A Critical Review of Electronic Effects in Enediamido and alpha-Diimino Complexes of the Group 4 Metals

The alpha-diimino chelating ligand 1,4-diaza-1,3-butadiene, or its dianionic enediamido form (generically defined as DAD), forms a variety of complexes with group 4 transition metals either with or without other co-ligands. The known species are either pseudo -octahedral [L4M(DAD)] or tetracoordinate [L2M(DAD)] with a lower degree of metal saturation in the latter case. Additionally, there are a number of known CpMLn species and homoleptic complexes with the formula [M(DAD)(3)] or [M(DAD)(2)]. The M-DAD metallacycle is generally either planar or folded at the (NN)-N-... vector, the latter arrangement being observed when the number of co-ligands is reduced and hence there is insufficient saturation at the metal. The conformation also depends on the relative electron population of DAD, which is a noninnocent ligand and exists as a continuum between the neutral and dianionic resonance forms. Herein some theoretical concepts, that have previously been developed to describe the bonding capabilities of DAD toward different triads of transition metals, are extended to complexes of the group 4 metals. New electronic aspects of the [M(DAD)(3)] complexes are described together with a completely new analysis of the [M(DAD)(2)] complexes. These exhibit the planar/bent conformational dichotomy, which may have an electronic and/or steric origin as it is affected by the type of substituent at the nitrogen atoms. A reasonable justification of the observed conformational preferences is found from a combination of density functional and QTAIM analyses. The latter clearly demonstrates that a peculiar intramolecular hydrogen bonding can contribute to the stabilization of planar-type structures