trans-[RhX(CO) L2] complexes (X = anionic ligand, L = tertiaryphosphine) from [Rh4(CO)12], phosphine (L), and acid (HX) is described. A plausible formation pathway is proposed. The electron density on the phosphorus atom in trans-[RhX(CO)L2] decreases and the length of the Rh–P bond increases with an increase in the electronegativity of the anionic ligand, X, in a cis position to the phosphine ligand. The rhodium