A Semi-Empirical Study on Metal Ion/Murexide Complexation

Computational chemistry has recently introduced the improved reaction field theory and now is capable of evaluating the bulk solvent effect quite accurately. It is time to expand this capability to mixed solvents, as it is often extremely useful through is still not well understood. For example, the determination of rates of complexations of divalent metal ions with a tridentate metal-ion indicator dye murexide (Figure 1) in water are greatly affected by the addition of glycerol. This empirical technique of using the mixed solvent water-glycerol in the determination of metal ion/ligand complexation rate constants has been used extensively. The kinetics of the complex formation have been explained in terms of an exchange of solvent molecules from the inner-coordination sphere of metal ions with the ligand in the outer sphere. It is not, however, clear how this exchange changes when mixed solvents are used. We undertook a preliminary study of the atomistic details of the ligand-solvent exchange equilibrium between the inner and outer coordination spheres of the metal ion by using a semi-empirical approach with the purpose of gaining guidelines to model the coordination sphere of solvation. Computational chemistry offers the opportunity to explain preferential solvation in terms of energetics.