8. Putative ligand field splittings for the ruthenium d 6 , d 5 , and d 4  electron configurations, assuming a change from pseudo-octahedral geometry to square pyramidal geometry. Despite these expectations, the two ruthenium dioxygen complexes do not appear to exhibit any low energy shoulder transitions. However, the main peak in the spectra for both dioxygen complexes is broad in contrast to spectra for the corresponding chloride species.  The most probable explanation for this observation is overlapping transitions that the spectroscopy is not capable of resolving. Thus, to aid in properly identifying the differences in electronic structure between the chloride and dioxygen complexes, Ru K-edge XAS was utilized.  A direct comparison between 6-Cl with 6-O2, and 7-Cl with 7-O2 are provided in Figure 4.9 and Figure 4.10 respectively. Inflection points, indicating the accepted edge position for each complex are also provided in the figures. From these spectra it is apparent that there is a shift to higher energy when the ligand is changed from chlorine to dioxygen. From the inflection points, it is evident that there is an increase of between 1-2 eV when going from the complex with the chloride ligand to that with the dioxygen ligand. This significant increase in 1s binding energy indicates an oxidation state change at the ruthenium metal center. However, whether the coordination of dioxygen has caused an oxidation 96  of one or two electrons is not clear, thus these complexes could be Ru(III) or Ru(IV) species. Additionally, the use of DFT calculations proved to be inconclusive as the mixing of ruthenium d character and NHCs was too overwhelming to provide useful insight. However, based on the fact that our collaborators reported O-O bond lengths for both species that were in agreement with typical peroxide lengths, and the fact that the complexes are not EPR active, an oxidation of two at the ruthenium center is most probable. It is important to note that we have the Cl K-edge XAS data for complexes 6-Cl and 7-Cl which could provide some useful insight into the complexes. However, this data was not utilized because of extraneous chlorine that was present in several of the complexes provided to us where no chlorine ligands were present. Unfortunately, extraneous chlorine makes it impossible to know what part of the Cl K-edge XAS data collected was due solely to the chlorine ligands and therefore, at this time, the Cl K- edge data was not pursued further.  Figure 4.9. A comparison of the normalized Ru K-edge XAS spectra of 6-Cl and 6-O2. 97   Figure 4.10. A comparison of the normalized Ru K-edge XAS spectra of 7-Cl and 7-O2. Despite the uncertainty in the overall oxidation state change that occurs when dioxygen coordinates to the ruthenium center, it is apparent that some oxidation of the ruthenium center occurs. Thus, neither of these two ruthenium dioxygen complexes exhibit the side-on singlet coordination mode that was observed for rhodium complexes 1 and 2 discussed in chapter 3. Unfortunately, this finding leaves no room for quantification of π-backbonding for these complexes as they most likely exhibit peroxo- bound ligands. 4.3.2 Discussion of 8-O2 and 8-CO Complexes The remaining two ruthenium complexes studied have significantly different ligand field environments than the previously analyzed ruthenium species, and thereby direct comparisons between them was not appropriate. However, the difference between the two complexes is simply the substitution of a carbonyl ligand for a dioxygen ligand trans to the hydride as shown previously in Figure 4.2. The fully processed and normalized L2-edge XAS spectra of 8-CO and 8-O2 are shown in Figure 4.11. From these spectra 98  two notable observations are apparent. First of all, there is a large increase in intensity of the main peak of 8-CO in comparison to 8-O2. The second, and perhaps more exciting observation, is the presence of a low energy shoulder in 8-O2 that is not present in the spectrum of 8-CO.  Figure 4.11. A comparison of the normalized Ru L2-edge XAS spectra of 8-CO and 8-O2. Table 4.2 contains important parameters extracted from the fully fitted Ru L2,3-edge XAS spectra of the two complexes. From this table it is evident that from the fitting results, the energy of the main feature in both spectra is essentially unchanged. Because their ligands are identical with the exception of the carbonyl and the dioxygen, this suggests that the ruthenium center in these complexes may have the same oxidation state. Conversely, the fitting results also indicate that the intensity of the carbonyl complex is significantly larger (8.51 ± 0.82 eV) than that of the dioxygen species (5.28 ± 1.41 eV). Also, the dioxygen complex exhibits a low energy shoulder that is not apparent in the spectrum of the carbonyl compound. These two findings make this situation complicated as several other issues need to be considered. 99  Complex Energy of Shoulder Peak (eV) Intensity of Shoulder Peak (eV) Energy of Main Peak (eV) Intensity of Main Peak (eV)  8-CO  -  -  2843.6 ± 0.0 (˂0.1)  8.51 ± 0.82 8-O2 2840.8 ± 0.2 1.83 ± 1.15 2843.2 ± 0.1 5.28 ± 1.41  Table 4.2. Important parameters extracted from fully fitted L2,3-edge XAS spectra of 8-CO and 8-O2. As was explained in section 4.3.1, analysis of the ruthenium complexes is not as straight forward as the rhodium case because of the Ru d orbital occupations. Assuming that 8-CO has a Ru(II) metal center, and thus a d6 electron configuration, one fairly intense feature would be expected in the Ru L2.3-edge due to the transition of the 2p electron to the two empty eg orbitals.  