The reaction of the metal carbonyl hydrides (eta5-C5H4CH3)2M(CO)H(M=Nb
(1), Ta (2)) with dioxygen produces the metal formato complexes (eta5
-C5H4CH3)2-Nb(O)(eta1-O2CH) (3) and (eta5-C5H4CH3)2Ta(O)(eta1-O2CH) (4
), apparently through the intermediacy of the corresponding CO2 comple
xes (eta5-C5H4CH3)2M(eta2-CO2)H.