The boron-boron single bond in diborane(4) as a non-classical electron donor for hydrogen bonding.

An ab initio study of an isomer of diborane(4) [B(2)H(4)] has been carried out at MP2/aug-cc-pVTZ to investigate the ground-state properties of this unusual molecule, a derivative of which has been described in the recent literature. The geometric, electronic and orbital characteristics of B(2)H(4)(4) have been analyzed using AIM, NBO, and ELF methodologies. A region with a high concentration of electron density is located near and along the B-B bond, on the opposite side of this bond relative to the bridging H atoms. This site serves as an electron-donor site to electrophiles, resulting in hydrogen-bonded complexes of B(2)H(4) with proton donors HF, HNC, HCl, HCN, and HCCH, and a van der Waals complex with H(2). These complexes have C(2v) symmetry and stabilization energies that vary from 2 to 27 kJ mol(-1). The SAPT2 energy decomposition analysis shows that the relative importance of the various terms that contribute to the interaction energy depends on the strength of the interaction.