Kinetic, thermodynamic and X-ray crystal studies on N-alkyl-α-amineoxime ligand exchange between planar nickel(II) complexes

Equilibrium and kinetic parameters have been obtained for bidentate ligand exchange between the complexes [Ni(RAO)(2)-H](+) and [Ni(R ' AO)(2)-H](+) to give [Ni(RAO)(R ' AO)-H](+) by analyzing aqueous solutions using reverse-phase HPLC chromatography; R = H, Me, Et, n-Pr and n-Bu. It was found that the reactions were primarily entropy-controlled with the major rate-controlling step being dissociation of the strongest bound bidentate ligand. In organic solvents the exchange reaction proceeds at comparable rates probably by the same basic mechanism. Similar measurements were made for mixed ligand complex formation from the free ligand and the symmetrical complex ion. Using measured and estimated rate constants, the concentration versus time profile for one exchange reaction was simulated using the GEPASI method and closely followed the experimental data. With (H, n-propyl) and (H, i-propyl), single crystals of the Ni(II) complexes were isolated as the bromide ion salt and identified by HPLC, X-ray structure solution and H-1, C-13 NMR. A comparison of bond distances and angles of the isomeric ligands coordinated to the same metal ion is made. Structural comparisons between these complexes and those having alkyl groups on both AO-ligands show no differences which could be related to their kinetic or thermodynamic stability in aqueous solution. (C) 2001 Elsevier Science Ltd. All rights reserved.