Trifluoroacetate complexes of cobalt(II), nickel(II), and copper(II) with pyridine-type ligands. Part 5. Dynamic nuclear magnetic resonance studies of cis–trans isomerism

Abstract

Dynamic ¹H n.m.r. studies of complexes of general type [ML₂(O₂CR)₂][M = Co^II or Ni^II; L = 3Me-py, 4Me-py, or 4Et-py (py = pyridine); R = CF₃, CF₂H, CFH₂, C₂F₅, or C₃F₇] have yielded accurate energies for the cis–trans interconversion of these complexes. For cobalt(II) complexes ΔG^‡ values lie in the range 47–60 kJ mol^–1, whereas for nickel(II) complexes values are substantially higher being 67–71 kJ mol^–1 for trifluoroacetate complexes. Data for ΔS^‡ imply that the mechanisms of the isomerisation are dependent on the carboxylate ligand O₂CR and become more dissociative in nature in the order R = CF₃ < CF₂H < CFH₂ < C₂F₅ < C₃F₇, this order also applying to the relative magnitudes of ΔH^‡ values. Isomerism in the related [ML₄(O₂CR)₂] complexes, where the O₂CR ligands are unidentate, is shown to be a much more facile, dissociative process, detectable amounts of two additional species thought to be cis- and trans-[ML₂(O₂CR)₂] also being present.