Question

The rate of reduction of [Co(OH2)(NH3)5] 3+ by Cr2+ is 7 orders of magnitude slower  than the reduction of [Co(OH)(NH3)5] 2+ by Cr2+.  For the corresponding reduction with [Ru(NH3)6] 3+, the two differ by less than a factor of 10. What do these observations suggest about the mechanism?

Answer 1

due to a change in the reaction mechanism large difference in the rates of the reactions of [Co(NH3)5(H2O)]³⁺ with [Cr(H2O)6] ²⁺ and [Co(NH3)5(OH)]2+ with [Cr(H2O)6] ²⁺ .

[Cr(H2O)6] ²⁺ is a labile complex: the Jahn-Teller distorted geometry of the high-spin d⁴ ion leads to long and weak axial bonds which are readily replaced by other ligands.

[Co(NH3)5(OH)]²⁺ contains the OH- ligand which is able to form an additional bond to a second metal. The reaction of these two complexes proceeds via the formation of an inner sphere complex:

[Co(NH3)5(OH)]²⁺ + [Cr(H2O)6] ²⁺ <--------> [Co(NH3)^5-OH- Cr(H2O)5] ⁴⁺

The close approach of the two metals ions facilitates e- transfer which probably proceeds via the orbitals of the ligand.

[Co(NH3)5(H2O)]³⁺ does not possess any ligands capable of bridging in this way. Thus, although [Cr(H2O)6] ²⁺ is labile, there is no ligand on the other complex in the reaction which can form an inner sphere reaction. Electron transfer must occur through the slower, outer sphere route.

[Ru(NH3)6] ²⁺ contains a low spin d⁶ Ru²⁺ ion. (The lower pairing energy and larger ∆oct values for 4d and 5d metals ensure that all their complexes are low spin). This leads to a large LFSE. As this LFSE is partially lost in exchanging ligands, this complex is inert. Thus, although [Co(NH3)5(OH)]²⁺ possesses a ligand capable of forming a bridged complex, the other complex in the reaction is inert and no inner sphere complex forms.

Electron transfer must occur through the slower outer sphere route for both [Co(NH3)5(OH)]²⁺ and [Co(NH3)5(H2O)]³⁺ and there is very little difference in the rate. (The small difference in the rate is probably due to the higher charge of Co(NH3)5(H2O)]³⁺ which leads to greater repulsion with the [Ru(NH3)6] ²⁺ ion.