Structure and bonding of III/V compounds X2Y2, with X=B, Al, Ga, and Y=N, P, As

The geometries and electronic states of group III/V (13/15) X2Y2 compounds were investigated by B3LYP and MP2 calculations, using a 6-311+G(3df) basis set. Twelve different starting geometries for B2N2, B2P2, B2As2, Al2N2, Al2P2, Al2As2, Ga2N2, Ga2P2, and Ga2As2 were chosen for both singlet and triplet states. For aluminum and gallium compounds, the lowest energy structure was a rhombus in a 1Ag ground state, having the group V atoms on the short diagonal. Al2N2 and Ga2N2 have a linear X-N-N-X Σg−3 state that lies only 0.05 and 0.08 eV, respectively, higher than the 1Ag rhombus. B2N2 prefers a linear B-N-B-N arrangement in a triplet state (3Π), but a 3B2g rhombus, having the B atoms on the short diagonal, lies 0.14 eV (B3LYP) or 0.07 eV (MP2) higher. B2P2 and B2As2 were found to be most stable in a rhombus (3B2g) structure, with B on the short diagonal. Bonding in the rhombus and in linear symmetric structures is described.