Redox poteftials of the aquo, cyano and EDTA complexes of some transition metal ions by CNDO calculations

By using an approximate molecular orbital method standard redox potentials were calculated for the series M(CN) 6 4−,5− , M(CN) 6 3−,4− (M = Cr, Mn, Fe and Co), M(EDTA) 2−,3− , (M = Cr, Co, Ni and Cu), M(H 2 O) 6 2+,+ (M = Cr, Mn, Fe, Co, Ni and Cu) and M(H 2 O) 6 3+,2+ (M = Cr, Mn, Fe and Co). Reasonable agreement with experimental values was obtained in the case of the M(CN) 6 3−,4− couples. From the results for the M(CN) 6 4−,5− series the redox potentials for the couples Cr(CN) 6 4−,5− and Fe(CN) 6 4−,5− were estimated to be −1.7 V and −2.5 V respectively. The agreement with the experimental data is poor for the M(H 2 O) 6 3+,2+ couples, both absolute and relative. It is concluded that this discrepancy is caused by the large bond length differences for the M(H 2 O) 6 3+,2+ couples. The corresponding energy differences are estimated to be 6–9 eV for the M(H 2 O) 6 3+,2+ series. For the M(H 2 O) 6 2+,+ series these energy differences may also be large (up to 6 eV for Cu 2+,+ , whereas they are smaller for the M(EDTA) 2−,3− series: about 3–5 eV. For the M(CN) 6 4−,5− and M(CN) 6 3−,4− series the energy differences due to relaxation are of the order of 1 eV.