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Studies in 57Fe and 121Sb Mossbauer spectorscopy Scott, James Charles Stewart

Abstract

A number of iron carbonyl complexes of the general formula LFe₂(CO)₆ (L = fluoroalicyclic-bridged di (tertiary arsine or phosphine)) have been investigated by Mossbauer spectroscopy and the results are consistent with the known structure of f₄farsFe₂(CO)₆. As well, a number of derivatives of the general formulae L[sup m]LFe₂(CO)₅ (L[sup m] = monodentate ligand), L[sup c]LFe₂(CO)₄ (L[sup c] = bidentate ligand, chelating), and L[sup b]LFe₂(CO)₄ (L[sup b] = bidentate ligand, bridging) have been examined. The Mossbauer parameters are consistent with substitution trans to the iron-iron bond in L[sup m]LFe₂(CO)₅. Mossbauer and infra-red data show complexes of the type L[sup c]LFe₂(CO)₄ have structures similar to f₄AsP[sup c]f₄AsPFe₂ (CO)₄. Mossbauer spectroscopy shows that in complexes of the type L[sup b]LFe₂(CO)₄ the ligands bridge the two iron atoms and are coordinated cis to the iron-iron bond. The usefulness of the magnetic perturbation technique for removal of ambiguities in the assignment of Mossbauer spectral parameters in low-spin iron compounds having two iron sites has been demonstrated. ¹²¹Sb Mossbauer spectra were obtained for the following compounds: (C₆H₅)₄SbX (X = Cl, OH, NCS), (C₆H₅)₃SbX₂ (X = OCOCH₃, NCS, NO₃, 1/2(OCrO₄)), (C₆H₅)₄SbCl₃ and (C₆H₅)₂Sb(O)OH. The data for the (C₆H₅)₄SbX and (C₆H₅)₃SbX₂ complexes are consistent with trigonal bipyramidal structures for these compounds (except for the acetate) with the X groups in the axial positions. The additive model for quadrupole splittings has been successfully applied to these and some related compounds. The linear relationship between the isomer shift and e²qQ for many of the compounds of the type (C₆H₅)₃SbX₂ suggests σ-bonding plays a dominant role in determining their Mossbauer parameters. Possible structures are examined for both (C₆H₅)₂SbCl₃ and (C₆H₅)₂Sb(O)OH. For (C₆H₅)₃Sb(OCOCH₃)₂, octahedral structures with one ester-like and one bidentate acetate group are compatible with the observed parameters. A number of cations of the general formula X[sub n]Sb(Fe(CO)₂π-C₅H₅)[sub 4-n]+. (X = Cl, Br, I, CF₃, C₆H₅, n-C₄H₉; n = 1, 2, 3, but not all combinations) have been studied by ⁵⁷Fe and ¹²¹Sb Mossbauer spectroscopy. These compounds are nominally isoelectronic with the extensively studied neutral tin derivatives X[sub n]Sn(Fe(CO)₂(π-C₅H₅))[sub 4-n] and a number of correlations between their respective spectral parameters have been investigated. The ¹²¹Sb isomer shifts were found to overlap with the ranges of isomer shifts characteristic of Sb(III) and Sb(V), hence the assignment of a formal oxidation state to antimony in these compounds has little justification. For the complexes R₃SbFe(CO)₂(π-C₅H₅)⁺ (R = C₆H₅, n-C₄H₉) the ¹²¹Sb quadrupole coupling constants e²qQ are positive while the coupling constant in the corresponding tin derivative (n-C₄H₉)₃SnFe(CO)₂(π-C₅H₅)has been found to be negative. Isomer shift data for ⁵⁷Fe, ¹¹⁹Sn and ¹²¹Sb as well as the carbonyl stretching frequencies for the Fe(CO)₂(π-C₅H₅) group indicate Fe-Sb π-bonding is more important than Fe-Sn π-bonding, although σ-bonding effects are the dominant factor in determining the Mossbauer spectral parameters.

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