Organometallic Solutions: #14

14.* (1993 3 2)

Which of the following reactions do you expect to be the fastest; which the slowest. Explain your answer.

(1) [Ru(dipy)₃]²⁺ + [Ru(phen)₃]³⁺ -->
(2) [Co(en)₃]³⁺ + [Co(en)₃]²⁺ -->
(3) [Ru(NH₃)₆]²⁺ + [Ru(NH₃)₆]³⁺ -->

These are all outersphere e- transfers. The less that nuclear rearrangement of the ligands (geometry change in bond lengths) is required, the less the E_a will be and the faster the reaction. So t_2g --> t_2g e- transfers will be fastest.

[Ru(dipy)₃]²⁺ + [Ru(phen)₃]³⁺ -->

t2g --> t2g
(p* --> p *)

Fastest. Minimal change in metal-ligand bond lengths.
Also aromatic bidentate ligands are more rigid.
remember: 4d and 5d complexes are always low spin.

[Co(en)₃]³⁺ + [Co(en)₃]²⁺ -->

e_g --> e_g
(p * --> p *)

Slowest. Substantial change in metal-ligand bond lengths is required.
Co³⁺ is low spin, Co²⁺ is high spin

[Ru(NH₃)₆]²⁺ + [Ru(NH₃)₆]³⁺ -->

t2g --> t2g
(p * --> p *)

Also little change in metal-ligand bond lengths.
Saturated monodentate ligands are less rigid.

B. Copper(I) disproportionates into copper(II) and Cu^o

(E^o Cu⁺/Cu^o = +0.52 V; E^o Cu²⁺/Cu⁺ = +0.15 V).

Given the following E^o values for Ag, calculate the feasibility of Ag⁺ disproportionating into Ag²⁺ and Ag^o.

(E^o Ag⁺/Ag^o = +0.799 V; E^o Ag²⁺/Ag⁺ = +1.98 V).

If everything is in standard states (temperature and pressure at STP, everything aqueous having 1M concentration), E = E^o. Therefore, we can use E^o > 0 as the criteria for a spontaneous reaction.

From the given potentials, we can write the following:

Ag⁺ + e^- --> Ag^o E^o = +0.80 V
Ag⁺ --> Ag²⁺ + e^- E^o = -1.98 V
--------------------------
2Ag⁺ --> Ag²⁺ + Ag^o E^o = -1.18 V

E^o < 0 so this disproportionation is nonspontaneous.

Instead, the reverse, "comproportionation" is spontaneous.

Ag^o + Ag²⁺ --> 2Ag⁺ E^o = +1.18 V