THE MECHANISMS OF PROTONATION OF [M(ETA-5-C5H5)2H2] (M = MO OR W)

The reaction between [M(eta5-C5H5)2H2] (M = MO or W) and an excess of anhydrous HCI occurs by essentially the same mechanism for the two com plexes. Protonation of the tungsten complex involves initial.attack at a hydride ligand to generate the spectroscopically detected [W(eta5-C 5H5)2H(eta2-H-2)]+, which subsequently undergoes an intramolecular oxi dative cleavage of the dihydrogen ligand to give the trihydride, [W(et a5-C5H5)2H3]+. The kinetic and thermodynamic isotope effects associate d with these elementary reactions have been determined. Protonation of the analogous molybdenum complex also involves initial attack at a hy dride ligand. However, in this system kinetic and spectroscopic studie s indicate that the subsequent cleavage of the dihydrogen ligand to fo rm [Mo(eta5-C5H5)2H3]+ occurs via the rapid formation of a detectable binuclear species which is probably [{Mo(eta5-C5H5)2H2}2(mu-H)]