Trifluoroacetate complexes of cobalt(II), nickel(II), and copper(II) with pyridine-type ligands. Part IV. Nuclear magnetic resonance studies of nickel(II) complexes and theoretical treatment of spin-density distributions in cobalt(II) and nickel(II) complexes

Abstract

¹ H and ¹⁹F N.m.r. studies have been carried out on complexes of nickel(II) trifluoroacetate with pyridine and alkylpyridine ligands (L), having stoicheiometries NiL₄T₂ and NiL₂T₂. cis- and trans-Isomers of these complexes have been detected, although in some cases the chemical-shift distinction is very small. cis–trans-Interconversion is slow on the n.m.r. time scale at temperatures < ca. 60 °C, in contrast to corresponding Co^II complexes. EHMO Calculations have been carried out to determine the conformational preferences of the free ligands and to explain the measured n.m.r. isotropic shifts in both the Co^II and Ni^II series. The results indicate that the contact shifts are dominated by a σ-delocalisation mechanism but that π-spin polarisation is important particularly for the alkyl-substituent shifts. Limitations of the EHMO method are discussed.