Homo- and heterobimetallic .mu.(.eta.1-O:.eta.1-O') formate complexes (M-OCHO-M')+PF6-[M,M' = (.eta.5-C5H5)(CO)(NO)Re, (.eta.5-C5H5)(CO)3W, and (.eta.5-C5H5)(CO)2Fe]: their synthesis, solution lability, and reactivity toward hydride donors