Also, as previously mentioned, if oxidation occurred at the ruthenium center, an extra low energy feature would be expected because of transitions to partially occupied t2g orbitals. Using this justification, we would rationalize our experimental findings as 8-CO having a ruthenium(II) metal center (as it has only one intense feature) and 8-O2 having a more oxidized metal center due to the extra low energy shoulder feature. However, this explanation does not account for the differences in intensity observed for the main feature of the two complexes. To gain more insight into this situation, the Ru K-edge data of the two complexes were investigated. The calibrated and normalized Ru K-edge XAS spectra for 8-CO and 8-O2 are shown in Figure 4.12. By comparing these spectra, it is clear that the inflection point of the rising edge is essentially identical for both complexes (22,127.5 eV). This provides strong support that these two species have a ruthenium metal center with the same oxidation state. 100   Figure 4.12. A comparison of the normalized Ru K-edge XAS spectra of 8-CO and 8-O2. Considering the Ru K-edge data has provided evidence that the oxidation state of these two complexes is the same, it is important to rationalize the findings from the Ru L2,3-edge data. The fact that a carbonyl ligand was simply substituted for a dioxygen ligand without formal oxidation of the ruthenium metal center implies that the dioxygen is, as was in the case for rhodium complexes 1 and 2, bound as a side-on singlet to the ruthenium metal center such that no formal oxidation at the metal center or formal reduction of the ligand occurred. This finding would support not only the spectroscopy which indicated no change in oxidation state, but also the short O-O bond length and low O-O stretching frequency reported by Häller et al. in their 2009 paper. If this is the case, then the low energy shoulder in the spectrum of 8-O2 represents a RuO2 π- backbonding peak (with an intensity of 1.83 ± 1.15 eV), similar to what was seen with rhodium-dioxygen complexes 1 and 2 in chapter 3. Similar to the case for the rhodium complexes, we cannot report an exact percentage for π-backbonding because we do not have an estimate of the pure transition dipole integral for a Ru 4d ← 2p transition. 101  However, the shoulder in the spectrum is from a transition to a RuO2π* orbital, analogous to the RhO2π* orbital discussed earlier. Therefore, 8-O2 would represent the first ruthenium dioxygen complex where no formal oxidation occurred at the metal center upon coordination of the dioxygen ligand. Additionally, this would be the first reported example of π-backbonding between a ruthenium metal and dioxygen ligand. However, one problem still remains; the spectrum of 8-CO was significantly more intense than that of 8-O2. The major factor that hasn‟t been taken into consideration is that carbonyl ligands are well known to undergo π-backbonding with transition metal centers. If the carbonyl ligand is behaving in this manner, then a shoulder, either at slightly higher or slightly lower energy should be present indicating a transition to one or both of the ruthenium carbonyl π-backbonding orbitals. However, this is not observed in the XAS spectrum (Figure 4.11). However, an extra feature can only be observed if enough separation is present between the π-backbonding orbital and the empty Ru eg set of orbitals. Specifically, as mentioned for the previous two ruthenium dioxygen complexes discussed in section 4.3.1, the separation between the partially filled t2g orbitals and the empty eg orbitals was insufficient and could not be distinguished as a separate feature in either spectra. Consequently, this resulted in both a broadening of the main feature, as well as an increase in the total intensity of the feature extracted from the quantitative fits. Therefore, the non-observable π-backbonding feature in the XAS spectrum of the carbonyl complex can be explained by exploiting the same rationale used to account for the missing transitions to partially occupied t2g orbitals for the ruthenium dioxygen complexes discussed previously. In the current case, the RuCOπ* orbital, from the π- backbonding interaction between the ruthenium metal center and the carbonyl ligand, is too close in energy to the empty Ru eg set of orbitals to be detected as an extra feature. Therefore, only one intense peak is apparent in the Ru L2.3-edge XAS spectrum and it is both broader and more intense than would be expected if the transition was only occurring to the eg set of orbitals as in the case of complexes 6-Cl, 7-Cl, and 8-O2. The reason for the observation of the transition to the RuO2π* orbital as a low energy feature whereas the transition to the RuCOπ* orbital is not, must be addressed. This involves providing a rationale for the energy of the carbonyl π-backbonding orbital being 102  higher (thus closer to the metal d manifold) than that of a dioxygen π-backbonding orbital.  The most reasonable explanation for this involves the ligand field strength of the carbonyl ligand in comparison with the dioxygen ligand. Carbonyls are known to be strong field ligands, therefore antibonding orbitals are closer in energy to the d orbitals than weaker field ligands such as dioxygen. Since the metal and ligand orbitals are closer together in the case of the carbonyl, the π-backbonding interaction should be stronger (better metal-ligand mixing), which can account for the π-backbonding orbital being very close to the empty ruthenium eg orbitals.  