Nuclear magnetic resonance contact shifts and spin delocalization mechanisms in tris(acetylacetone) nickel(II) perchlorate

Abstract

The NMR contact shifts for the complex tris(acetylacetone) Ni(II) perchlorate have been found to be +4.7 ppm for the methyl group and +35.6 for the methylene group. INDO molecular orbital calculations are used to show that the major spin delocalization is into the highest filled ligand orbital which is of sigma symmetry, with a minor contribution from delocalization into the lowest empty pi orbital. In the case of Ni(acac) 3 − and Ni(acac) 2(py) 2 the major spin delocalization is into the highest filled ligand orbital, of π symmetry, with a minor contribution from the next highest filled orbital, which is a sigma type. For Co(acac) 2(py) 2 an additional mechanism, which places spin in the third highest filled orbital, also sigma, is found to be consistent with the contact shift results. The implications of these results on the “ratio method” of separating contact and dipolar shifts in complexes of the type M(acac) 2L 2, M  Co, Ni, is discussed and the donor properties of the acetylacetone and acetylacetonate ligands are compared.