The multifarious world of transition metal hydrides
Transition metal (TM) hydrides display a remarkable range of bonding types, encompassing classical M–H moieties, dihydrogen complexes containing the η2-H2 ligand, and trihydrides which display quantum mechanical site exchange. Furthermore, C–H, Si–H and B–H moieties can bind to TM centres in an η2-manner, to give σ-bond complexes with a spectrum of M⋯H contributions. In addition to these primary bonding modes, TM complexes also indulge in a wide spectrum of hydrogen-bonding interactions, including both M⋯H–X and the unique type M–H⋯H–X. This review begins with a historical perspective of the development of TM hydride chemistry, and proceeds to focus on three significant developments of the past two decades: the discovery of σ-bond and dihydrogen complexes, the involvement of TM hydrides in hydrogen bonding, and the role played by quantum mechanical phenomena in the chemistry and dynamics of TM hydrides. The account concludes with an overview of the interrelationship between these apparently disparate novel aspects of TM hydride chemistry.