Thermodynamic study of steric effects on the formation of tetrahedral pyridine-base complexes of cadmium(II) halides

Abstract

The formation of tetrahedral complexes of cadmium(II) chloride or bromide with pyridine (py) or methyl-substituted pyridines (R-py)(R-py = 3-methyl-, 4-methyl-, 2-methyl-, 2,4-dimethyl-, 2,6-dimethyl- or 2,4,6-trimethyl-pyridine) and the steric effect of methyl substituents at pyridine on the formation of the complexes in 1,2-dichloroethane have been studied by means of liquid–liquid extraction at 25.0 °C. The formation of pyridine-base complexes from the tetrahedral trioctylphosphine oxide (topo) complex is given by equations (i) and (ii) where X refers to Cl^– or Br^–. The logarithmic formation [CdX₂(topo)₂]+ R-py [graphic omitted] [CdX₂(topo)(R-py)]+ topo (i)[CdX₂(topo)₂]+ 2R-py [graphic omitted] [CdX₂(R-py)₂]+ 2 topo (ii) constants, log β₁ and log β₂, of the complexes having no steric hindrance show a good linear free energy relationship with pK_a of the conjugate acid of the pyridine base. The formation constants of the 2-and 2,6-substituted pyridine complexes are lowered by the steric hindrance of the substituents. Thermodynamic parameters for the formation of bis(pyridine) complexes, [CdX₂(R-py)₂], have been determined from the temperature dependence of the formation constants. For the bromo complexes, the formation of the pyridine complexes is enthalpy driven and the increase in the formation constant with increase in pK_a is due to the increase in enthalpy of the reaction, -Δ_rH°. The decrease in the formation constants for the sterically-hindered complexes is caused predominantly by the decrease in entropy, i.e., a decrease in a freedom of rotation of the co-ordinating pyridine. The formation of bis(pyridine) complexes of cadmium(II) chloride is entropy driven and the introduction of a steric effect causes a decrease in both of the enthalpy and entropy terms.