97403-42-4Relevant academic research and scientific papers
Kinetics and mechanism of ligand substitution in four-coordinate nickel(II) chelate complexes: Study on the reactivities of planar and tetrahedral configurational isomers and octahedral adducts
Schumann, Manfred,Elias, Horst
, p. 3187 - 3192 (2008/10/08)
Stopped-flow spectrophotometry has been used to study the kinetics of ligand substitution in neutral four-coordinate bis chelate nickel(II) complexes NiA2 (=bis(N-alkylsalicylaldiminato)nickel(II) (Ni(R-sal)2), bis(N,N′-dialkyl-2-aminotropone iminato)nickel(II) (Ni(R2-ati)2), bis(N-alkylbenzoylacetone iminato)nickel(II) (Ni(R-bza)2)) by acetylacetone (=HB) and by N,N′-disalicylideneethylenediamine (=H2B) at different temperatures. Substitution by HB in methanol takes place in two observable steps, both following a two-term rate law; rate = (ks + kHB[HB])[NiA2]. For substitution by H2B in acetone only one step is observed, which follows a simple rate law; rate = kH2B[H2B][NiA2]. Complexes Ni(R-sal)2, Ni(R2-ati)2, and Ni(R-bza)2 are subject to a fast configurational equilibrium: planar ? tetrahedral. The equilibrium constant Kp,t for this isomerization was determined by 1H NMR spectroscopy in acetone for several complexes Ni(R2-ati)2 differing in the alkyl group R. It is shown by correlation of the data obtained for Kp,t, kH2B, and the activation parameters ΔH? and ΔS? that the entering ligand H2B attacks only the planar configurational isomer and that the tetrahedral isomer is inert toward ligand substitution by H2B. For several complexes Ni(R-sal)2 the equilibrium constants K1 and K2 for adduct formation with pyridine in acetone were determined by spectrophotometric titration. It is found that K1(Ni(R-sal)2py) ? K2(Ni(R-sal)2(py)2) and that substituents in the 5-position of the salicylaldehyde ring have a marked effect on the size of K1 and K2. Correlation of K1 and K2 with rate constant kH2B as determined in the presence of pyridine proves quantitatively that in the presence of pyridine the substitution reaction Ni(R-sal)2 + H2B → NiB + 2R-salH takes place exclusively via the pyridine-free planar complex, which is in a fast equilibrium with Ni(R-sal)2py and Ni(R-sal)2(py)2. The octahedral complex Ni(R-sal)2(py)2 is completely inert toward ligand substitution by H2B. The mechanistic implications of these results are discussed.
