Z-H Bond Activation in (Di) hydrogen Bonding as a Way to Proton/Hydride Transfer and H2 Evolution

The ability of neutral transition-metal hydrides to serve as a source of hydride ion H- or proton H+ is well appreciated. The hydride ligands possessing a partly negative charge are proton accepting sites, forming a dihydrogen bond, M-Hd(delta-) center dot center dot center dot (delta+) HX (M = transition metal or metalloid). On the other hand, some metal hydrides are able to serve as a proton source and give hydrogen bond of M-H delta+ center dot center dot center dot X type (X = organic base). In this paper we analyse recent works on transition-metal and boron hydrides showing i) how formation of an intermolecular complex between the reactants changes the Z-H (M-H and X-H) bond polarity and ii) what is the implication of such activation in the mechanisms of hydrides reactions.