Isotropic NMR Shifts in Pyridine-Type Bases Complexed with Paramagnetic NiII and CoII Acetylacetonates

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Abstract
Isotropic contact interaction shifts in the proton NMR spectra of pyridine‐type molecules coordinated with paramagnetic nickel (II) and cobalt (II) acetylacetonates have been observed. The role of π‐ and σ‐ bonding frameworks in the transfer of unpaired electron spin density from the metal ion to ligand protons is considered. Pseudocontact interactions appear to be significant in the cobalt systems but not in the nickel systems. When the pseudocontact contribution to isotropic shifts in cobalt systems is factored out there remain contact shifts which are in good agreement with the observed nickel shifts. It is concluded that the unpaired electrons in metal eg orbitals of both cobalt and nickel systems cause contact shifts in the pyridine‐type ligands whereas the t2g electron in the cobalt system does not. The t2g electron gives isotropic shifts by contributing to anisotropy in the electronic g factor.