The crystal field strength of the nitro ligand and the chemistry of the hexanitrocobaltate(III) anion

Abstract

The chemical and spectroscopic properties of the hexanitrocobaltate(III) anion are not in accord with the classification of the N-bonded nitrite ion as a strong field ligand. The nitro groups are rapidly displaced by other ligands, including water, and in dilute aqueous solution spontaneous reduction to Co(II) occurs. Comparison of solid state and solution vibrational and 59Co nmr spectra demonstrates that the principal species in solution is the same as in the solid. All ligands are N-bonded. However, within 2 or 3 min of dissolution new species appear. An electron transfer mechanism for ligand exchange is suggested. It is shown that the band at 480 nm arises from the first d–d transition of the pentanitroaquacobaltate(III) ion and not from a hexanitrocobaltate(III) transition. The composition of the aged solutions has been studied by 59Co, 14N, and 17O nmr. At least 10 different species are apparent in the 59Co spectra. They have been assigned to mixed nitro/nitrito/aqua ions. Electron transfer can also lead to the formation of Co2+ and NO3−, ions, both ions being detected by 14N and 17O nmr spectroscopy. The Co complexed nitro ligand has been detected for the first time in the 14N nmr spectrum. Analysis of 59Co chemical shifts shows that the crystal field strength of the nitro ligand falls steadily with the number of nitro groups in the molecule. The cis groups are four times more effective than the trans groups in causing this change. The cyano ligand shows the opposite behaviour — the crystal field strength increases with substitution and trans groups have a larger effect than cis groups. The reactions of the hexanitrocobaltate(III) ion with ethylenediamine and with cyanide ions have been studied by 59Co nmr. Mixed nitro complexes are formed with ethylenediamine but mixed nitrito complexes predominate with cyanide.