Outer-sphere effects on the redox properties of the system Co(en) 33+/Co(en) 32+

Abstract

The half-wave potentials for the polarographic reduction of Co(en) 3 3+ to Co(en) 3 2+ have been determined in the solvents: nitromethane, acetonitrile, propanediol-1,2-carbonate, acetone, dimethylformamide, dimethylacetamide, dimethylsulfoxide and hexamethylphosphoric acid triamide. A nearly linear relationship was found between the half-wave potentials E 1/2 and the donor numbers of the solvents which shows that the outer-sphere solvation of Co(en) 3 3+ and to a lesser extent of Co(en) 3 2+ involves donor-acceptor interactions between the solvent molecules and the hydrogen atoms of the coordinated ethylenediamine ligands. The observed shift of E 1/2 to more negative potential values corresponds to an increase in reducing properties of the system Co(en) 3 3+/Co(en) 3 2+ with increasing donor numbers of the solvents. A correlation was found between the E 1/2 values and the 59Co n.m.r. chemical shifts of Co(en) 3(ClO 4) 3 in solution which proves that the outer-sphere solvation of Co(en) 3 3+ is accompanied by an increase in electron density at the Co atoms with increasing donor numbers of the solvents. The maximum difference in E 1/2 for the solvents investigated is 0.51 V which corresponds to a free energy difference of 49.5 kJ mol −1.