Kinetics and mechanism of the formation and isomerization of some propionyl complexes of iridium(III)

Abstract

Reactions of the complex [Ir(CO)₂Cl₂Et]₂ with phosphorus ligands [L = PMe₂Ph, PMePh₂, P(OMe)₃, P(OMe)₂Ph, and P(OMe)Ph₂] yield complexes [Ir(CO)L₂Cl₂(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)L₂Cl₂(COEt)] from [Ir(CO)₂Cl₂Et]₂ is discussed in the context of the findings for the rearrangement reaction and a mechanism is proposed which is based on the large trans-effect of the propionyl ligand.