Spectrochemical Series for the Outersphere Coordination of Carboxylatocobaltate(III) Complexes with Solvent Molecules

Abstract

Abstract The d–d transition spectra of K[Co(edta)]·2H2O, K[Co(mal)2en]·H2O, K[Co(ox)2en]·1.5H2O, K2[Co(ox)2gly]·3H2O, and K3[Co(ox)3]·3H2O (edta=ethylenediaminetetraacetate, mal=malonate, en=ethylenediamine, ox=oxalate, and gly=glycinate) were measured in a variety of solvents: H2O, HOCH2CH2OH, CH3OH, HCONH2, C2H5OH, i-C3H7OH, CHCl3, CH3NO2, CH2Cl2, (CH3)2SO, CH3CN, HCON(CH3)2, C6H5CN, C6H5NO2, CH3CON(CH3)2, (CH3)2CO, and [(CH3)2N]3PO. The 59Co NMR spectra of K[Co(edta)]·H2O and K3[Co(ox)3]·3H2O were also recorded at 25°C in these solvents. It was found that the peak positions (λmax) of the first and second d–d transition bands were greatly influenced by solvent molecules; the peak positions (λmax) of all the cobalt(III) complexes were linearly correlated to the electrophilic ability (Gutmann’s acceptor number) of the solvent and those of K[Co(edta)]·2H2O and K3[Co(ox)3]·3H2O were linearly correlated to the 59Co NMR chemical shifts in the corresponding solvents. This suggests that the d–d transition energy (ΔE) is influenced by solvent molecules as electron acceptors since 59Co NMR chemical shifts are theoretically correlated linearly to the peak positions in the first d–d transition (A1g–T1g). The 13C NMR measurements indicated that solvent molecules directly interact with the carboxyl groups of cobalt(III) complexes. Therefore, the influence of the solvent on the d–d transition energy of the carboxylatocobaltate(III) complex is due to the electron-withdrawing effect of solvent molecules on the carboxyl oxygens of the cobalt(III) complex. A spectrochemical series for the outersphere coordination of the cobalt(III) complex with solvent molecules is proposed.