An illustration demonstrating these differences is provided in Figure 4.13. Although from classical coordination chemistry considerations, the two COπ* orbitals would generally be slightly higher in energy than the empty Ru eg orbitals, for 8-CO the 10Dq is very large because ruthenium is a second row transition metal and because of the NHCs create a strong ligand field. The combination of these factors would cause the Ru 4dζ* orbitals to be very high in energy, and therefore the COπ* orbitals may be lower than the empty Ru d orbitals (as depicted in Figure 4.13).  Figure 4.13. An illustration exemplifying how the different strengths of the ligand-metal bonds can affect the energy position of the backbonding orbital. (Note the RuCOπ* orbital could be slightly above the Ru 4dσ*, but the representation shown is in agreement with DFT predictions). 103  To further investigate this situation, DFT calculations were utilized. The software package ORCA was used to perform both geometry optimization and single point calculations for 8-CO and 8-O2 complexes. For these calculations, the same basis set, and functionals were used as for the rhodium calculations previously discussed in chapter 3. The valence MO diagrams for 8-CO and 8-O2 are shown in Figure 4.14 and Figure 4.15 respectively. The diagrams show both pictorial representations for the important π*-backbonding orbitals as well as their bonding counterparts, with two in the case of the carbonyl due to the two perpendicular interactions.  Figure 4.14. A valence MO diagram extracted from single point DFT calculation of 8-CO (H atoms left out for clarity). 104   Figure 4.15. A valence MO diagram extracted from a single point DFT calculation of 8-O2 (H atoms left out for clarity). The DFT predicted valence MO diagram for 8-CO, shown in Figure 4.14, suggests that our arguments for the increased intensity in its L-edge spectrum due to transitions to the empty RuCOπ* orbital may be well founded. From the figure, it is apparent that DFT predicts that the RuCOπ* orbitals are close in energy to the two empty Ru d (eg) orbitals. DFT predicts a splitting of only 0.6 eV between these orbitals. In the case of the rhodium complexes, there was almost an energy splitting of 2 eV between the empty Rh d orbital and the RhO2π*. With a separation that large, we were able to observe a small shoulder in the lower energy side of the main feature. However, a splitting of only 0.6 eV, strongly suggests that the spectroscopy will not be capable of resolving these transitions as separate features. Instead, one broader, more intense feature is most likely to be observed, as was the case for the L2,3-edge XAS of 8-CO. The valence MO diagram shown for 8-O2, also supports our arguments. In this case, DFT predicts that the LUMO is indeed dominated by dioxygen character (52% O p) rather than by ruthenium character (36% Ru d). This provides the rationale for the low energy shoulder feature observed in the L-edge spectrum, similar to what was seen in the cases of rhodium complexes 1 and 2 discussed earlier in this thesis. Additionally, 105  DFT predicts that the LUMO, the RuO2π* orbital (analogous to the RhO2π* orbital in 1 and 2), is quite well separated from the DFT predicted energy of the empty Ru 4d orbitals. The results of this calculation indicate an energy splitting of approximately 1.8 eV between the π-backbonding orbital and the two main empty Ru 4d orbitals. With an energy splitting this large, we would expect to be able to observe the transition to the π- backbonding peak as it should be distinct from the main transitions. In the L-edge XAS spectrum of 8-O2, a low energy feature was present below the main feature, and based on DFT predictions, as well as the Ru K-edge data which indicate no change in oxidation state between 8-CO and 8-O2, it can be assigned to the RuO2π* orbital. Therefore these findings strongly support that 8-O2 is the first fully characterized complex exhibiting a Ru(II)-1O2 bonding, where side-on singlet dioxygen is coordinated to a ruthenium metal center, in the same manner that was first recognized in the Rh(I)- 1O2 adducts discussed in chapter 3. Despite the positive findings for this ruthenium dioxygen complex, the investigation of the ruthenium carbonyl complex demonstrates a possible limitation of utilizing our new XAS fitting methodology to extract quantitative π-backbonding information. If the π- backbonding orbital is not energetically well separated from the empty d orbitals which are responsible for the transition of the main feature, no discrete low energy feature will be observed. Subsequently, the fitted quantitative data of the spectrum would generate results indicating a larger intensity of the main feature than would be expected if no π- backbonding was present. Since this is the most probable case for 8-CO, we cannot extract quantitative information regarding the carbonyl π-backbonding interaction in the complex. Additionally, the quantitative data for the main energy peak is comprised of the intensity from both transitions to the eg orbitals as well as the π-backbonding orbital. Unfortunately the quantitative intensity of each transition cannot be accurately separated because deconvolution cannot be performed accurately when the separation between the features is not adequate. 