Structure and Bonding in Cyclic Isomers of BAl₂H_n^m (n=3–6, m=−2 to +1): Preference for Planar Tetracoordination, Pyramidal Tricoordination, and Divalency

Graphical Abstract

The most stable structures of BAl₂H₃²⁻, BAl₂H₄⁻, BAl₂H₅ and BAl₂H₆⁺ at the B3LYP/6-311+G** level of theory exhibit several unusual coordination modes around boron and aluminium.

Abstract

The structure and energetics of cyclic BAl₂H_n^m (n=3–6, m=−2 to +1), calculated at the B3LYP/6-311+G** and QCISD(T)/6-311++G** levels, are compared with their corresponding homocyclic boron and aluminium analogues. Structures in which the boron and aluminium atoms have coordination numbers of up to six are found to be minima. There is a parallel between structure and bonding in isomers of BAl₂H₃²⁻ and BSi₂H₃. The number of structures that contain hydrogens out of the BAl₂ ring plane is found to increase from BAl₂H₃²⁻ to BAl₂H₆⁺. Double bridging at one bond is common in BAl₂H₅ and BAl₂H₆⁺. Similarly, species with lone pairs on the divalent boron and aluminium atoms are found to be minima on the potential energy surface of BAl₂H₃²⁻. BAl₂H₄⁻ (2 b) is the first example of a structure with planar tetracoordinate boron and aluminium atoms in the same structure. Bridging hydrogen atoms on the BAl bond prefer not to be in the BAl₂ plane so that the π MO is stabilised by π–σ mixing. This stabilisation increases with increasing number of bridging hydrogen atoms. The order of stability of the individual structures is decided by optimising the preference for lower coordination at aluminium, a higher coordination at boron and more bridging hydrogen atoms between BAl bonds. The relative stabilisation energy (RSE) for the minimum energy structures of BAl₂H_n^m that contain π-delocalisation are compared with the corresponding homocyclic aluminium and boron analogues.