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 CoII 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 CoII and NiII 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.