Using rhodium complexes of tertiary phosphines with carbonyl groups β to the P atom, ethene and CO react in methanol to give products involving increased chain growth (octane-3,6-dione, methyl 4-oxohexanoate) compared with PEt₃ complexes and unsaturated products (methyl propenoate, penten-3-one and 1-methoxypentan-3-one from addition of methanol to penten-3-one); mechanistic studies suggest that the ligand carbonyl group prevents coordination of the keto group in the growing chain.