Is statistical lack of fit a reliable criterion for chemical complexity?

Complexations of Fe-III and Co-II ions with thiocyanate in water-acetone-ethanol ternary mixtures are investigated using multivariate statistical techniques. Simplex lattice and hexagonal mixture designs lead to model equations resulting in similar conclusions. The Fe-III thiocyanate complex exhibits a very simple dependence (quadratic) on the ternary mixture composition. On the other hand this dependence is much more complex for the Co-II thiocyanate complex since even a full cubic mixture model shows a significant lack of fit at the 95% confidence level. These results indicate that the different possible symmetries of the Co-II complexes are more sensitive to the ternary mixture composition than are the multistep Fe-III complex equilibria.