Comparison of classical and non-classical MH2 bonding as applied to the structure and bonding of the first non-classical dihydrogen complex discovered, Mo(CO)3(PPr3)2H2

Classically, the MH2 bonding is viewed as cis-M(H)2 (or dihydride complex), however in complexes such as Mo(CO)3(PPr3)2H2, the dihydrogen (η-H2) ligand was found with it’s H-H bond intact (dihydrogen complex). To compare and contrast the classical and nonclassical views of the MH2 bonding, neutron diffraction analysis of classical and nonclassical complexes were compared. A proton NMR method based on the assumption that it takes much longer for hydrogen nuclei involved in non-classical coordination to relax than those involved in classical coordination, has also been found useful in determining the type of bonding. To determine the cause of non-classical bonding as opposed to classical bonding, the molecular orbital picture and backbonding will be discussed. Introduction: Classically, the MH2 bond has been viewed as each hydrogen donating one electron to the metal, and the metal donating an electron to each hydrogen. This is shown below as the end product in Eq. 1. However in 1983 the first dihydrogen, or non-classical complex, Mo(CO)3(PPr3)2H2, was discovered. This complex exists with it’s H-H bond intact. In this case, each hydrogen is donating it’s electron to each other and the metal is donating electrons to the hydrogen.