Reaction of cis-[FeH2{P(CH2CH2PPh2)3}] with acids in tetrahydrofuran solution leads first to cis-[FeH(H2){P(CH2CH2PPh2)3}]+ followed by slower substitution of co-ordinated H2 by N2 or the anion of the acid. The kinetics of formation of the dihydrogen complex has been studied electrochemically by following the time dependence of the current intensity under conditions at which the starting dihydride is oxidised and the dihydrogen complex is not. The protonation reactions are first order with respect to the concentration of both the iron complex and the acid HX, with second-order rate constants at 25 °C ranging from 1.7 × 10–4 (HBF4·Et2O) to 3.4 × 10–2 dm3 mol–1 s–1 (HBr). Reactions with DX are faster and the resulting values of the kinetic isotope effect (k.i.e.) are 0.45 (CF3SO3H), 0.62 (HCl) and 0.64 (HBr). The inverse k.i.e. for these reactions differs markedly from the normal k.i.e. observed in reactions of metal hydrides with bases and suggests a late transition state with a structure close to that of the dihydrogen complex. Chemical transformations along the reaction coordinate are proposed to consist of a series of hydrogen-bonded structures ranging from a weakly hydrogen-bonded metal hydride to a weakly hydrogen-bonded dihydrogen complex.