4.4 COMPARING DIOXYGEN BACKBONDING Overall, from all of the complexes investigated in this work, three were found to exhibit side-on singlet dioxygen coordinated to transition metal centers. It was shown that this previously unknown dioxygen coordination involves no formal oxidation of the 106  metal and no formal reduction of the dioxygen ligand; instead, synergistic bonding, where both the ligand and metal donate electronic density to one another through a two part π-backbonding interaction was observed. Complexes 1, 2, and 8-O2 were the three species shown to exhibit this bonding and a π-backbonding orbital was identifiable and quantified for each utilizing the new fitting methodology of the L-edge XAS data described herein. Although all three complexes exhibited the low energy shoulder indicative of transitions to the π-backbonding orbital, the energy position relative to the main feature as well as the normalized fractional intensity of the shoulder were not the same for each complex. Table 4.3 lists the important quantitative parameters from the π-backbonding feature extracted from the fully fitted L-edge XAS spectra. The RuO2π* orbital for 8-O2 appears to be significantly lower in energy relative to the main transition than the RhO2- π* orbital for either complexes 1 or 2. Within the error of the fits, the fractional shoulder intensity, indicating the intensity of the π-backbonding, is similar for all three complexes. Although it is tempting to try to draw comparisons regarding the amount of π- backbonding present, this cannot be done accurately between different transition metals (ruthenium versus rhodium), especially when the pure transition dipole moment for both are not known. Therefore, absolute intensities, in terms of orbital composition (percentage of π-backbonding occurring), are not known at this time. Complex  Energy of Shoulder Peak Relative to Main Peak (eV) Normalized Fractional Shoulder Intensity  1  -1.41 ± 0.10 eV  0.43 ± 0.09 2  -1.49 ± 0.16 eV 0.54 ± 0.14 8-O2  -2.78 ± 0.26 eV 0.30 ± 0.14  Table 4.3. Various extracted parameters related to backbonding for 1, 2, and 8-O2.  107  Chapter 5 : CONCLUSIONS AND FUTURE WORK For the work carried out in this thesis, various X-ray spectroscopic techniques, in combination with theoretical DFT calculations, were employed to investigate the electronic structure of several rhodium and ruthenium complexes. The combination of techniques used to assess the dioxygen bonding in these complexes was of utmost importance for this work. In particular, identifying a bonding motif where dioxygen was coordinated to the metal centers as a side-on singlet, without formal oxidation of either transition metal was a significant accomplishment. Through this bonding motif, it was shown that synergistic bonding between the rhodium and ruthenium metal centers to dioxygen was occurring through π-backbonding, which is not generally thought to occur with dioxygen ligands. Such bonding mimics the DCD model discussed earlier, with a two part interaction comprising this bonding motif, as demonstrated in Figure 5.1.  Figure 5.1. An illustration of transition metal-dioxygen synergistic bonding, similar to DCD model for alkenes. The experimental ruthenium and rhodium L2,3-edge XAS spectra provided the means by which to identify this newly discovered side-on singlet dioxygen binding. The discovery of this bonding motif was essential for the method developed leading to extraction of quantitative π-backbonding information directly from experimental data. Over the years, the desire to obtain quantitative π-backbonding information for transition metal complexes has been an inaccessible goal. However, by using the fitting methodology described herein, this thesis provides the first example of obtaining quantitative π-backbonding information directly from experimental data rather than from estimations and predictions from theoretical calculations. Thus, a methodology to extract quantitative information from ruthenium and rhodium L2,3-edge XAS spectra for 108  the first time without relying on simulations from multiplet calculations was demonstrated. Additionally, both rhodium and ruthenium K-edge XAS was employed to help support the findings from the L-edge spectra and DFT calculations were utilized for corroboration. The K-edge data was valuable to support the non-simplistic evaluation of the formal oxidation state of the metal centers. Through the combination of experimental techniques employed to examine the compounds, we successfully identified two rhodium dioxygen complexes (1 and 2) and one ruthenium dioxygen complex (8-O2) which demonstrated this side-on singlet dioxygen coordination mode. By implementing our new fitting methodology for the sets of complexes, we were able to quantify the π-backbonding interaction between both rhodium and ruthenium with dioxygen.  Quantification of these interactions was one of the most important achievements of this thesis work. Understanding the bonding between transition metals and coordinated ligands is of essential importance for understanding the reactivity of transition metal complexes. To date, only qualitative π- backbonding information has been accessible, however from the work presented herein, we provide a means whereby direct quantitative data can be extracted. Future application of our methods to other complexes should provide invaluable insight into their chemical bonding. Despite the contributions and potential our approach brings for further understanding π-backbonding in inorganic complexes, there are a few limitations worth mentioning. First and foremost, the error bars determined from the results of the Monte Carlo fitting procedure were fairly large in some cases. In order to try to decrease these errors, it is desirable to improve the quality of the data being analyzed. Additionally, as demonstrated with complexes 8-O2 and 8-CO in chapter 4, the π-backbonding ligand being examined is important in order for our methods to work. Our experiments demonstrated that although our methodology works for identification of π-backbonding in O2 ligands, and presumably the same may be true for alkenes, it was unsuccessful at isolating the π-backbonding of a carbonyl ligand. Since this side-on singlet dioxygen bonding motif offers a potential for these complexes to be excellent oxidation catalysts because of the facile and reversible dioxygen coordination, an understanding of what stabilizes this interaction is desirable. 