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.<ref>
Brown, M. P.; Heseltine, R. W. "Co-ordinated BH3 as a proton acceptor group in hydrogen bonding" Chemical Communications 1968, page 1551-2. DOI: 10.1039/C19680001551</ref> They observed intense absorptions in the [[IR spectroscopy|IR band]]s at 3300 and 3210 cm<sup>-1</sup> for a solution of (CH<sub>3</sub>)<sub>2</sub>NHBH<sub>3</sub>. 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<sup>-1</sup> band increased in intensity and the 3210 cm<sup>-1</sup> band decreased, indicative of intermolecular association.
Interest in dihydrogen bonding was reignited upon the crystallographic characterization of the molecule [[ammonia borane|H<sub>3</sub>NBH<sub>3</sub>]]. In this molecule, like the one studied by Brown and Hazeltine, the hydrogen atoms on nitrogen have a partial positive charge, denoted H<sup>δ+</sup>, and the hydrogen atoms on boron have a partial negative charge, often denoted H<sup>δ-</sup>.<ref>
A New Intermolecular Interaction: Unconventional Hydrogen Bonds with Element-Hydride Bonds as Proton Acceptor" Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold, A. L.; Koetzle, T. F. ''Acc. Chem. Res.'' '''1996''', ''29(7)'', 348 - 354.</ref> In other words, the amine is a protic acid and the borane end is hydridic. The resulting B-H<sup>...</sup>H-N attractions stabilize the molecule as a solid. In contrast, the related substance ethane, H<sub>3</sub>CCH<sub>3</sub>, 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 H<sub>2</sub> from the reaction of a hydride and a protic acid. A very short dihydrogen bond is observed in NaBH<sub>4</sub><sup>'''.'''</sup>2H<sub>2</sub>O with H---H contacts of 1.79, 1.86, and 1.94 Å.
<ref> Custelcean, R.; Jackson, J. E. "Dihydrogen Bonding: Structures, Energetics, and Dynamics" Chemical Reviews 2001, volume 101, page 1963-1980.</ref>
This kind of H---H interaction is distinct from the H---H bonding interaction in [[transition metal]] complexes having dihydrogen bound to a metal.<ref>
Kubas, G. J., "Metal Dihydrogen and σ-Bond Complexes", Kluwer Academic/Plenum Publishers: New York, 2001.</ref> Another so-called [[hydrogen hydrogen bond]] is postulated to exist in certain compounds between two neutral non-bonding hydrogen atoms.