ClCH2C(O)Co(CO)4 (I) is prepd. by the reaction of ClCH2COCl with Na[Co(CO)4]. I readily decarbonylates to ClCH2Co(CO)4 (II). I is formed from II at .apprx.5 bar of CO pressure and room temp. or at 1 bar of CO by cooling to .apprx.-10 Deg in a reversible reaction. II with PPh3 gives ClCH2C(O)Co(CO)3PPh3 (III) that can be decarbonylated at 50 Deg to the corresponding alkyl complex ClCH2Co(CO)3PPh3 (IV). IV takes up CO even under 1 bar of pressure at room temp. I was characterized by an IR spectrum. II-IV were isolated, and the structures of III and IV were detd. by x-ray diffraction. II reacts with the methoxide ion to yield CH2(CO2Me)2. These results represent the first example of the carbonylation/decarbonylation reaction couple of an XCH2M vs. XCH2C(O)M (X = Cl, Br, iodo; M = transition metal) pair.