The cobalt(III) pentammine complex, [Co(NH3)5Cl]2+, consists of a central cobalt cation with coordinate covalent bonds to five neutral ammonia molecules and a chloride anion, which act as ligands.  In an aqueous solution with a pH > 10, the complex readily undergoes a ligand exchange via base hydrolysis as shown in Reaction 1.[Co(NH3)5Cl]2++OH−⟶[Co(NH3)5(OH)]2++Cl−CoNH35Cl2++OH-⟶CoNH35OH2++Cl-Reaction 1Researchers have proposed that Reaction 1 might proceed by either a bimolecular nucleophilic substitution (an SN2 mechanism) as shown in Figure 1, or by a unimolecular nucleophilic substitution of the complex's conjugate base (an SN1CB mechanism) as shown in Figure 2.Figure 1  SN2 mechanism proposed for Reaction 1Figure 2  SN1CB mechanism proposed for Reaction 1In the proposed SN2 mechanism, the OH− ion functions as a nucleophile that attacks the central Co atom in the complex.  Accordingly, the nucleophilic attack results in a transition state in which the existing Co–Cl bond breaks and a new Co–OH bond forms in a single step.In contrast, the SN1CB mechanism proposes that the reaction occurs in multiple steps in which OH− initially acts as a base to deprotonate one of the coordinately bonded ammine ligands to form a hexa-coordinated intermediate with an amido (NH2−) ligand (Step 1).  The Cl− ligand in the amido intermediate then dissociates to form a pentacoordinated intermediate (Step 2).  A water molecule from the solution then coordinates with the Co atom of the pentacoordinated intermediate to form an aquo complex (Step 3).  A solvent-mediated proton transfer from the aquo to the amido ligand yields the final base-hydrolysis product (Step 4). Question 38To evaluate the proposed mechanisms, researchers attempted to modify Reaction 1 by replacing the [Co(NH3)5Cl]2+ complex with the complex shown above, which contains C5H5N (pyridine) ligands instead of NH3 ligands, but no reaction occurred.  Assuming that steric hindrance is not a factor in the reaction, this result provides evidence in support of the:A.SN2 mechanism, because the experimental change resulted in no reaction.B.SN2 mechanism, because the pyridine ligands have a negative formal charge that repels the OH− ion.C.SN1CB mechanism, because the leaving group is the same in both the original and the modified reaction.D.SN1CB mechanism, because an N–H bond is required to form the proposed intermediate.