In recent years, there has been growing interest in the use of transition metal complexes bearing main group ligands for small molecule activation. Several systems incorporating B, Al-In, , Zn and Sn ligands have been reported for H2 activation. A number of potential mechanisms for H2 activation, some of which involve cooperative behaviour of the main group Scheme 1. Reversible reaction of 1 with H2 to form Pd-Mg and [2]2 ligand, have been discussed. Defined examples of reversible behaviour are more limited, the majority of these involve boron-based ligands. For example, Sabo-Etienne and co-workers reported the reversible addition of H2 to a ruthenium complex supported by a borylene ligand. Peters and co-workers documented reversible reactions of H2 to a nickel-borane, iron-borane, and cobaltborane complexes. More recently, Okuda and co-workers reported the reversible reaction of H2 with a Ga-Zn compound, mediated by changes in solvent polarity. Reversible dihydrogen activation by these systems offers a potential approach to catalytic H/D-exchange.