Kinetics of protonation of cis-[FeH2(dppe)2]:: formation of the dihydrogen complex trans-[FeH(H2)(dppe)2]+ (dppe = Ph2PCH2-CH2PPh2)

Reaction of cis-[FeH2(dppe)(2)] (dppe = Ph2PCH2CH2PPh2) with acids in tetrahydrofuran solution led to formation of trans-[FeH(H-2)(dppe)(2)](+), although co-ordinated H-2 was substituted for N-2 or the anion of the acid in a slower process. The kinetics of the initial protonation reaction with several acids has been studied using an electrochemical procedure. The protonations are first order with respect to both the concentrations of the iron complex and the acid, with second-order rate constants (in dm(3) mol(-1) s(-1)) at 25 degrees C of 9.7 x 10(-3) (HBF4. Et2O), 1.39 x 10(-2) (CF3CO2H), 2.14 x 10(-2) (CF3SO3H), 4.8 x 10(-2) (HCl) and 1.48 x 10(-1) (HBr). The ordering of rate constants within this series of acids is similar to that found for the reactions with cis-[FeH2{P(CH2CH2PPh2)(3)}]. There is a good correlation between the values of log k(HX) for the two complexes, which suggests a common mechanism for the formation of dihydrogen complexes in these cases. Kinetic data show that reactions of the dppe complex with all acids are faster than those of the analogous P(CH2CH2PPh2)(3) compound. The reaction of cis-[FeH2(dppe)(2)] with deuteriated acids DX is faster than with HX, and the values of the kinetic isotope effect (k.i.e.), expressed as k(HX)/k(DX), are 0.21 (CF3SO3H), 0.36 (HCl) and 0.55 (HBr). The inverse k.i.e. suggests that protonation occurs through a late transition state, and comparison with cis-[FeH2{P(CH2CH2PPh2)(3)}] and with theoretical values indicates that protonation of cis-[FeH2(dppe)(2)] occurs through a transition state closer to the dihydrogen complex than in the case of the related P(CH2CH2PPh2)(3) compound.