Steric and conformational effects on the kinetics of ligand substitution in bis(salicylaldiminato)nickel(II) complexes

Article abstract of DOI:10.1021/ic00132a028

Stopped-flow spectrophotometry has been used to study the kinetics of ligand substitution in bis(N-R-salicylaldiminato)nickel(II) complexes I (R = Et, i-Pr, t-Bu) by bidentate ligands HB (acetylacetone, benzoylacetone, dibenzoylmethane, trifluoroacetylacetone, 8-hydroxyquinoline, N-ethylsalicylaldimine) in methanol, 2-propanol, and toluene. A two-term rate law, rate = (k_S + k_HB[HB])[complex], has been found. The substitution of the first ligand in I is rate determining. Rate constant k_S, describing the solvent path, and the corresponding activation parameters ΔH^≠ and ΔS^≠ do not depend on the nature of the entering ligand for I with R = t-Bu studied in methanol. Rate constant k_HB is strongly dependent on the nature of the entering ligand HB. The relative contributions of the two pathways to the overall rate are governed by the conformational equilibrium planar ? tetrahedral of complexes I: the planar isomer favors the ligand-dependent path k_HB and the tetrahedral one the solvent path k_S. For both pathways mechanisms are derived, which have in common the rate-determining rupture of the Ni-O bond. They differ in that the solvent path is initiated by the attack of an alcohol molecule at the donor oxygen of a coordinated ligand through hydrogen bonding, whereas ligand attack occurs at the metal. The factors influencing the ligand path are the donor ability, acid strength, and stereochemical properties of the entering ligand as well as the Lewis acidity of the substrate. The discussion focuses on a comparison of the nickel system studied with corresponding copper(II) systems and with ligand substitution in square-planar d⁸ complexes. Additional kinetic information is presented from studies carried out in the solvent mixtures toluene/methanol and toluene/pyridine. The equilibrium constant for the addition of pyridine to complexes I (R = Et, n-Pr, i-Pr, allyl, n-Bu, i-Bu, t-Bu, phenyl) has been determined spectrophotometrically in toluene at 298 K. The individual equilibrium constants for the formation of the mono- (K₁) and bis(pyridine) (K₂) adduct were calculated (K₁ ? K₂). The effect of the conformational equilibrium and of self-association on K₁ and K₂ is discussed.