Reactions of the complex [Ir(CO),CI,EtJ, with phosphorus ligands [L = PMe,Ph, PMePh,, P(OMe),, P(OMe),Ph, and P(OMe)Ph,] yield complexes [Ir(CO) L,CI,(COEt)] which subsequently rearrange in solution to a more stable isomeric form. The rearrangement is inhibited by the presence of free ligand L and kinetic data support a mechanism in which a five-co-ordinate intermediate (formed by loss of L from the less stable isomer) rearranges to a second intermediate. The latter then recombines with L to produce the more stable isomer. The reason for the exclusive formation of the less stable isomer of [Ir(CO)L,CI,(COEt)] from [Ir(CO),C12Et], is discussed in the context of the findings for the rearrangement reaction and a mechanism is proposed which is based on the large mns-effect of the propionyl iigand.