RhCl(CO)(PEt 3 ) 2 ] catalyses the carbonylation of methanol in the presence of MeI and water at a rate 1.8 times that for [RhI 2 (CO) 2 ] Ϫ at 150 ЊC. The reaction is first order in [MeI] and zero order in p CO . However, the phosphine complex degrades to [Rh(CO) 2 I 2 ] Ϫ during the course of the reaction. Stoichiometric studies show that the rate of oxidative addition of MeI to [RhI(CO)(PEt 3 ) 2 ] is 57 times faster than to [RhI 2 (CO) 2 ] Ϫ at 298 K and that [RhMeI 2 (CO)(PEt 3 ) 2 ] can be isolated and crystallographically characterised. Combination of the methyl and carbonyl ligands to give the acyl intermediate occurs 38 times slower for [RhMeI 2 (CO)(PEt 3 ) 2 ] than for [RhMeI 3 (CO) 2 ] Ϫ but the steady state concentration of the intermediates is different in that [Rh(COMe)I 2 (PEt 3 ) 2 ] is thermodynamically less stable than [RhMeI 2 (CO)(PEt 3 ) 2 ]. In CH 2 Cl 2 , [Rh(COMe)I 2 (CO)(PEt 3 ) 2 ] reductively eliminates MeCOI. [RhI(CO)(PEt 3 ) 2 ] reacts with CO to give [RhI(CO) 2 (PEt 3 ) 2 ]. Catalyst degradation occurs via [RhHI 2 (CO)(PEt 3 ) 2 ], formed by oxidative addition of HI to [RhI(CO)(PEt 3 ) 2 ], which reacts further with HI to give [RhI 3 (CO)(PEt 3 ) 2 ] from which [Et 3 PI] ϩ reductively eliminates and is hydrolysed to give Et 3 PO. In the presence of water, much less [RhI 3 (CO)(PEt 3 ) 2 ] and Et 3 PO are formed so the catalyst is more stable, but loss of [Et 3 PMe]ϩ and [Et 3 PH] ϩ from [RhMeI 2 (CO)(PEt 3 ) 2 ] or [RhHI 2 (CO)(PEt 3 ) 2 ], respectively, lead to catalyst deactivation. The rate determining step of the catalytic reaction in the presence of water is MeI oxidative addition to [RhI(CO)(PEt 3 ) 2 ], but in the absence of water there is evidence that it may be reductive elimination of MeCOI from [Rh(COMe)I 2 -(CO)(PEt 3 ) 2 ]. [RhMeI 2 (CO)(PEt 3 ) 2 ] has mutually trans phosphines and the methyl group trans to I.

Under mild conditions, [RhI(CO)(PEt 3 ) 2 ], is more active for the carbonylation of methanol to ethanoic acid than [Rh(CO) 2 I 2 ] 2 , which is widely used industrially; intermediates in the catalytic cycle have been identified and characterised.