On the origin of apparently short carbon-carbon double bonds in transition-metal vinyl complexes
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abstract
A survey of carbon-carbon double bond lengths in transition metal vinyl complexes reveals a number of complexes with surprisingly short C=C bonds. Accurate calculations with restricted Hartree-Fock (RHF) and density functional theory (DFT) methods on three examples with extremely short C=C double bonds, Cp2ZrCl(-CH=CHSiMe3), Cp*Ir(PMe3)(C2H3)(H) and Pt(PPh3)3(MeC=CHMe)+, show that the short double bonds cannot be due to electronic or intramolecular steric effects. However, model calculations on Cp2ZrCl(-CH=CHSiMe3) reveal that a disorder between two configurations of the C=C moiety could result in the apparent shortening of the C=C distance. The simulated structural parameters under this disorder model are completely consistent with the measured structure. A corresponding disorder model can be used to rationalize the shortening of the C=C bond in Cp*Ir(PMe3)(C2H3)(H) and Pt(PPh3)3(MeC=CHMe)+. Thus, these structures and perhaps other vinyl transition metal structures with short C=C bonds could be subject to a similar disorder problem that results in an apparent shortening of the C=C bonds.