Solvent Influence on UV, 59Co, and 13C NMR of [Co(CN)6]3− and Its Solvation Site

Abstract

Abstract The electronic absorption (UV) spectrum, 59Co, and 13C NMR of potassium hexacyanocobaltate(III), K3[Co(CN)6], were measured in water and a variety of organic solvents: ethylene glycol, methanol, formamide, ethanol, chloroform, nitromethane, dichloromethane, dimethyl sulfoxide, acetonitrile, N,N-dimethylformamide, benzonitrile, and nitrobenzene. The complex salt was dissolved in organic solvents by macrobicyclic polyether, cryptand 222. It was found that the spectra are influenced by the solvent, and vary according to the electrophilic ability (Gutmann’s acceptor number) of solvents. Especially, the 13C NMR chemical shifts of coordinated cyano carbons (octet) have shifted downfield as the acceptor number becomes larger. This means that the solvent influence is caused by direct interaction between the ligand CN− and solvent molecules as electron acceptors. The result concerning the linear correlation between the solvent influence on the peak position (nm) of the first d–d transition band and on the chemical shift of 59Co NMR suggests that this influence is due to the d–d transition-energy change of [Co(CN)6]3 by solvation. Finally, the plausible structure of the outersphere complex of [Co(CN)6]3− with solvent molecules is proposed on the basis of the result obtained and an examination of molecular models.