Substituents Effect on the Kinetics and Mechanisms of Formation of Copper (II) and Nickel (II) Complexes of Some Β-Diketones

Abstract

Chelates of β – diketones (R¹COCH₂COR²) with transition metals have been found to possess a wide range of properties and application, some of which are dependent on the substituents (R¹ and R²).  Copper(II) and Nickel(II) complexes of β – diketones in which R¹= C₆H₅ and R² = C₆H₅ (dbm), R² = CH₃ (bza) R² = CF₃ (tfpbd) and R¹ = C₄H₃S, R² = CF₃₍tta) were synthesised and characterised using UV, IR, elemental analysis and magnetic susceptibility. The probable influence of the substituents on the kinetics and mechanisms of formation of the complexes were investigated with the aid of a Thermostated Schimadu 1800 uv-visible spectrophotometer. The reactions were studied at wavelengths characteristics of each complex. Results show the six coordinate copper(II) and nickel(II) complexes have probable distorted octahedral geometry while the four coordinate Ni(dbm)₂ is tetrahedral. The kinetics data suggests an influence of the diketonate substituents on the copper(II) and nickel(II) complexes in solution is in the order C₆H₅ > CH₃ >CF₃ > C₄H₃S and CF₃ > C₆H₅ > CH₃ > C₄H₃S respectively. Solvent influence (k_-1) was constant in the range 0.007 – 0.008 for fluorosubstituted complexes and least at 0.004 – 0.005 mol^-1s^-1 in the alkyl substituted complexes. The steady state approximation gave solvent independent rates k₁k₂ at 25^oC, in the copper(II) complexes, k₂ is greater than k₁ (k₂ > k₁) , with k₂ constant while in the nickel(II) complexes, the values of k₂ was less than k₁ and ascribed to rate determining and varies with the nickel(II) - ligand systems studied.

Keywords: β-diketones, Copper II and Nickel II Complexes, Substituents effect, Kinetics.