Fundamentals of H2 binding and reactivity on transition metals underlying hydrogenase function and H2 production and storage

In order to understand the fact that H 2 molecule is only useful chemically when the two H's are split apart in controlled fashion, there is a need to be aware of how activation of H 2 occurs on metal complexes and on enzymes in naturelike hydrogenases. Recently, the detailed mechanism at the molecular level by which the H-H union splits to form, for instance, a metal dihydride complex, has been established. The breakthrough is due to the fact that H 2 contains only a strongly bonded electron pair always assumed to be inert to further chemical interaction. The H 2 binds side-on to the metal center primarily via donation of its two σ electrons to a vacant d orbital and forms a stable dihydrogen complex. Such a complex can encompass interaction of any σ bond with a metal center and therefore termed σ complex. It is notable to mention that such relationships can be defined facts, like octohedral Fe(II) d 6 centers are favorable for reversible molecular H 2 binding rather than irreversible formation of catalytically inactive hydride complexes. In addition, to increase the electrophilicity of the metal center to promote reversible H 2 binding rather than irreversible formation of catalytically inactive hydride complexes must be presumed under the CO ligands.