The kinetics and mechanisms of reactions involving the dihydrogen complex trans-[FeH(H2)(DPPE)2]+ and related compounds

The kinetic and mechanistic aspects of reactions involving the dihydrogen complex trans-[FeH(H2)(DPPE)2]+ and related Fe(II) and Ru(II) complexes are reviewed. Despite the observation that substitution of coordinated H2 usually goes through a limiting dissociative mechanism, the reactions of the title complex involve associative activation and are proposed to occur through the initial opening of a DPPE chelate ring followed by rate-determining attack by the entering ligand. The kinetics of reactions between cis-[MH2(diphosphine)2] compounds and acids to form dihydrogen complexes is also reviewed. The rate of protonation is strongly dependent on the nature of the acid and shows an inverse kinetic isotope effect; the mechanism proposed consists of attack by the acid to yield a transition state involving a dihydrogen-bonded adduct. For these complexes, the kinetics of protonation can be summarised in two parameters, R and S, that measure the intrinsic reactivity and selectivity of the complexes towards acids. The lack of reaction of [CpRuH(diphosphine)] complexes with some acids poses some questions about the validity of an aqueous pKa scale to measure the acidity of dihydrogen complexes.