Reactions between Co(CH3)(PMe3)(4) and various ortho-substituted aromatic aldehydes (Ar(R-1,R-2)CHO) and related benzylic alcohols (Ar(R-1,R-2)CH2OH) proceed with high selectivity to give the mono carbonyl complexes Aryl(R-1,R-2)(PMe3)(3)Co(CO): R-1 = H, R-2 = CH3 (1), R-1 = H, R-2 = ethyl (2), R-1 = CH3, R-2 = CH3 (3), R-1 = H, R-2 = NH2 (4), R-1 = F, R-2 = F (5), R-1 = H, R-2 = CF3 (6), and R-1 = H, R-2 = benzo (7). This tandem C-H bond activation/decarbonylation reaction provides easy and rapid access under mild conditions (-70 degrees C) to the first isolated trimethylphosphine stabilized aryl-monocarbonyl complexes of cobalt. X-ray diffraction studies were performed on ortho-amino substituted complex 4, ortho-trifluormethyl 6, and naphthyl derivative 7.