Thermodynamic and electronic properties of copper(II) five-coordinate cations in nonaqueous solution

Equil. consts. and heats of reaction were detd. by spectrophotometry and calorimetric titrn., resp., for the reaction [CuL2]2+ + X- ↹ [CuL2X]+ in MeOH at 25° (L = en, N-methyl-, N,N'-dimethyl-, and N,N-dimethylethylenediamine; X = Cl, Br, I, SCN). Std. free energies and entropies of reaction were calcd. The spectroscopic and thermodynamic parameters of the 5-coordinate cations (except for cation A, where L = N,N-dimethylethylenediamine with X = I) showed a regular trend in each series of anions considered, indicating that the [CuL2X]+ cations have the same square-pyramidal stereochem. for a given L. The anomalous values for cation A were probably due to a large distortion towards a trigonal bipyramidal configuration. The reactions were all entropy-stabilized, indicating desolvation as the driving force. There was a linear correlation between enthalpies of reaction and enthalpies of anion desolvation; the different slopes found for each [CuL2]2+ complex were explained in terms of steric hindrance by Me groups to the introduction of X- into the coordination sphere of Cu(II).