Dihydrogen bond: Difference between revisions

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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 other [[proton]] donor. With a van der Waals radius of 1.2 Å, hydrogen atoms do not usually approach other hydrogen atoms closer than 2.4 Å. Close approaches near 1.8 Å, are however characteristic of dihydrogen bonding.<ref name=ChemRev/>
[[File:DihydrogenBondingBH4-H2O.png|160 px|thumb|Dihydrogen bonding is evident in the close H---H contacts between water of crystallization and the borohydride anion in the salt NaBH<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>.<ref name=ChemRev/>]]
 
==Boron hydrides==
An early example of this phenomenon is credited to Brown and Heseltine.<ref>{{Cite journal
| issue = 23
}}</ref> In other words, the amine is a protic acid and the borane end is hydridic. The resulting B&minus;H<sup>...</sup>H&minus;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 centers are involved, the interaction is termed a dihydrogen bond.
 
[[File:DihydrogenBondingBH4-H2O.png|thumb|Dihydrogen bonding is evident in the close H---H contacts between water of crystallization and the borohydride anion in the salt NaBH<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>.<ref name=ChemRev/>]]
 
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>·2H<sub>2</sub>O with H&minus;H contacts of 1.79, 1.86, and 1.94 Å.<ref name=ChemRev>{{Cite journal
}}</ref>
 
==Coordination chemistry==
This kind of H&minus;H interaction is distinct from the H&minus;H bonding interaction in [[transition metal]] complexes having dihydrogen bound to a metal.<ref>{{Cite book
[[File:DihydrogenBondingIrCmpx.png|thumb|140 px|Dihydrogen bonding exists between the hydroxypyridine and a hydride ligand in this iridium complex.]] Protonation of transition metal hydride complexes is generally thought to occur via dihydrogen bonding.<ref>Natalia V. Belkova, Elena S. Shubina, and Lina M. Epstein "Diverse World of Unconventional Hydrogen Bonds" Acc. Chem. Res., 2005, vol. 38, pp 624–631{{DOI|10.1021/ar040006j}}</ref> This kind of H&minus;H interaction is distinct from the H&minus;H bonding interaction in [[transition metal]] complexes having dihydrogen bound to a metal.<ref>{{Cite book
| edition = 1
| publisher = Springer
| date = 2001-08-31
}}</ref> Another so-called [[hydrogen hydrogen bond]] is postulated to exist in certain compounds between two neutral non-bonding hydrogen atoms.
 
==References==
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