Metal carbonyl syntheses. 18. Hydrogen-bridged heterobimetallic complexes: structural characterization of (.eta.-C5H5)2(CO)Nb(.mu.-H)Cr(CO)5, [((.eta.-C5H5)2(CO)Nb)2(.mu.-H)]+ [Mn(CO)5]-, and (.eta.-C5H5)2(CO)Ta(.mu.-H)Mn2(CO)9