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

Abstract

A recent publication concerning the synthesis and structure of the compound Na₂{GaC₆H₃-2,6-Trip₂}₂ (Trip = C₆H₂-2,4,6-ⁱPr₃), which has a trans-bent geometry, has generated considerable discussion owing to the description of its gallium–gallium bond as a triple one. 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 GeGe 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 π orbital and an antibonding σ* orbital in a second-order Jahn–Teller effect.