109  Such an understanding would be useful for rational design of new complexes that also exhibit this new dioxygen coordination. For the ruthenium complexes, altering the NHC ligand from 8-O2 to the phosphorus pincer-type ligands in 6-O2 and 7-O2 indicated a destabilization of the singlet dioxygen and the later two complexes exhibited more typical dioxygen coordination to a metal center. Thus, future work involving the synthesis of new ruthenium dioxygen complexes where the NHC ligand present in 8-O2 is slightly altered, or where the hydride ligand is changed, could provide better insight into the stabilization of the side-on singlet dioxygen coordination exhibited in 8-O2. For the rhodium complexes, DFT calculations were employed to help explain the stabilization of the singlet dioxygen in 1 and 2 as opposed to the more traditional peroxo-bound dioxygen present in 3. It was initially thought that the ligand trans to the dioxygen could readily affect this stabilization, however, DFT predicted that although the trans ligand will strongly affect the π-backbonding interaction, the overall bonding description does not appear to be altered. This finding paves the way for future investigations involving the synthesis of new, similar complexes where only one ligand is systematically changed. Performing similar XAS experiments and applying the methodology described herein will allow for the type of dioxygen coordination to be assessed and any further π-backbonding to be quantified. Due to the confirmed presence of coordinated side-on singlet dioxygen, exciting new reactivity should be expected from these complexes. At the current time, our collaborators in the Crudden group are actively exploring the reactivity of these side-on singlet dioxygen complexes. Preliminary results, indicating these complexes perform chemistry not previously observed by dioxygen, provides exciting new possibilities for dioxygen reactivity. Additionally, since the dioxygen ligand coordinated in 8-O2, 1, and 2, binds easily and reversibly, and the complexes are air and moisture stable, they may be excellent candidates to be utilized for the much needed and desired novel „green‟ oxidation catalysts. Since the work within clearly indicates their unique electronic description, where the dioxygen is coordinated as a singlet to the metal centers, they may prove to be efficient catalysts as the energetic spin state flip is not required. Future experiments 110  testing the possibility of these complexes to be used as catalysts in various types of oxidation reactions should be executed. In addition to being used as oxidation catalysts, we propose the possibility of utilizing 1, 2, and 8-O2 as potential fuel cell catalysts to lower the overpotential of the oxygen reduction reaction at the cathode. Preliminary studies have indicated that 8-O2 is not a good candidate for this as it was not stable in solution with applied electrochemical potential. However, 1 and 2 appear to be more promising and controlled experiments testing its electrochemical activity towards dioxygen reduction should be carried out in the future. Overall, the experimental and theoretical techniques employed in conjunction with the novel fitting methodology for the analysis of the complexes presented in this work have proven to be effective for providing direct experimental evidence for discovering and quantifying π-backbonding in both rhodium and ruthenium dioxygen complexes. Although the experimental methods and overall fitting procedure employed herein were specifically utilized to assess dioxygen π-backbonding, they lay the foundation for new investigations of other second row transition metal complexes containing different ligands capable of π-backbonding.  Of particular importance are alkenes, as many transition metal complexes used for all types of catalysis utilize alkene ligands. 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IL2  (Intensity L2-edge) 0.01 0.2 0.088 ± 0.005 B (L3:L2 ratio) 1 2 1.92 ± 0.05 Δ ≈ 3/2·spin orbital coupling 135 150 142.7 ± 0.4 Rhodium L2.3-edges We (hwhm) 0.1 3 2.07 ± 0.73 Ge (Gaussian/Lorentian) 0 1 0.080 ± 0.136 EL2 (Energy of L2-edge) 3006 3015 3011.7 ± 2.4 Argon K-edge IAr (Intensity Ar K-edge) -0.01 0.09 -0.0617 ± 0.0020 EAr (Energy of Ar K-edge) 3204 3209 3205.4 ± 0.4 WAr 0.1 3 0.24 ± 0.29 GAr 0 1 0.95 ± 0.13 Rhodium Peaks V1 and V1ʹ Total Normalized Intensity 0.5 40 5.07 ± 0.46 Fractional intensity (for shoulder from total) 0 1 0.43 ± 0.09 E1 (relative to E2 position) -3 -1.2 -1.41 ± 0.10 W1 0.1 2 1.33 ± 0.20 G1 0 1 0.063 ± 0.097 V2 and V2ʹ E2 (Energy position of main peak) 3007.8 3008.8 3008.2 ± 0.1 W2 0.1 2 1.13 ± 0.15 G2 0 1 0.131 ± 0.201 V3 and V3ʹ I3 (normalized intensity) 0.1 5 1.14 ± 0.63 E3 (Energy of Peak position) 3009.5 3011.5 3011.2 ± 0.6 W3 0 2 1.89 ± 0.36 G3 0.3 1 0.889 ± 0.197  Table A1. List of parameters from the fitting of complex 1 with average and standard deviation values obtained from a population of 86 successful fits. 120   Figure A1. Comparison of the average fit of 1 (solid line) to the experimental data (dotted line), and the residual is displayed at the bottom.          