Metal–dihydrogen and σ-bond coordination: the consummate extension of the Dewar–Chatt–Duncanson model for metal–olefin π bonding

There is a marvelous analogy between the metal–olefin π bonding model first brought to light by Dewar 50 years ago and that of σ-bond coordination discovered by us 17 years ago. In some ways it is surprising that 33 years elapsed between the two parallel bonding situations. However this difference pales in comparison to that between the actual synthesis of the first olefin complex, Zeiseʹs salt in 1837, and the first recognized dihydrogen complex nearly 150 years later. This article delineates the principles of σ-bond coordination and activation inspired by the Dewar–Chatt–Duncanson model and illuminates the often-spectacular interplay between theory and experiment in this field. Aside from HH bond coordination and activation towards cleavage, the structure and bonding principles apply to SiH, CH, and virtually any two-electron XH or XY bond. Metal d to σ* XH backdonation is the key to stabilizing σ-bond coordination and is also crucial to homolytic cleavage (oxidation addition). There are some differences in bonding depending on X, and, in the case of BH bond coordination, in metal–borane complexes, backdonation to boron p orbitals occurs. For electrophilic complexes, particularly cationic systems with minimal backdonation, heterolytic cleavage of XH is common and is a key reaction in industrial and biological catalysis. Thus there are two separate pathways for σ-bond activation that directly depend on the electronics of the metal σ-ligand bonding.