Metal Metal Bonds in Edge Sharing Bioctahedra

Abstract

This lecture reviews and illustrates our understanding of how pairs of metal atoms interact within the structural framework of an edge-sharing bioctahedron. Examples of d⁰-d⁰, d¹-d¹, d²-d², d³-d³, d⁴-d⁴ and d⁵-d⁵ systems are cited. The simplest approach to the bonding is to consider only the overlaps of pairs of d orbitais to give σ, π, δ, δ*, π* and σ* orbitais, increasing in energy in that order. This picture is unsatisfactory because there are interactions of d orbitais with filled ligand orbitais that may alter this ordering of the levels. Some results of MO calculations at the Fenske-Hall level illustrate this. It is also important to realize that the structures of compounds cannot be meaningfully compared unless the compounds are homologous, in the sense of having the same set of ligand atoms and the same formal oxidation state for the metal atoms. One such series is provided by the M₂Cl₆(R₂PCH₂PR₂)₂ molecules. The structural results for these suggest a level order of σ << π< δ* < δ < π* << σ*.