On the origin of apparently short carbon-carbon double bonds in transition-metal vinyl complexes

A survey of carbon-carbon double bond lengths in transition metal vinyl com plexes reveals a number of complexes with surprisingly short C=C bonds. Acc urate calculations with restricted Hartree-Eock (RHF) and density functiona l theory (DFT) methods on three examples with extremely short C=C double bo nds, 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 intramol ecular steric effects. However, model calculations on Cp2ZrCl(-CH=CHSiMe3) reveal that a disorder between two configurations of the C-alpha=C-beta moi ety could result in the apparent shortening of the C-alpha=C-beta distance. The simulated structural parameters under this disorder model are complete ly consistent with the measured structure. A corresponding disorder model c an be used to rationalize the shortening of the C=C bond in Cp*Ir(PMe3)(C2H 3)(H) and Pt(PPh3)(3)(MeC=CHMe)(+). Thus, these structures and perhaps othe r vinyl transition metal structures with short C=C bonds could lie subject to a similar disorder problem that results in an apparent shortening of the C=C bonds. (C) 1999 Elsevier Science Ltd All rights reserved.