The complexestrans-[RhX(CO)PPh3)2] (1, X = Cl, Br, I) react with CO to form [RhX(CO)2(PPh3)2] (2), and not to formcis-[RhX(CO)(PPh3)2]. Complexes2contain equivalent axial carbonyl ligands. It is proposed that a series of dicarbonyl complexes, including2, is formed during decarbonylation reactions catalyzed by1.