INDO–MO Calculations and Proton Isotropic Shifts of Methyl-Substituted Pyridine Ligands in Ni(II) Complexes

Relative p.m.r. isotropic shifts for the ligands in the complexes NiL 4 (NO 3 ) 2 (L = 4-picoline and 3,5-lutidine), NiL 3 (NO 3 ) 2 (L = 3-picoline and 3,4-lutidine), and NiL 2 (NO 3 ) 2 (L = 2-picoline, 2,3-lutidine, 2,4-lutidine, and 2,5-lutidine) were determined in CDCl 3 . Proton hyperfine coupling constants for corresponding pyridine cations and analogous phenyl radicals were calculated by the INDO–MO method. Analysis of the p.m.r. shifts and subsequent correlation with the INDO data revealed that the spin transfer process between metal and ligand was dominated by a σ-mechanism, as indicated by ring protons at the α- and β-positions. The γ-ring proton and methyl protons at all positions indicated a π-mechanism was also operative through σ–π spin polarization at the nitrogen atom. The INDO calculations based on spin densities of the hydrogen Is orbitals, however, predicted that all ring protons would be dominated by a σ-mechanism, and that methyl protons at the α- and γ-positions should manifest a π-mechanism. Both mechanisms should be operative for the β-CH 3 protons, with 50–60% σ-contribution in the pyridine cations and 30–50% σ-contribution in the phenyl radicals. The pattern of spin polarization (i.e., negative at α-C and γ-C, positive at β-C) is not altered by a small excess of α or β spin in the π-system. However, the π-spin density ratios, especially at the α- and γ-positions, are affected, as is the magnitude of the π-spin densities on each carbon. A comparison of the observed isotropic shifts for the methyl protons and for the γ-ring proton with the INDO calculated π-spin densities allows a determination of the spin delocalization in the ligand π-system.