The complex (CO)(5)Cr[C(OMe)(Me)] is known to react photochemically with N-benzylidenemethylamine (CH3NC(Ph)H) to form a beta-lactam (1,3-dimethyl-3-methoxy-4-phenyl-2-azetidinone), whereas our study slows that (CO)(5)W[C(OMe)(Me)] does not. Irradiation of the complexes in argon and nitrogen matrices (lambda > 390 nm) resulted in a geometrical isomerization and the trapping of the syn isomers. There was no evidence from the matrix isolation experiments for the formation of a metal-ketene complex, a likely intermediate in the solution photochemistry of (CO)(5)Cr[C(OMe)(Me)] with respect to beta-lactam formation. Three differences were observed in the matrix and solution photochemistry of (CO)(5)Cr-[C(OMe)(Me)] and (CO)(5)W[C(OMe)(Me)]. (i) (CO)(5)Cr[C(OMe)(Me)] undergoes CO loss upon irradiation with UV light more readily than its tungsten analogue. (ii) Upon UV irradiation of (CO)(5)Cr[(C(OMe)(Me] in a nitrogen matrix bands were observed in the IR spectrum which indicated that two (CO)(4)(N-2)Cr[C(OMe)(Me)] isomers were formed, whereas irradiation of (CO)(5)W[C(OMe)(Me)] under the same conditions produced only one nitrogen adduct, (iii) Irradiation of (CO)(5)Cr[C(OMe)(Me)] in solution and in a solid CO matrix showed that this complex underwent lass of the carbene ligand more readily than the tungsten analogue, These findings are consistent with the proposal that, upon irradiation of chromium carbenes, a ketene transient could be formed by a cleavage of the chromium-carbene sigma bond and intramolecular nucleophilic attack by the carbene on a metal carbonyl. The photochemistry of the complex (CO)(5)Cr(OMe)(biphenyl) was studied in both argon and nitrogen matrices, This complex underwent CO loss very readily, and it is likely that this step is involved in at least one pathway of its solution photochemistry.