A 48-electron oxo-capped dimolybdenum-iron cluster, (.eta.5-C5Mo5)2Mo2Fe(CO)7(.mu.3-O): its synthesis via the photolytic reaction of Fe2(CO)g with the molybdenum-molybdenum triple-bonded Mo2(.eta.5-C5Me5)2(CO)4 dimer and its stereochemical analysis