Structure and bonding in cyclic isomers of B₂AlH_n^m (n = 3-6, m = -2 to +1): a comparative study with B₃H^m_n, BAl₂H^m_n and Al₃H^m_n

Abstract

The structure, bonding and energetics of B₂AlH^m_n (n = 3-6, m = -2 to +1) are compared with corresponding homocyclic boron, aluminum analogues and BAl₂H^m_n using density functional theory (DFT). Divalent to hexacoordinated boron and aluminum atoms are found in these species. The geometrical and bonding pattern in B₂AlH^-₄ is similar to that for B₂SiH₄. Species with lone pairs on the divalent boron and aluminum atoms are found to be minima on the potential energy surface of B₂AlH₃^2-. A dramatic structural diversity is observed in going from B₃H_n^m to B₂AlH^m_n, BAl₂H^m_n and Al₃H^m_n and this is attributable to the preference of lower coordination on aluminum, higher coordination on boron and the higher multicenter bonding capability of boron. The most stable structures of B₃H⁺₆, B₂AlH₅ and BAl₂H^-₄ and the trihydrogen bridged structure of Al₃H^2-₃ show an isostructural relationship, indicating the isolobal analogy between trivalent boron and divalent aluminum anion.