Analysis of why boron avoids sp2 hybridization and classical structures in the BnHn+2 series.

We performed global minimum searches for the B(n) H(n+2) (n=2-5) series and found that classical structures composed of 2c-2e B-H and B-B bonds become progressively less stable along the series. Relative energies increase from 2.9 kcal mol(-1) in B(2) H(4) to 62.3 kcal mol(-1) in B(5) H(7). We believe this occurs because boron atoms in the studied molecules are trying to avoid sp(2)  hybridization and trigonal structure at the boron atoms, as in that case one 2p-AO is empty, which is highly unfavorable. This affinity of boron to have some electron density on all 2p-AOs and avoiding having one 2p-AO empty is a main reason why classical structures are not the most stable configurations and why multicenter bonding is so important for the studied boron-hydride clusters as well as for pure boron clusters and boron compounds in general.