On the nature and incidence of β-agostic interactions in ethyl derivatives of early transition metals:: Ethyltitanium trichloride and related compounds

In light of the structures and spectroscopic properties of the ethyltitanium compounds EtTiCl3 (1), and EtTiCl3(dmpe) (dmpe = Me(2)PCH(2)CH2PMe(2)) (2), extensive DFT calculations have been carried out to explore the structures, energetics, potential energy surfaces, and spectroscopic properties not only of these but also of related ethyl derivatives of early transition metals, M. The analysis has sought to assess the true nature of so-called beta-agostic interactions in these systems and how such interactions depend on the atomic number, coordination number, valence electron (VE) count, and net charge of M. The calculations reproduce well the experimentally determined properties of 1 and 2. Analysis of wave functions indicates that, where significant beta-interactions occur, the M-C-alpha bonding electrons are delocalized over the entire ethyl group, the marked reduction of the MCC valence angle allowing the metal atom to establish covalent bonding interactions with the beta-C atom and, perhaps to a lesser extent, with its appended proximal H atom. A beta-agostic alkyl group may be considered a two-electron ligand, and the main contribution to the stabilization arises from Ti ... B-beta bonding. Barriers to conformational change of the M-Et unit depend inter alia on the space available in the coordination sphere of the central M atom, with rearrangement of the ethyl group to accommodate significant beta-interaction being opposed by interligand repulsion. Such interaction is found not in 1, but in its dmpe adduct 2 because the increase in coordination number is offset by elongation of the Ti-C and Ti-Cl bonds and by the extra pliability of the EtTiCl3 moiety resulting from complexation by the dmpe ligand.