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Crystal and molecular structures of some group VI organometallic complexes Yee, Vivien Chia


The molecular structures of 18 Group VI organometallic compounds were determined by single crystal X-ray diffraction. Most of these complexes either contain the 'Cp'M(NO)' fragment (where Cp'=Cp(ɳ⁵-C₅H₅ ) or Cp⁻(ɳ⁵-C₅Me₅ ) ) or were derived from such compounds. The purpose of this work was to determine the structures of representative types of Group VI nitrosyl complexes and related compounds and to compare their features. Also, an effort was made to study any structural changes that accompanied reactions involving ancillary ligands. Fourteen of the structures that were determined contain a metal-nitrosyl linkage that is nearly linear in all cases. One of these compounds is the first example of a Group VI nitrosyl complex containing two different alkyl groups. The structures of two of its reaction products show that one of the alkyl groups can form a second metal to carbon bond via an orthometallation process. Five other nitrosyl alkyl complexes that were studied contain benzyl groups. In these structures, the benzyl ligands bond to the central metal atoms in ɳ²-type linkages, through both the methylene carbon and the ipso carbon atoms. In the compounds studied, this unusual mode of attachment is not influenced by the nature of other ligands on the metal or by methyl substitution on the phenyl ring. Other neutral nitrosyl complexes studied were the products of reactions involving Group VI diene compounds with acetone. Two structures are the result of similar insertion of an acetone molecule into one of the metal-diene bonds with the formation of a new metal to oxygen bond. The orientation of the diene ligands, however, is different in these structures due to steric interactions between these groups and cyclopentadienyl ligands in the complex. A third compound, produced upon prolonged reaction of the parent diene nitrosyl complex with acetone, has a trimeric structure containing bridging acetone groups and no diene ligands. Three cationic nitrosyl structures showed some small differences from the other nitrosyl complexes studied. The metal to nitrosyl linkages are nearly linear but the bond lengths are slightly different from those found in other compounds. Metal to nitrogen distances are slightly longer and nitrogen to oxygen bond lengths are slightly shorter in these cationic complexes. This observation suggests that in these cations, the metal to nitrosyl π backbonding is weaker than in other complexes. Three of the structures presented were not derivatives of nitrosyl containing complexes. One of the compounds was isolated upon oxidation of a dioxoalkyltungsten complex. Its structure is a rare example of an organometallic oxo peroxo complex. Reaction of this product with tetracyanoethylene gave a charge-transfer complex; structural studies of this compound identified a 2:1 organometallic:TCNE complex with interactions involving the peroxo groups of the organometallic molecules and the central ethylene carbon atoms of the TCNE molecule. Treatment of a dioxoalkyltungsten complex with a substituted isocyanate did not give the anticipated diamido compound. Instead, the starting complex reacted with three isocyanate molecules; the structure of the product is essentially that of a solvated diamido complex, with one amido group and a substituted urea ligand. In the nitrosyl complexes studied, a range of ancillary ligands (alkyl groups such as trimethylsilylmethyl, triphenylethyl, benzyl; diene ligands; chloro or acetonitrile groups) does not alter the linear nature of the metal-nitrosyl fragment. In all cases, the nitrosyl group is attached to the metal centre through only the nitrogen atom and the M-N-O bond angle does not deviate significantly from 180°.

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