Stepwise formation of triosmium edge double-bridged .mu.-H,.mu.-O:C(Nu) decacarbonyl complexes, Nu = NRR', OR, or R. Reversal of barrier heights in the fluxional behavior of the anions [Ru3(.mu.-O:CNMe2)(.mu.-CO)3(CO)7]- and [Os3(.mu.-O:CNMe2)(CO)10]-