A mercurial route to a cobalt dihydrogen complex.

The report by Kubas and co-workers in 1984 of a metal complex with dihydrogen as a ligand heralded the advent of a new era in organometallic and inorganic chemistry. Hydrogenations are a cornerstone of homogeneous catalysis, so reactions of metal complexes with H2 had been studied for decades, but the idea that H2 could bind to the metal as a ligand, without complete cleavage of the H H bond to make a dihydride, was a surprising concept. Traditional ligands use lone pairs of electrons in their bonding, but in dihydrogen complexes, the bonding to the metal comes from donation of electron density from the nonpolar H H s bond to d orbitals on the metal. Backbonding from filled metal d orbitals to the s* orbital on H2 also contributes to the bonding; in cases where this backbonding is large, cleavage of the H H bond can occur, thus leading to a dihydride with twoM Hbonds, and no remaining H H bond (Scheme 1). The discovery that H2 could be bound