Theoretical studies of multiple bonds in gallium-gallium and germanium-germanium compounds

A recent publication concerning the synthesis and structure of the compound Na-2{GaC6H3-2,6-Trip(2)}(2) (Trip=C6H2-2,4,6-Pr-i(3)), which has a trans-bent geometry, has generated considerable discussion owing to the description of its gallium-gallium bond as a triple one.(1) To provide a theoretical perspective on this subject, we have studied a series of model compounds by the methods of molecular electronic structure theory. For the species trans-Li2MeGaGaMe we find a Ga-Ga bond order somewhat less than two, instead of a triple bond, owing to the antibonding character of one of the molecular orbitals. In the isoelectronic trans-MeGeGeMe we find an essentially Ge=Ge double bonded structure. The neutral trans-MeGaGaMe molecule has a weak Ga-Ga single bond rather than a Ga-Ga double bond. Each of these molecules features a lone pair orbital of b(u) symmetry, with the main regions of electron density located on the gallium or germanium centers, formed by mixing a bonding pi orbital and an antibonding sigma* orbital in a second-order Jahn-Teller effect.