OUTER-SPHERE EFFECTS ON THE REDOX PROPERTIES OF THE SYSTEM CO(EN)3(3+)-CO(EN)3(2+)

The half-wave potentials for the polarographic reduction of Co(en)33+ to Co(en)32+ 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 E1/2 and the donor numbers of the solvents which shows that the outer-sphere solvation of Co(en)33+ and to a lesser extent of Co(en)32+ involves donor-acceptor interactions between the solvent molecules and the hydrogen atoms of the coordinated ethylenediamine ligands. The observed shift of E1/2 to more negative potential values corresponds to an increase in reducing properties of the system Co(en)33+/Co(en)32+ with increasing donor numbers of the solvents. A correlation was found between the E1/2 values and the 59Co n.m.r. chemical shifts of Co(en)3(ClO4)3 in solution which proves that the outer-sphere solvation of Co(en)33+ is accompanied by an increase in electron density at the Co atoms with increasing donor numbers of the solvents. The maximum difference in E1/2 for the solvents investigated is 0.51 V which corresponds to a free energy difference of 49.5 kJ mol-1. © 1979 Elsevier Sequoia S.A.