121  Parameter Lower Limit Upper Limit Result (± St. Dev.) IL2  (Intensity L2-edge) 0.01 0.2 0.094 ± 0.004 B (L3:L2 ratio) 1 2 1.89 ± 0.05 Δ ≈ 3/2·spin orbital coupling 135 150 142.6 ± 0.1 Rhodium L2.3-edges We (hwhm) 0.1 3 1.56 ± 0.80 Ge (Gaussian/Lorentian) 0 1 0.417 ± 0.309 EL2 (Energy of L2-edge) 3006 3015 3011.2 ± 2.9 Argon K-edge IAr (Intensity Ar K-edge) -0.2 0.01 -0.0629 ± 0.0021 EAr (Energy of Ar K-edge) 3204 3209 3205.4 ± 0.4 WAr 0.1 3 0.20 ± 0.23 GAr 0 1 0.99 ± 0.05 Rhodium Peaks V1 and V1ʹ Total Normalized Intensity 0.1 40 5.08 ± 0.52 Fractional intensity (for shoulder from total) 0 1 0.54 ± 0.13 E1 (relative to E2 position) -2.2 -0.5 -1.49 ± 0.16 W1 0.1 2 1.39 ± 0.24 G1 0 1 0.087 ± 0.131 V2 and V2ʹ E2 (Energy position of main peak) 3007.8 3008.8 3008.3 ± 0.1 W2 0.1 2 0.99 ± 0.16 G2 0 1 0.167 ± 0.236 V3 and V3ʹ I3 (normalized intensity) 0.1 5 1.19 ± 0.84 E3 (Energy of Peak position) 3009.5 3011.5 3011.2 ± 0.6 W3 0 2 1.88 ± 0.37 G3 0.3 1 0.826 ± 0.362  Table A2. List of parameters from the fitting of complex 2 with average and standard deviation values obtained from a population of 77 successful fits. 122   Figure A2. Comparison of the average fit of 2 (solid line) to the experimental data (dotted line), and the residual is displayed at the bottom.          123  Parameter Lower Limit Upper Limit Result (± St. Dev.) IL2  (Intensity L2-edge) 0.01 0.2 0.072 ± 0.004 B (L3:L2 ratio) 1 2 1.63 ± 0.03 Δ ≈ 3/2·spin orbital coupling 135 150 142.7± 0.0 (˂0.1) Rhodium L2.3-edges We (hwhm) 0.1 3 2.51 ± 0.51 Ge (Gaussian/Lorentian) 0 1 0.015 ± 0.059 EL2 (Energy of L2-edge) 3006 3015 3013.4 ± 2.1 Argon K-edge IAr (Intensity Ar K-edge) -0.2 -0.01 -0.0288 ± 0.0031 EAr (Energy of Ar K-edge) 3204 3209 3204.9 ± 0.1 WAr 0.1 3 1.38 ± 0.72 GAr 0 1 0.11 ± 0.14 Rhodium Peaks V1 and V1ʹ I1 (normalized intensity) 0.1 40 4.99 ± 1.09 E1 (energy position of main peak) 3007.2 3009.3 3008.2 ± 0.0 (˂0.1) W1 0.1 2 1.19 ± 0.09 G1 0 1 0.24 ± 0.15 V2 and V2ʹ I2 (normalized intensity) 0.1 5 2.39 ± 1.23 E2 3010.2 3012.5 3011.0 ± 0.3 W2 0.1 4 1.41 ± 0.47 G2 0 1 0.80 ± 0.21  Table A3. List of parameters from the fitting of complex 3 with average and standard deviation values obtained from a population of 75 successful fits.  124   Figure A3. Comparison of the average fit of 3 (solid line) to the experimental data (dotted line), and the residual is displayed at the bottom.               125  Parameter Lower Limit Upper Limit Result (± St. Dev.) IL2  (Intensity L2-edge) 0.01 0.2 0.074 ± 0.003 B (L3:L2 ratio) 1 2 1.62 ± 0.02 Δ ≈ 3/2·spin orbital coupling 135 150 142.7± 0.0 (˂0.1) Rhodium L2.3-edges We (hwhm) 0.1 3 2.77 ± 0.37 Ge (Gaussian/Lorentian) 0 1 0.018 ± 0.081 EL2 (Energy of L2-edge) 3006 3015 3013.8 ± 2.0 Argon K-edge IAr (Intensity Ar K-edge) -0.2 -0.01 -0.0278 ± 0.0023 EAr (Energy of Ar K-edge) 3204 3209 3204.9 ± 0.1 WAr 0.1 3 1.09 ± 0.66 GAr 0 1 0.10 ± 0.11 Rhodium Peaks V1 and V1ʹ I1 (normalized intensity) 0.1 40 5.45 ± 0.60 E1 (energy position of main peak) 3007.2 3009.3 3008.3 ± 0.0 (˂0.1) W1 0.1 2 1.22 ± 0.05 G1 0 1 0.16 ± 0.08 V2 and V2ʹ I2 (normalized intensity) 0.1 5 1.96± 0.78 E2 3010.2 3012.5 3011.0 ± 0.3 W2 0.1 4 2.66 ± 0.04 G2 0 1 0.71 ± 0.21  Table A4. List of parameters from the fitting of complex 4 with average and standard deviation values obtained from a population of 64 successful fits.  126    Figure A4. Comparison of the average fit of 4 (solid line) to the experimental data (dotted line) and the residual is displayed at the bottom.             127   Parameter Lower Limit Upper Limit Result (± St. Dev.) IL2  (Intensity L2-edge) 0.01 0.2 0.110 ± 0.005 B (L3:L2 ratio) 1 2 1.62 ± 0.01 Δ ≈ 3/2·spin orbital coupling 135 150 142.3± 0.0 (˂0.1) Rhodium L2.3-edges We (hwhm) 0.1 3 1.29 ± 0.81 Ge (Gaussian/Lorentian) 0 1 0.081 ± 0.120 EL2 (Energy of L2-edge) 3006 3015 3010.0 ± 2.2 Argon K-edge IAr (Intensity Ar K-edge) -0.01 0.09 0.0635 ± 0.0016 EAr (Energy of Ar K-edge) 3204 3207 3204.9 ± 0.0 (˂0.1) WAr 0.1 3 1.54 ± 0.34 GAr 0 1 0.94 ± 0.09 Rhodium Peaks V1 and V1ʹ I1 (normalized intensity) 0.1 40 3.27 ± 0.38 E1 (energy position of main peak) 3007.2 3009.3 3008.3 ± 0.0 (˂0.1) W1 0.1 2 1.38 ± 0.05 G1 0 1 0.83 ± 0.14 V2 and V2ʹ I2 (normalized intensity) 0.1 5 2.08 ± 0.78 E2 3010.2 3012.5 3010.5 ± 0.2 W2 0.1 4 2.86 ± 0.44 G2 0 1 0.78 ± 0.18  Table A5. List of parameters from the fitting of complex 5 with average and standard deviation values obtained from a population of 89 successful fits. 128   Figure A5. Comparison of the average fit of 5 (solid line) to the experimental data (dotted line) and the residual is displayed at the bottom.           129  APPENDIX B: Fitting Results of Ruthenium Complexes Parameter Lower Limit Upper Limit Result (± St. Dev.) IL2  (Intensity L2-edge) 0.01 0.2 0.041 ± 0.004 B (L3:L2 ratio) 1 2 1.34 ± 0.07 Δ ≈ 3/2·spin orbital coupling 125 135 129.3 ± 0.2 Ruthenium L2.3-edges We (hwhm) 0.1 3 1.27 ± 0.78 Ge (Gaussian/Lorentian) 0 1 0.582 ± 0.354 EL2 (Energy of L2-edge) 2840.2 2850.2 2845.0 ± 2.6 Chlorine K-edge ICl (Intensity Cl K-edge) 0.01 0.5 0.138 ± 0.006 ECl (Energy of Cl K-edge) 2820.2 2826.7 2826.5 ± 0.5 WCl 0.1 3 2.80 ± 0.40 GCl 0 1 0.548 ± 0.254 Ruthenium Peaks   V1 and V1ʹ I1 (normalized intensity) 0.5 40 3.75 ± 0.87 E1  2841.2 2842.1 2841.5 ± 0.1 W1 0.1 2 1.26 ± 0.16 G1 0 1 0.332 ± 0.112 V2 and V2ʹ I2 (normalized intensity) 0.5 20 1.61 ± 1.11 E2 (Relative to E1) 0.9 2 1.51 ± 0.22 W2 0.1 2 1.46 ± 0.41 G2 0 1 0.213 ± 0.281 V3 and V3ʹ I3 (normalized intensity) 0.5 20 2.59 ± 0.90 E3 (Relative to E1 ) 2.5 3.3 3.27 ± 0.08 W3 0.1 2 1.84 ± 0.19 G3 0 1 0.540 ± 0.246 