Photolysis of M(CO)6, where M = Cr,Mo, and W, in Nujol mulls at 77K resulted in the formation of “free” CO and C4v M(CO)5 photoproducts. This reaction was found to be photochemically and thermally reversible. Similarly, the photolysis of the metal “half-sandwich” complexes (η6-C6H6)Cr(CO)3, (η5-C5H5)Mn(CO)3, (η5-CH3C5H4)Mn(CO)3 and (η5-C5H5)Re(CO)3 in Nujol at 77 K was found to form the corresponding dicarbonyl photoproducts and “free” CO. These reactions were found to be photochemically reversible. (η6-C6H6)Cr(CO)3 and (η5-C5)Re(CO)3 were also found to produce small amounts of dinuclear products directly upon photolysis, while (η5-C5H5)Mn(CO)3 and (η5-CH3C5H4)Mn(CO)3 formed similar species upon annealing.