Question

The rate of reduction of [Co(NH3)5(OH2)]3 by Cr(II) is seven orders of magnitude slower than reduction of its conjugate base, [Co(NH3)5(OH)]2, by Cr(II). For the corresponding reductions with [Ru(NH3)6]2, the two differ by less than a factor of 10. What do these observations suggest about their mechanisms?

Answer 1

In answering this question I am assuming that Cr(II) points to [Cr(H₂O)₆]²⁺ complex.

Slow reaction rates in the case of coordination complexes involving oxidation and reduction imply that the reaction is happening via outer sphere mechanism which are slow.

On the other hand fast reaction rates in the case of coordination complexes involving oxidation and reduction imply that the reaction is happening via inner sphere mechanism which are fast, as they allow the metals involved in the reaction to interact directly.

The difference between [Co(NH₃)₅(OH₂)]³⁺ and [Co(NH₃)₅(OH)]²⁺ is that [Co(NH₃)₅(OH)]²⁺ contains a ligand that can bridge with the incoming complex.

[Cr(H₂O)₆]²⁺ complex consists of ligands that can readily dissociate due to Jahn- Teller distortions. Thus

[Co(NH₃)₅(OH)]²⁺ can formed bridging complex with [Cr(H₂O)₆]²⁺ thus allowing inner sphere elctron transfer which are significantly faster than outer sphere electron transfer which happens in the case with [Co(NH₃)₅(OH₂)]³⁺ as it cannot form bridging complex with [Cr(H₂O)₆]²⁺.

[Ru(NH₃)₆]²⁺ contains a low spin d⁶ Ru²⁺ ion. This complex is inert due to large ligand field stabilization energy and thus reduction of both [Co(NH₃)₅(OH)]²⁺ and [Co(NH₃)₅(OH₂)]³⁺ have to occur via out sphere mechanism and is thus slow, hence there is very little difference in the rate.