Ligand field effects in the nuclear magnetic shielding of nitrogen-15 and cobalt-59 in bent nitrosyl complexes of cobalt(III)

Abstract

A range of square-pyramidal complexes of cobalt(III) with a bent apical nitrosyl ligand has been prepared and examined by VN and VZCo NMR spectroscopy, in a study of nephelauxetic and spectrochemical effects at the metal and nitrogen nuclei in the bent Co-NO chromophore. The basal ligands in this comparison include dithiocarbamate, quadridentate Schiff base or porphine, and bis-chelating diamine or oximate, so as to give S4, S2N2, N4, OONN, or ONON coordination in the plane and a range of substituents in the chelate and phenylene rings. The shielding of both cobalt and nitrogen tends to decrease with decrease in the M(d) *(NO) back-bonding, as indicated by the MN and NO bond distances, the MNO angle and the NO stretching frequency. The shieldings decrease from sulfur to nitrogen to oxygen coligators and also with electron withdrawal by ring substituents (and vice versa), i.e. with decrease in the ligand field splitting and in the nephelauxetism of the coligands. These parallelisms of the cobalt and nitrogen shielding accord with the orbital theory that was developed to explain the bending of the MNO ligand and influences of the metal and coligands. Significant interdependence of spectrochemical and nephelauxetic effects at cobalt and nitrogen arises frommore » the degree of overlap and similarity in energies of the frontier orbitals for the paramagnetic circulation at nitrogen (n(N) * (NO)) and at cobalt (d-d). 43 references, 5 figures, 2 tables.« less