Organometallic Solutions: #10

10.* (1994 3 2) Which of the following redox reactions (A or B) do you expect to be faster? Explain your answer.

A. Ru(en)₃²⁺ + Ru(en)₃³⁺ --> Ru(en)₃³⁺ + Ru(en)₃²⁺

B. Co(en)₃²⁺ + Co(en)₃³⁺ --> Co(en)₃³⁺ + Co(en)₃²⁺

Because the ligands are not very open to substitution, which is required for an inner-sphere electron transfer mechanism, both reactions have an outer-sphere mechanism. Ru³⁺ is d5 and Ru²⁺ is d6, and both ions are low spin because Ruthenium is below the 3d level. Therefore, the transfer occurs from one t_2g orbital to another t_2g orbital, and there is very little change in bond lengths because the t_2g orbitals are non-bonding.

For cobalt, on the other hand, the situation is different. Co³⁺ is d6 while Co²⁺ is d7. Therefore, it is necessary to move an electron from an e_g orbital to a t_2g orbital in order to do this reaction, and there will have to be a concomitant change in bond lengths because e_g is antibonding while t_2g is nonbonding.

Therefore, the Ruthenium electron transfer reaction is faster.