KINETICS OF FORMATION OF DIHYDROGEN COMPLEXES - PROTONATION OF CIS-[FEH2(P(CH2CH2PPH2)(3)) WITH ACIDS IN TETRAHYDROFURAN

Reaction of cis-[FeH2{P(CH2CH2PPh2)(3)}] with acids in tetrahydrofuran solution leads first to cis-[FeH(H-2){P(CH2CH2PPh2)(3)}](+) followed by slower substitution of co-ordinated H-2 by N-2 or the anion of the acid. The kinetics of formation of the dihydrogen complex has been stu died electrochemically by following the time dependence of the current intensity under conditions at which the starting dihydride is oxidise d and the dihydrogen complex is not. The protonation reactions are fir st order with respect to the concentration of both the iron complex an d the acid HX, with second-order rate constants at 25 degrees C rangin g from 1.7 x 10(-4) (HBF4.Et2O) to 3.4 x 10(-2) dm(3) mol(-1) s(-1) (H Br). Reactions with DX are faster and the resulting values of the kine tic isotope effect (k.i.e.) are 0.45 (CF3SO3H), 0.62 (HCl) and 0.64 (H Br). 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 s uggests a late transition state with a structure close to that of the dihydrogen complex. Chemical transformations along the reaction coordi nate are proposed to consist of a series of hydrogen-bonded structures ranging from a weakly hydrogen-bonded metal hydride to a weakly hydro gen-bonded dihydrogen complex.