The preparation of cis- and trans-[RhI(CO)(PAr 3) 2] (Ar= Aryl) and their ready dissociation in solution

Abstract

Attempts to prepare [RhX(CO)(PAr 3) 2] complexes have shown that when X = I these complexes are far less stable than the well-known [RhCl(CO)(PPh 3) 2]. The bromo complexes [RhBr(CO)(PAr 3) 2] (Ar = C 6H 5, p-EtC 6H 4) can be prepared by simple halide exchange from their respective chloro complexes. However a similar attempt to prepare the iodo complexes was frustrated by dissociative equilibria; in the absence of oxygen dimers were formed, whereas in the presence of oxygen polymeric oxygen complexes were formed. The ease of dissociation of phosphine can be attributed to the greater steric crowding in the iodo complexes than in the chloro and bromo complexes. The complex [RhI(CO)PPh 3) 2] could only be obtained in the presence of excess PPh 3, which inhibits the dissociation. The identification of this monomer was further complicated by the previously unnoticed presence of both cis and trans isomers in the solid state.