V4 and V4ʹ I4 (normalized intensity) 0.5 20 0.889 ± 0.575 E4 (Energy of Peak position) 2848.2 2850 2848.7 ± 0.4 W3 0.1 2 1.97 ± 0.10 G3 0 1 0.101 ± 0.203 Chlorine Peaks V5 I5 (normalized intensity) 0.05 10 0.146 ± 0.037 E5 (Energy of Peak position) 2822.7 2823.4 2823.3 ± 0.1 W5 0.1 2 1.04 ± 0.15 G5 0 1 0.873 ± 0.181 V6 I6 (normalized intensity) 0.01 10 0.034 ± 0.040 E6 (Relative to E5) -1.5 -0.8 -0.915 ± 0.207 W6 0.1 2 1.84 ± 0.36 G6 0 1 0.363 ± 0.373 V7 I7 (normalized intensity) 0.01 10 0.237 ± 0.063 E7 (Energy of Peak position) 2826.0 2826.8 2826.0 ± 0.0 (˂0.1) W7 0.1 2 1.42 ± 0.23 G7 0 1 0.858 ± 0.247 V8 I8 (normalized intensity) 0.1 20 0.176 ± 0.078 E8 (Energy of Peak position) 2829.4 2831.0 2830.2 ± 0.4 W8 0.1 2 1.54 ± 0.27 G8 0 1 0.602 ± 0.354  Table B1. List of parameters from the fitting of complex 6-Cl with average and standard deviation values obtained from a population of 71 successful fits. 130   Figure B1. Comparison of the average fit of 6-Cl (solid line) to the experimental data (dotted line) and the residual is displayed at the bottom.                             131   Parameter Lower Limit Upper Limit Result (± St. Dev.) IL2  (Intensity L2-edge) 0.01 0.2 0.018 ± 0.002 B (L3:L2 ratio) 1 2 1.61 ± 0.07 Δ ≈ 3/2·spin orbital coupling 125 135 129.3 ± 0.1 Ruthenium L2.3-edges We (hwhm) 0.1 3 2.94 ± 0.24 Ge (Gaussian/Lorentian) 0 1 0.017 ± 0.108 EL2 (Energy of L2-edge) 2840.2 2850.2 2846.5 ± 2.6 Ruthenium Peaks V1 and V1ʹ I1 (normalized intensity) 0.5 40 7.95 ± 0.98 E1  2841.2 2842.7 2842.5 ± 0.1 W1 0.1 2 1.97 ± 0.05 G1 0 1 0.494 ± 0.137 V2 and V2ʹ I2 (normalized intensity) 0.5 20 1.00 ± 0.284 E2  2844.8 2846.2 2845.5 ± 0.4 W2 0.1 2 1.97 ± 0.08 G2 0 1 0.927 ± 0.179  Table B2. List of parameters from the fitting of complex 6-O2 with average and standard deviation values obtained from a population of 86 successful fits.   132   Figure B2. Comparison of the average fit of 6-O2 (solid line) to the experimental data (dotted line) and the residual is displayed at the bottom.          133  Parameter Lower Limit Upper Limit Result (± St. Dev.) IL2  (Intensity L2-edge) 0.01 0.2 0.061 ± 0.005 B (L3:L2 ratio) 1 2 1.16 ± 0.11 Δ ≈ 3/2·spin orbital coupling 125 135 129.2 ± 0.3 Ruthenium L2.3-edges We (hwhm) 0.1 3 1.24 ± 0.83 Ge (Gaussian/Lorentian) 0 1 0.976 ± 0.067 EL2 (Energy of L2-edge) 2840.2 2850.2 2845.9 ± 2.3 Chlorine K-edge ICl (Intensity Cl K-edge) 0.01 0.5 0.486 ± 0.013 ECl (Energy of Cl K-edge) 2820.2 2826.7 2825.4 ± 0.6 WCl 0.1 3 1.49 ± 0.68 GCl 0 1 0.447 ± 0.325 Ruthenium Peaks   V1 and V1ʹ I1 (normalized intensity) 0.5 40 4.26 ± 0.92 E1  2841.2 2842.1 2841.4 ± 0.1 W1 0.1 2 1.27 ± 0.16 G1 0 1 0.338 ± 0.150 V2 and V2ʹ I2 (normalized intensity) 0.5 20 2.06 ± 1.42 E2 (Relative to E1) 0.9 2 1.46 ± 0.27 W2 0.1 2 1.53 ± 0.43 G2 0 1 0.181 ± 0.265 V3 and V3ʹ I3 (normalized intensity) 0.5 20 3.02 ± 1.15 E3 (Relative to E1 ) 2.5 3.3 3.19 ± 0.17 W3 0.1 2 1.77 ± 0.23 G3 0 1 0.392 ± 0.225 V4 and V4ʹ I4 (normalized intensity) 0.5 20 0.904 ± 0.581 E4 (Energy of Peak position) 2848.2 2850 2849.6 ± 0.6 W3 0.1 2 1.99 ± 0.04 G3 0 1 0.068 ± 0.167 Chlorine Peaks V5 I5 (normalized intensity) 0.05 10 0.359 ± 0.111 E5 (Energy of Peak position) 2822.7 2823.4 2822.9 ± 0.0 (˂0.1) W5 0.1 2 0.667 ± 0.099 G5 0 1 0.552 ± 0.230 V6 I6 (normalized intensity) 0.01 10 0.953 ± 0.216 E6 (Energy of peak position) 2824.2 2825.2 2824.8 ± 0.1 W6 0.1 2 0.938 ± 0.095 G6 0 1 0.800 ± 0.201 V7 I7 (normalized intensity) 0.01 20 0.341 ± 0.163 E7 (Energy of Peak position) 2829.4 2831 2830.0 ± 0.2 W7 0.1 2 1.25 ± 0.25 G7 0 1 0.821 ± 0.289  Table B3. List of parameters from the fitting of complex 7-Cl with average and standard deviation values obtained from a population of 74 successful fits.  134   Figure B3. Comparison of the average fit of 7-Cl (solid line) to the experimental data (dotted line) and the residual is displayed at the bottom.                            135  Parameter Lower Limit Upper Limit Result (± St. Dev.) IL2  (Intensity L2-edge) 0.01 0.2 0.029 ± 0.002 B (L3:L2 ratio) 1 2 1.55 ± 0.02 Δ ≈ 3/2·spin orbital coupling 125 135 129.3 ± 0.0 (˂0.1) Ruthenium L2.3-edges We (hwhm) 0.1 3 2.51 ± 0.65 Ge (Gaussian/Lorentian) 0 1 0.015 ± 0.060 EL2 (Energy of L2-edge) 2840.2 2850.2 2844.4 ± 1.4 Ruthenium Peaks V1 and V1ʹ I1 (normalized intensity) 0.5 40 6.02 ± 0.401 E1  2841.2 2842.7 2842.0 ± 0.0 (˂0.1) W1 0.1 2 1.70 ± 0.04 G1 0 1 0.589 ± 0.070 V2 and V2ʹ I2 (normalized intensity) 0.5 20 1.88 ± 0.40 E2  2844.7 2846.2 2844.7 ± 0.1 W2 0.1 2 1.97 ± 0.09 G2 0 1 0.671 ± 0.187  Table B4. List of parameters from the fitting of complex 7-O2 with average and standard deviation values obtained from a population of 98 successful fits.     136   Figure B4. Comparison of the average fit of 7-O2 (solid line) to the experimental data (dotted line) and the residual is displayed at the bottom. Parameter Lower Limit Upper Limit Result (± St. Dev.) IL2  (Intensity L2-edge) 0.01 0.2 0.023 ± 0.002 B (L3:L2 ratio) 1 2 1.45 ± 0.04 Δ ≈ 3/2·spin orbital coupling 125 135 129.2 ± 0.0 (˂0.1) Ruthenium L2.3-edges We (hwhm) 0.1 3 1.98 ± 0.72 Ge (Gaussian/Lorentian) 0 1 0.074 ± 0.159 EL2 (Energy of L2-edge) 2840.2 2850.2 2845.6 ± 0.0 (˂0.1) Ruthenium Peaks V1 and V1ʹ I1 (normalized intensity) 0.5 40 8.51 ± 0.82 E1  2842.7 2844.2 2843.6 ± 0.0 (˂0.1) W1 0.1 2 1.34 ± 0.06 G1 0 1 0.383 ± 0.091 V2 and V2ʹ I2 (normalized intensity) 0.5 20 0.868 ± 0.461 E2  2848.0 2850.0 2848.0 ± 0.1 W2 0.1 2 1.99 ± 0.03 G2 0 1 0.890 ± 0.247  Table B5. List of parameters from the fitting of complex 8-CO with average and standard deviation values obtained from a population of 74 successful fits. 137   Figure B5. Comparison of the average fit of 8-CO (solid line) to the experimental data (dotted line) and the residual is displayed at the bottom.            