Ligand substitution behavior of a simple model for coenzyme B-12

The ligand substitution reactions of trans- [Co-III(en)(2)(Me)H2O](2+), a s imple model for coenzyme B-12, were studied for cyanide and imidazole as en tering nucleophiles. It was found that these nucleophiles displace the coor dinated water molecule trans to the methyl group and form the six-coordinat e complex trans-[Co(en)(2)(Me)L]. The complex formation constants for cyani de and imidazole were found to lie (8.3 +/- 0.7) x 10(4) and 24.5 +/- 2.2 M -1 at 10 and 12 degrees C, respectively. The second-order rate constants fo r the substitution of water were found to be (3.3 +/- 0.1) x 10(3) and 198 +/- 13 M-1 s(-1) at 25 degrees C for cyanide and imidazole, respectively. F rom temperature and pressure dependence studies, the activation parameters Delta H double dagger, Delta S double dagger, and Delta V double dagger for the reaction of trans-[Co-III(en)(2)(Me)H2O](2+) with cyanide were found t o be 50 +/- 4 kJ mol(-1), 0 +/- 16 J K-1 mol(-1), and +7.0 +/- 0.6 cm(3) mo l(-1), respectively, compared to 53 +/- 2 kJ mol(-1), -22 +/- 7 J K-1 mol(- 1), and +4.7 +/- 0.1 cm(3) mol(-1) for the reaction with imidazole. On the basis of reported activation volumes, these reactions follow a dissociative mechanism in which the entering nucleophile could be weakly bound in the t ransition state.