The gold–hydrogen bond, Au–H, and the hydrogen bond to gold, Au⋯H–X
In the first part of this review, the characteristics of Au–H bonds in gold hydrides are reviewed including the data of recently prepared stable organometallic complexes with gold(I) and gold(III) centers. In the second part, the reports are summarized where authors have tried to provide evidence for hydrogen bonds to gold of the type Au⋯H–X. Such interactions have been proposed for gold atoms in the Au(−I), Au(0), Au(I), and Au(III) oxidation states as hydrogen bonding acceptors and H–X units with X = O, N, C as donors, based on both experimental and quantum chemistry studies. To complement these findings, the literature was screened for examples with similar molecular geometries, for which such bonding has not yet been considered. In the discussion of the results, the recently issued IUPAC definitions of hydrogen bonding and the currently accepted description of agostic interactions have been used as guidelines to rank the Au⋯H–X interactions in this broad range of weak chemical bonding. From the available data it appears that all the intra- and intermolecular Au⋯H–X contacts are associated with very low binding energies and non-specific directionality. To date, the energetics have not been estimated, because there are no thermochemical and very limited IR/Raman and temperature-dependent NMR data that can be used as reliable references. Where conspicuous structural or spectroscopic effects have been observed, explanations other than hydrogen bonding Au⋯H–X can also be advanced in most cases. Although numerous examples of short Au⋯H–X contacts exist in the literature, it seems, at this stage, that these probably make only very minor contributions to the energy of a given system and have only a marginal influence on molecular conformations which so far have most often attracted researchers to this topic. Further, more dedicated investigations will be necessary before well founded conclusions can be drawn.