Multinuclear NMR study of the crystal field strength of the nitro ligand and the empirical estimation of the cobalt-59 NMR chemical shifts of cobalt-nitro complexes

Abstract

The variable ligand field strength of the nitro ligand has been reexamined on the basis of a steric model. An improved empirical method is proposed for the estimation of nitro ligand field strength in terms of a single correction parameter {Delta}{sub s} (1.824 {times} 10{sup {minus}6} ppm{sup {minus}1}) to the shift parameter S{sub NO{sub 2}} (3.075 {times} 10{sup {minus}5} ppm{sup {minus}1}) for the estimation of the {sup 59}Co NMR chemical shifts of cobalt-nitro complexes. The reversed chemical shift trend of geometrical isomers of cobalt-nitro complexes are attributed to variations in the 10Dq of the nitro ligands. The model is applied to the assignment of different isomers of cobalt-nitro compounds obtained from ligand exchange reactions of the cobaltinitrite anion with the N{sub 3}{sup {minus}} and SCN{sup {minus}} ions. In all the ligand exchange reactions studied, it was found that mixed nitro complexes formed in the reaction predominately adopted the trans configuration.