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

The kinetic and mechanistic aspects of reactions involving the dihydrogen c omplex trans-[FeH(H-2)(DPPE)(2)](+) and related Fe(II) and Ru(II) complexes are reviewed. Despite the observation that substitution of coordinated H-2 usually goes through a limiting dissociative mechanism,the reactions of th e title complex involve associative activation and are proposed to occur th rough the initial opening of a DPPE chelate ring followed by rate-determini ng attack by the entering ligand. The kinetics of reactions between cis-[MH 2(diphosphine)(2)] compounds and acids to form dihydrogen complexes is also reviewed. The rate of protonation is strongly dependent on the nature of t he acid and shows an inverse kinetic isotope effect; the mechanism proposed consists of attack by the acid to yield a transition state involving a dih ydrogen-bonded adduct. For these complexes, the kinetics of protonation can be summarised in two parameters, R and S, that measure the intrinsic react ivity and selectivity of the complexes towards acids. The lack of reaction of [CpRuH(diphosphine)] complexes with some acids poses some questions abou t the validity of an aqueous pK(a) scale to measure the acidity of dihydrog en complexes. (C) 2000 Elsevier Science S.A. All rights reserved.