MECHANISM OF ASSOCIATIVELY CONTROLLED LIGAND SUBSTITUTION IN SQUARE-PLANAR BIS(N-ALKYLSALICYLALDIMINATO)NICKEL(II) COMPLEXES - KINETIC, SPECTROSCOPIC, AND THERMODYNAMIC CHARACTERIZATION OF ADDUCTS AND INTERMEDIATES

Spectrophotometric titration was used to determine the equilibrium constants and thermodynamic parameters DELTAH-degrees and DELTAS-degrees for the addition of mono- and bidentate N-bases to planar four-coordinate complexes I = bis(N-ethylsalicylaldiminato)nickel(II), II = bis(N-ethyl-5-nitrosalicylaldiminato)nickel(II), III = bis(N-n-propyl-5-nitrosalicylaldiminato) nickel(II), and IV = bis(N-ethyl-3-nitro-5-tert-butylsalicylaldiminato)nickel(II) in the solvent acetone. The visible absorption properties of the base adducts are reported. Single-wave length and multiwavelength stopped-flow spectrophotometry was applied to study the displacement of the two bidentate ligands in I and II by tetradentate ligands H-2salen = N,N'-disalicylideneethyienediamine, H-2[H-4]salen = N,N'-bis(2-hydroxybenzyl)-1,2-diaminoethane, and H-2BuMe[H-4]salen=N,N'-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-1,2-diaminoethane in acetone at ambient and reduced temperature. Rate laws and activation parameters DELTAH double dagger and DELTAS double dagger for the various substitution reactions are reported. The kinetic and spectroscopic results prove that substitution is initiated by the formation of the adducts (I.H2B) and (II.H2B), with H2B = H-2 salen, H-2[H-4]salen, or H-2BuMe[H-4]salen. It follows from the spectroscopic and thermodynamic properties of these adducts that the nickel is six-coordinate, the tetradentate ligands H2B being coordinated in a bidentate fashion through the two nitrogen atoms. The kinetic parameters describing the mono- or biphasic decay of the initially formed adducts to products are presented, and a three-step mechanism for ligand substitution is discussed.