Ligand substitution reactions on metal clusters are often found to occur by associative mechanisms, and it is postulated that these occur by initial breaking of an M–M bond, thereby providing an open coordination site for the incoming ligand. If the proposed mechanism is applicable, which would you expect to undergo the fastest exchange with added ¹³CO, [Co₄(CO)₁₂] or [Ir₄(CO)₁₂]? Suggest an explanation.