Phosphine modified rhodium complexes are currently the topic of considerable research as methanol carbonylation catalysts, but often suffer from poor stability. This paper reports on an investigation into how coordination mode affects the elimination of phosphonium salts from rhodium complexes, namely [trans-RhCl(CO)(PPh₃)₂] 1, [RhCl(CO)(dppe)] 2, [RhCl(CO)(dppb)]₂3, [Rh(TRIPHOS)(CO)₂]Cl 4. These complexes are all potential pre-catalysts for methanol carbonylation. The reaction of these complexes with methyl iodide at 140 °C under both N₂ and CO atmospheres has been studied and has revealed clear differences in the stability of the corresponding Rh(III) complexes. In contrast to both monomeric 2 and dimeric 3 that react cleanly with CH₃I to give stable Rh(III) acetyl complexes, 4 forms a novel bidentate complex after the elimination of the one arm of the ligand as a quaternised phosphonium salt. The structure of this complex has been determined spectroscopically and using X-ray crystallography. The mechanism of formation of this novel complex has been investigated using ¹³CH₃I and strong evidence that supports a dissociative mechanism as the means of phosphine loss from the rhodium centre is provided.