Spin density distribution in some conjugated hetero-atomic molecules from isotropic proton resonance shifts

Abstract

Isotropic proton resonance shifts are reported for a number of nickel(II) and cobalt(II) complexes of 1 : 3 bidentate Schiff's bases. The observed shifts are interpreted in terms of delocalization of a beta-polarized electron from the ligand π-bonding system into the metal 3d atomic orbitals of e(D 2d ) symmetry, with concurrent delocalization involving the ligand σ-bonding system. In addition to the above isotropic contact contributions, the isotropic shifts of the cobalt complexes contain contributions from the isotropic pseudocontact interaction. Variable temperature measurements show that the isotropic shifts have a Curie law dependence, that the relaxation mechanism contributing to the linewidths in the cobalt complexes is dominated by a temperature dependent interaction, and that the phenyl rings are undergoing rapid rotation at room temperature.