In the mid-1980s two major events renewed interest in the interaction between dihydrogen and transition metals. The most important was the discovery by Kubas that dihydrogen can coordinate to a transition metal without being dissociated.'s2 Besides this major discovery, another new phenomenon was reported by two groups, namely, the existence of large, temperature-dependent H-H couplings in some transition metal p~lyhydrides.~.~ The quantum mechanical origin of these couplings was independently recognized by Zilm5 and Weitekamp.6 However, the existence and magnitude of the couplings were shown to be favored by a reduction of the electronic density on the metal, as was the formation of the dihydrogen state, which indicated a link between these two p h e n~m e n a .~.~-~ Indeed, the two similar complexes CpzNbH3 and CpzTaH3 ' H.A. el M. is on leave from Laboratoire de Chimie ThCorique, FacultC des Sciences, Rabat, Morocco. (l)(a) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. Chaudret, B.; Commenges, G.; Fajardo, M.; Jalbn, F.; Moms, R. H.; Otero, A,; Schweitzer, C. T. J. Chem. Soc., Chem. Commun. 1988, 211. (c) Arliguie, T.; Border, C.; Chaudret, B.; Devillers, J.; Poilblanc, R. Organometallics 1989, 8, 1308. (d) Arliguie, T.; Chaudret, B.; Jal6n, F.; Otero, A.; Lbpez, J. A.; Lahoz, F.
