Monsanto process
The catalytically active species is the anion cis-[Rh(CO)2I2]− (1). The catalytic cycle has been shown to involve six steps, two of which do not involve rhodium: conversion of methanol to methyl iodide and the hydrolysis of the acetyl iodide to acetic acid. (These steps are in the middle of the above diagram.) The first organometallic step is the oxidative addition of methyl iodide to cis-[Rh(CO)2I2]− to form the hexacoordinate species [(CH3)Rh(CO)2I3]− (2). This anion rapidly transforms, by the migration of a methyl group to the carbonyl ligand, affording the pentacoordinate acetyl complex [(CH3CO)Rh(CO)I3]− (3). This five-coordinate complex then reacts with carbon monoxide to form the six coordinate dicarbonyl complex. (4) This decomposes by reductive elimination to form acetyl iodide (CH3COI) and regenerate the active form of the catalyst. Acetyl iodide is then hydrolyzed to acetic acid.