Outer-sphere coordination effects on the redox behaviour of the Fe(CN)63−/Fe(CN)64− couple in non-aqueous solvents

The polarographic behaviour of tetra-n-butylammonium hexacyanoferrate(II) has been studied in N-methylpyrrolidinone, N,N-dimethylformamide, acetonitrile, dimethylsulphoxide, N,N-dimethylthioformamide, 1,2-dichloroethane, propanediolcarbonate, nitromethane, ethanol, methanol and acetic acid using tetrabutylammonium perchlorate as supporting electrolyte. A polarographically reversible electrode process was found in all solvents but acetic acid. The large shift in half-wave potentials (vs. bisbiphenylchromium(I) as a reference ion) is interpreted on the basis of donor-acceptor interactions between the hexacyanoferrate ions and solvent molecules and also between the hexacyanoferrate ions and cations of the supporting electrolyte. The considerable difference in half-wave potentials between tetraethylammonium and tetrabutylammonium perchlorate is discussed within the framework of this concept.