Analysis of why boron avoids sp ² hybridization and classical structures in the B _nH _n+2 series

We performed global minimum searches for the B _nH _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 ₂H ₄ to 62.3 kcal mol ^-1 in B ₅H 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. Classical structures composed of 2c-2e B-H and B-B bonds become progressively less stable along the series B _nH _n+2 (n=2-5). Relative energies increase from 2.9 kcal mol ^-1 in B ₂H ₄ to 62.3 kcal mol ^-1 in B ₅H ₇. It is believed that this occurs because boron atoms in the studied molecules tend to avoid sp ² hybridization and trigonal structures at the boron atoms, as in that case one 2p-AO is empty, and thus highly unfavorable (see figure).