Strictly speaking, this reaction results in the formation of a rhodium carbonyl complex rather than free carbon monoxide.
The catalytic cycle is assumed to involve oxidative addition of the aldehyde (or acid chloride) to gives a 16e acyl Rh(III)-hydride intermediate, which undergoes migratory extrusion of CO proceed to form an 18-electron d6 Rh(III) carbonyl complex. Reductive elimination produces the decarbonylated product.  In the catalytic variant of the Tsuji–Wilkinson decarbonylation, RhCl(CO)(PPh) evolves CO above 200 °C, thereby regenerating RhCl(PPh). Otherwise, the reaction mechanism halts by formation of this thermodynamically stable carbonyl complex.