Mechanisms of reactions of dihydrogen complexes: Formation of trans-[RuH(H-2)(dppe)(2)](+) and substitution of coordinated dihydrogen

The reactions between cis-[RuH2(DPPE)(2)] and a number of acids in THF solu tion (DPPE = Ph2PCH2CH2PPh2) show biphasic kinetics, with initial formation of trans-[RuH(H-2)(DPPE)(2)](+) followed by slower substitution of coordin ated dihydrogen by the anion of the acid. The formation of the dihydrogen c omplex is a second-order process that occurs with an inverse kinetic isotop e effect and rate constants k(HX) strongly dependent on the nature of the a cid. There is a linear correlation between the values of log k(HX) for cis- [RuH2(DPPE)(2)] and the related cis-[FeH2(PP3)] [PP3 = P(CH2CH2PPh2)(3)] th at leads to two parameters, S and R, that can be used as a measure of the s electivity and intrinsic reactivity of the dihydride toward acids. The poss ible contributions to the values of these parameters are discussed, especia lly the role of the isomerization of the starting complex and the basicity of the reacting species. The substitution of coordinated dihydrogen in tran s-[RuH(H-2)(DPPE)(2)](+) occurs through a simple dissociative mechanism ins tead of the more complicated one previously proposed far substitutions in t he analogous Fe complex; the mechanistic change is associated with the rela tive strength of the M-H-2 and M-P(chelate) bonds.