Chemistry 332 Basic Inorganic Chemistry II

Acid Hydrolysis of [Co(NH3)5X]2+

[Co(NH3)5X]2+ + H2O [Co(NH3)5(OH2)]3+ + X-

rate = ka[Co(NH3)5X2+] (ka = acid hydrolysis rate constant, s-1)

From the rate law, what mechanism would you predict?

Evidence for the D mechanism:

The rate of aquation follows the bond strength of the Co-X bond; as the bond energy decreases the rate increases.

X = F- ka = 9x10-8 s-1

Cl- 2x10-6 s-1

Br- 6x10-6 s-1

I- 8x10-6 s-1

BECo-X

(HSAB theory)

Given Ka is a thermodynamic quantity a larger value means greater stability for [Co(NH3)5(X)]2+and implies a stronger bond energy. It is clear that as the Co-X bond energy increases, the (or the Keq for anation increases) ka for aquation/hydrolysis decreases.