PLANAR TETRACOORDINATE CARBONS IN DIMETALLIC COMPLEXES - QUANTUM-CHEMICAL INVESTIGATIONS

Extended Huckel (EH) calculation's have been carried out on V-2(DMP)(4) (DMP = dimethoxyphenyl) (1) and on [(C5H5)(2)Zr](2)(mu-CH3)(mu-DMP) (2). Ab initio Hartree-Fock calculations have been carried out on 1 and on a model complex of 2 with C-2 upsilon symmetry. In both complexes, the symmetrically:bridging DMP ligands are usually considered as examples of planar tetracoordinate carbons (PTCs). Orbital interaction diagrams (from EH calculations) and electron deformation density maps (from ab initio calculations) show that those specific examples of planar tetracoordinate carbons are best represented by the formal electronic structure sigma(4) pi(1) . The sigma lone pair of the PTC is stabilized through donation interactions toward fragment orbitals delocalized on the two metal atoms. The resulting two-electron, multicenter molecular;orbitals have their major weight on the. bridging ligands but display metal-metal bonding character. Some pi donation, not involving the PTC, is present in 2, but the pi interactions are negligible in 1. In the complex of (V-2)(4+), the out-of-phase combination of the sigma lone pairs of carbons displaces the two metal pi electrons in the,plane:of the PTCs. Those electrons are accommodated in the delta orbital, so the V-V triple bond corresponds to the electronic configuration sigma(2) pi(2) delta(2), not sigma(2) pi(4) as previously assumed.