Dihydrogen bond

In chemistry, a dihydrogen bond is a kind of hydrogen bond, an interaction between a metal hydride bond and an OH or NH group or another proton donor. The first example of this phenomenon is credited to Brown and Heseltine. They observed intense absorption bands in the IR bands at 3300 and 3210 cm-1 for a solution of (CH3)2NHBH3. The higher energy band is assigned to a normal N-H vibration whereas the lower energy band is assigned to the same bond, which is interacting with the B-H. Upon dilution of the solution, the 3300 cm-1 band increased in intensity and the 3210 cm-1 band decreased, indicative of intermolecular association.

Interest in dihydrogen bonding was reignited upon the crystallographic characterization of the molecule H3NBH3. In this molecule, like the one studied by Brown and Hazeltine, the hydrogen atoms on nitrogen have a partial positive charge, denoted H&delta;+, and the hydrogen atoms on boron have a partial negative charge, often denoted H&delta;-. In other words, the amine is protic acid and the borane end is hydridic. The resulting B-H...H-N attractions stabilize the molecule as a solid. In contrast, the related substance ethane, H3CCH3, is a gas with a boiling point 285 °C lower. Because two hydrogen atoms are involved, this is termed a dihydrogen bond.

Formation of a dihydrogen bond is assumed to precede formation of H2 from the reaction of a hydride and a protic acid. A very short dihydrogen bond is observed in NaBH4.2H2O with H---H contacts of 1.79, 1.86, and 1.94 Å.

This kind of H---H interaction is distinct from the H---H bonding interaction in transition metal complexes having dihydrogen bound to a metal. Another so-called hydrogen hydrogen bond is postulated to exists in certain compounds between two neutral non-bonding hydrogen atoms.