Nuclear magnetic resonance studies of Lewis acid–base interactions. Part II. A correlation between isotropic contact shifts and taft σ* values for some pyridine base adducts of nickel(II) bis(O-alkyl dithiocarbonates) and bis(β-diketonates)

Abstract

The Lewis acid–base interaction between pyridine bases and various O-alkyl dithiocarbonates and β-diketonates of Ni^II has been studied by n.m.r. spectroscopy. There is correlation between the isotropic contact shifts of the H² and H³ protons of pyridine bases and the Taft σ* values of the substituent group R in [Ni(S₂COR)₂]. An explanation, based on a detailed study of the change in energy levels of the individual metal d orbitals, is proposed for the observed correlation which implies the relative strength of the Lewis acid–base interaction can be inferred from contact-shift studies. The observed trend in relative isotropic shifts of the protons at the 3-Me, 4-Me, and H⁴ positions and the absolute isotropic shifts of the H² protons on the spin-delocalization mechanism is also discussed.