The carbonylation of methanol catalysed by [RhI(CO)(PEt₃)₂]; crystal and molecular structure of [RhMeI₂(CO)(PEt₃)₂]

[RhCl(CO)(PEt3)(2)] catalyses the carbonylation of methanol in the presence of MeI and water at a rate 1.8 times that for [RhI2(CO)(2)](-) at 150 degrees C. The reaction is first order in [MeI] and zero order in p(CO). However, the phosphine complex degrades to [Rh(CO)(2)I-2](-) during the course of the reaction. Stoichiometric studies show that the rate of oxidative addition of MeI to [RhI(CO)(PEt3)(2)] is 57 times faster than to [RhI2(CO)(2)](-) at 298 K and that [RhMeI2(CO)(PEt3)(2)] can be isolated and crystallographically characterised. Combination of the methyl and carbonyl ligands to give the acyl intermediate occurs 38 times slower for [RhMeI2(CO)(PEt3)(2)] than for [RhMeI3(CO)(2)](-) but the steady state concentration of the intermediates is different in that [Rh(COMe)I-2(PEt3)(2)] is thermodynamically less stable than [RhMeI2(CO)(PEt3)(2)]. In CH2Cl2, [Rh(COMe)I-2(CO)(PEt3)(2)] reductively eliminates MeCOI. [RhI(CO)(PEt3)(2)] reacts with CO to give [RhI(CO)(2)(PEt3)(2)]. Catalyst degradation occurs via [RhHI2(CO)(PEt3)(2)], formed by oxidative addition of HI to [RhI(CO)(PEt3)(2)], which reacts further with HI to give [RhI3(CO)(PEt3)(2)] from which [Et3PI](+) reductively eliminates and is hydrolysed to give Et3PO. In the presence of water, much less [RhI3(CO)(PEt3)(2)] and Et3PO are formed so the catalyst is more stable, but loss of [Et3PMe](+) and [Et3PH](+) from [RhMeI2(CO)(PEt3)(2)] or [RhHI2(CO)(PEt3)(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)(PEt3)(2)], but in the absence of water there is evidence that it may be reductive elimination of MeCOI from [Rh(COMe)I-2(CO)(PEt3)(2)]. [RhMeI2(CO)(PEt3)(2)] has mutually trans phosphines and the methyl group trans to I.