Electrochemical Evidence for a Three-Center, Three-Electron Agostic Interaction in Tungstenocene Dialkyl Cations and Estimation of the Magnitude of the Interaction

Article abstract of DOI:10.1021/ja00277a024

The role of three-electron agostic interactions in the chemistry of d¹ tungstenocene alkyl complexes has been explored electrochemically.Cyclic voltammograms in acetonitrile of the dialkyls (R=R'=CH3, Ch2CH3, or CH2C(CH3)3; R=CH3, R'=CH2CH3, CH(CH3)2, or Ph; R=CH2CH3, R'=CH2Ph), of the alkyl and aryl halides (R=CH3, X=Cl or I; R=CH2CH3, X=Cl or I; R=CH(CH3)2, X=Cl; R=CH2C(CH3)3, X=Cl; R=Ph, X=Cl, Br, or I), and of exhibit a reversible one-electron oxidation between -450 and +125 mV vs.SCE.The substituent effects are additive and result from a combination of inductive effects and ?-effects involving donation into the partially occupied frontier orbital of the metal.Methoxide is the strongest ?-donor, and only has a low potential, reversible second oxidation.The donor interactions of the halides (Cl > Br > I) reverse the usual electronegativity trend, probably because of unusually good d-p ?-overlap.The ?-donor effects of the alkyl ligands suggest a stabilizing three-electron agostic interaction in the cations, controlled by steric repulsion between the alkyls and the cyclopentadienyl ligands.Partitioning the substituent effects in branched alkyls into inductive and agostic components suggests that agostic interactions with methyl and ethyl ligands stabilize the d¹ metal center by up to 2.7 and 0.7 kcal mol^-1, respectively.