Dynamics of the molecular hydrogen ligand in metal complexes

The recent discovery that certain metal complexes can bind hydrogen in molecular form as they do other small molecules has presented a unique opportunity for neutron scattering to assist in the determination of electronic details of the novel chemical bond formed between this dihydrogen ligand and the metal. This can be accomplished by comparative studies of the barriers to rotation for the dihydrogen ligand in various complexes where either the metal center or the other metal ligands are changed together with appropriate theoretical analyses. This information can be extracted from vibrational and rotational inelastic neutron scattering spectra. Results from a wide variety of molecular hydrogen complexes are reviewed and their implications for the metal-dihydrogen chemical bond are discussed along with reference to various theortical approaches to this problem.