Mechanistic study on methanol carbonylation catalyzed by chelated square planar carbonyl rhodium complex is reported in this paper. Chelated square planar cationic complex was formed via the ligand of two donors with different complexing capacity with rhodium. Experimental results showed that the active species of the complex differed from literature-reported [Rh(CO)2I2]- in methanol carbonylation. Other than conventional concept the N→Rh bond formed from rhodium in the complex and nitrogen at pyridine ring didn’t break in the course of reaction, it formed into new active species instead, i.e. the ligand and rhodium as a whole completed the oxidative addition of CH3I and the formation of CH3COI. The result is of significance towards the molecular design of such catalysts and the solution to their deactivation due to precipitation in industrial application as well.

Key words: Rhodium complex, Methanol carbonylation, Reaction mechanism, Active species