The reaction of CrCl3 with liquid NH3 to give yellow [Cr(NH3)6]Cl3 often has a low yield, since the reaction tends to stop with the production of pink [Cr(NH3)5Cl]Cl2. However, adding a small piece of Na metal to the liquid NH3 results in complete conversion of the CrCl3 to [Cr(NH3)6]Cl3. Offer two plausible explanations of this rate enhancement for the final step of the substitution process.