138  Parameter Lower Limit Upper Limit Result (± St. Dev.) IL2  (Intensity L2-edge) 0.01 0.2 0.03 ± 0.005 B (L3:L2 ratio) 1 2 1.41 ± 0.09 Δ ≈ 3/2·spin orbital coupling 125 135 129.2 ± 0.8 Chlorine K-edge ICl (Intensity Cl K-edge) 0.01 0.5 0.096 ± 0.006 ECl (Energy of Cl K-edge) 2820.2 2826.7 2826.4 ± 0.3 WCl 0.1 3 1.06 ± 0.83 GCl 0 1 0.151 ± 0.229 Ruthenium L2.3-edges We (hwhm) 0.1 3 1.26 ± 0.832 Ge (Gaussian/Lorentian) 0 1 0.542 ± 0.399 EL2 (Energy of L2-edge) 2840.2 2850.2 2844.7 ± 2.2 Ruthenium Peaks V1 and V1ʹ I1 (normalized intensity) 0.1 20 1.83 ± 1.15 E1  2840.2 2841.2 2840.8 ± 0.161 W1 0.1 2 1.07 ± 0.19 G1 0 1 0.319 ± 0.246 V2 and V2ʹ I2 (normalized intensity) 0.5 40 5.28 ± 1.41 E2  2842.9 2843.9 2843.2 ± 0.1 W2 0.1 2 1.21 ± 0.11 G2 0 1 0.494 ± 0.200 V3 and V3ʹ I3 (normalized intensity) 0.5 20 2.01 ± 0.86 E3  2844.7 2846.2 2845.5 ± 0.4 W3 0.1 2 1.89 ± 0.21 G3 0 1 0.522 ± 0.354 Chlorine Peak V4 and V4ʹ I4 (normalized intensity) 0.01 20 0.399 ± 0.127 E4  2827.2 2828.7 2828.0 ± 0.1 W4 0.1 3 1.63 ± 0.22 G4 0 1 0.889 ± 0.189  Table B6. List of parameters from the fitting of complex 8-O2 with average and standard deviation values obtained from a population of 74 successful fits. 139   Figure B6. Comparison of the average fit of 8-O2 (solid line) to the experimental data (dotted line) and the residual is displayed at the bottom.               140  APPENDIX C: X-ray Crystallography Compared with DFT Results for Geometries Selected Crystallographic Bond Lengths and Angles Corresponding Results from DFT Calculations Rh-O1 1.995 Å Rh-O 1  2.037 Å Rh-O2 2.020 Å Rh-O 2  2.037 Å Rh-C1 2.057 Å Rh-C 1  2.095 Å Rh-C28 2.060 Å Rh-C 28  2.095 Å Rh-Cl 2.289 Å Rh-Cl  1.860 Å O-O 1.315 Å O-O 1.394 Å O1-Rh-O2 38.22º O 1 -Rh-O 2  40.0º O1-Rh-C1 90.95º O 1 -Rh-C 1  90.3º O2-Rh-C1 89.74º O 2 -Rh-C 1  89.5º O1-Rh-C28 87.46º O 1 -Rh-C 28  89.5º O2-Rh-C28 88.57º O 2 -Rh-C 28  90.3º C1-Rh-C28 178.25º C 1 -Rh-C 28  179.7º O1-Rh-Cl 157.89º O 1 -Rh-Cl 151.7º O2-Rh-Cl 163.89º O 2 -Rh-Cl 165.5º C1-Rh-Cl 89.62º C 1 -Rh-Cl 83.0º C28-Rh-Cl 92.13º C 28 -Rh-Cl 97.3º O1-O2-Rh 71.90º O 1 -O 2 -Rh 70.0º O2-O1-Rh 69.88º O 2 -O 1 -Rh 70.0º Table C1. Comparison of selected bond lengths and angles from crystallographic data of 1 with the results from DFT calculations. (Crystallographic data obtained from supplementary from reference 1).         141  Selected Crystallographic Bond Lengths and Angles Corresponding Results from DFT Calculations Rh-C1 2.041 Å Rh-C 1  2.092Å Rh-C22 2.045 Å Rh-C 22   2.092Å Rh-O1 2.113 Å Rh-O 1  2.037 Å Rh-O2 2.122 Å Rh-O 2  2.038Å Rh-Cl 2.216 Å Rh-Cl 2.357 Å O-O 1.267 Å O-O 1.395 Å C1-Rh-C22 179.01º C 1 -Rh-C 22  130.8º C1-Rh-O1 89.8º C 1 -Rh-O 1  89.9º C1-Rh-O2 90.4º C 1 -Rh-O 2  90.3º C22-Rh-O1 91.2º C 22 -Rh-O 1  88.9º C22-Rh-O2 90.3º C 22 -Rh-O 2  90.5º O1-Rh-O2 34.8º O 1 -Rh-O 2  40.0º C22-Rh-Cl 88.01º C 22 -Rh-Cl 90.4.º C1-Rh-Cl 91.11º C 1 -Rh-Cl 90.4º O1-Rh-Cl 163.0º O 1 -Rh-Cl 159.8º O2-Rh-Cl 162.1º O 2 -Rh-Cl 160.2º O2-O1-Rh 73.0º O 2 -O 1 -Rh 70.0º O1-O2-Rh 72.2º O 1 -O 2 -Rh 69.9º Table C2. Comparison of selected bond lengths and angles from crystallographic data of 2 (using structural parameters A) with the results from DFT calculations. (Crystallographic data obtained from supplementary from reference 1).          142  Selected Crystallographic Bond Lengths and Angles Corresponding Results from DFT Calculations Rh-O1 1.974 Å Rh-O 1  2.020 Å Rh-O2 1.980 Å Rh-O 2  2.020 Å Rh-C1 2.085 Å Rh-C 1  2.146 Å Rh-C2 2.098 Å Rh-C 2  2.147 Å O-O 1.428 Å O-O 1.421 Å Rh-N5 2.091 Å Rh-N 5  2.098 Å Rh-N6 2.105 Å Rh-N 6  2.098 Å O1-Rh-O2 42.3º O 1 -Rh-O 2  41.2º O1-Rh-N5 117.9º O 1 -Rh-N 5  118.6º O1-Rh-N6 159.6º O 1 -Rh-N 6  159.4º O1-Rh-C2 87.6º O 1 -Rh-C 2  85.6º O1-Rh-C28 89.1º O 1 -Rh-C 28  86.1º O2-Rh-N5 159.9º O 2 -Rh-N 5  159.9º O2-Rh-N6 117.8º O 2 -Rh-N 6  117.8º O2-Rh-C2 89.6º O 2 -Rh-C 2  86.1º O2-Rh-C28 87.5º O 2 -Rh-C 28  85.5º N5-Rh-N6 82.2º N 5 -Rh-N 6  81.8º N5-Rh-C1 85.0º N 5 -Rh-C 1  89.8º N5-Rh-C28 97.1º N 5 -Rh-C 28  97.0º N6-Rh-C1 98.3º N 6 -Rh-C 1  97.0º N6-Rh-C28 84.4º N 6 -Rh-C 28  89.7º C1-Rh-C28 176.7º C 1 -Rh-C 28  171.0º Table C3. Comparison of selected bond lengths and angles from crystallographic data of 3 with the results from DFT calculations. (Crystallographic data obtained from CIF provided by Crudden group).         143  Selected Crystallographic Bond Lengths and Angles Corresponding Results from DFT Calculations Ru-O1 2.088 Å Ru-O 1  2.128 Å Ru-O2 2.087 Å Ru-O 2  2.127 Å Ru-C1 2.171 Å Ru-C 1  2.207 Å Ru-C12 2.176 Å Ru-C 12  2.208 Å Rh-C23 2.162 Å Rh-C 23  2.241 Å Ru-C34 2.138 Å Ru-C 34  2.243 Å O-O 1.354 Å O-O 1.364 Å O1-Ru-O2 37.9º O 1 -Ru-O 2  37.4º O1-Ru-C1 79.8º O 1 -Ru-C 1  80.7º O1-Ru-C12 87.5º O 1 -Ru-C 12  85.7º O1-Ru-C23 111.5º O 1 -Ru-C 23  115.3º O1-Ru-C34 103.4º O 1 -Ru-C 34  100.3º O2-Ru-C1 113.6º O 2 -Ru-C 1  115.3º O2-Ru-C12 105.1º O 2 -Ru-C 12  100.4º O2-Ru-C23 78.7º O 2 -Ru-C 23  80.8º O2-Ru-C34 86.0º O 2 -Ru-C 34  85.9º C1-Ru-C12 87.7º C 1 -Ru-C 12  88.6º C1-Ru-C23 167.8º C 1 -Ru-C 23  163.9º C1-Ru-C34 90.6º C 1 -Ru-C 34  90.8º C12-Ru-C23 88.3º C 12 -Ru-C 23  88.0º C12-Ru-C34 168.5º C 12 -Ru-C 34  173.5º C23-Ru-C34 91.0º C 23 -Ru-C 34  90.8º Ru-O1-O2 71.0º Ru-O 1 -O 2  71.4º Ru-O2-O1 71.1º Ru-O 2 -O 1  71.2º Table C4. Comparison of selected bond lengths and angles from crystallographic data of 8-O2 with the results from DFT calculations. (Crystallographic data obtained from supplementary from reference 127).  

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