Chemical bonding of hydrogen molecules to transition metal complexes

Abstract

An overview of the recently discovered binding of molecular hydrogen (H₂) to transition metal complexes is given. Emphasis is placed on the features which distinguish non-classical metal-dihydrogen bonding from classical atomic metal-hydride systems. Crystallographic and spectroscopic data (IR, nuclear magnetic resonance (NMR), inelastic neutron scattering) and theoretical aspects are discussed. The subtle factors which influence cleavage of bound H₂ to hydride are identified and examples are given demonstrating that a compound containing a dihydrogen “pair” can exist in facile equilibrium with the “isomeric” compound derived from it containing two separated hydrides. Metal-H₂ systems show a uniquely rich dynamic behavior, including rotation of the side-on bonded H₂. The relatively low energy barrier to rotation is studied experimentally by inelastic neutron scattering and theoretically by ab initio methods, in relation to the bonding model for M-H₂. Quantum mechanical tunneling and exchange processes are identified for bound dihydrogen and certain trihydride systems.