Evaluation of thermodynamic parameters on higly correlated chemical systems: a spectrophotometric study of the 1:1 and 1:2 equilibria between I2 and R4todit = 4,5,6,7-tetrathiocino[1,2-b:3,4-b′]diimidazolyl-1,3,8,10-tetraalkyl-2,9-dithione; (R = Bu, Me (new data); Et, Ph (reinvestigation)). Crystal and molecular structure of the charge-transfer complex Bu4 todit•2I2

The reactions between Bu4todit and I2 in CHCl3 solution have been investigated by spectrophotometry at different temperatures. Evidence for the stepwise formation of the 1:1 and 1:2 charge-transfer complexes has been obtained by computer analysis of the experimental data. The stability constants of both complexes and the corresponding standard enthalpy and entropy changes have been calculated. The 1:2 complex has been characterized in the solid state by an X-ray structural study: the crystals of Bu4todit 2I2 are monoclinic, space group C2/c, a = 16.352(6), b = 9.376(5), c = 23.470(7) Å, β = 99.78(2)°, Z = 4,R = 0.0579. The molecular structure shows that the two thionic sulfur atoms coordinate two diiodine molecules, the bond distances and angles being similar to those observed previously in Et4todit•2I2. On the contrary, the solution studies disagree with the previous investigations on Et4todit and Ph4todit, where the absorbance data were explained assuming the formation of the 1:1 complex only. A new calculation procedure for the least-squares minimization of spectrophotometric data, based on Powell's direct search method, has been developed. This procedure allows for the refinement of the standard enthalpy and entropy changes from a set of absorbance data at different temperatures. Convergence has been attained for the reaction of diiodine with Bu4todit and Me4todit (new data) and also for Et4todit and Ph4todit (old data). Keywords: equilibrium constants, charge-transfer complexes, Fourier transform – Raman, X-ray, I2-thioamides.