The catalytically active species is the anions cis-[Rh(CO)I] (top of scheme). The first organometallic step is the oxidative addition of methyl iodide to cis-[Rh(CO)I] to form the hexacoordinate species [(CH)Rh(CO)I]. This anion rapidly transforms, via the migration of a methyl group to an adjacent carbonyl ligand, affording the pentacoordinate acetyl complex [(CHCO)Rh(CO)I]. This five-coordinate complex then reacts with carbon monoxide to form the six-coordinate dicarbonyl complex, which undergoes reductive elimination to release acetyl iodide (CHC(O)I). The catalytic cycle involves two non-organometallic steps: conversion of methanol to methyl iodide and the hydrolysis of the acetyl iodide to acetic acid and hydrogen iodide.
The reaction has been shown to be first-order with respect to methyl iodide and [Rh(CO)I]. Hence the oxidative addition of methyl iodide is proposed as the rate-determining step.