The compounds (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/M(H)(C/sub 2/H/sub 4/) and (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/M(H)(CO) (M = Nb, Ta) reversibly bind trialkylaluminum reagents to give 1:1 adducts in which the aluminum is bonded to the metal-bound ethylene or carbonyl oxygen in preference to the hydride ligand. The mixed-ring compound (eta/sup 5/-C/sub 5/Me/sub 5/)(eta/sup 5/-C/sub 5/H/sub 5/)Ta(H)(CO), in contrast, binds aluminum at the hydride ligand. The nature of the interactions has been established by IR and NMR studies and by determination of the molecular structure of (eta/sup 5/-C/sub 5/Me/sub 5/)/sub 2/Ta(H)(C/sub 2/H/sub 4/xAlEt/sub 3/) at reduced temperature (P2/sub 1//c, a = 14.877 (5) A, b = 12.455 (7) A, c = 15.017 (4) A, ..beta.. = 101.08 (13)/sup 0/, V = 2730 (2) A/sup 3/, T = 220 K, Z = 4, 2496 reflections, S (goodness-of-fit) = 2.75, 283 parameters, R = 0.054 (2033 reflections, I > 3 sigma/sub I/)). This structure displays an unusual ethylene bridge between the aluminum and tantalum centers. The significance of these adducts in the context of Ziegler-Natta catalysis and migratory insertion is discussed.