The formation of the complex pentacyano(3-pyrazincarboxylate)ferrate(II) in various water-cosolvent mixtures

This article presents the kinetic of formation of pentacyano(3-pyrazincarboxylate)ferrate(II) from pyrazincarboxylate and pentacyanoaquoferrate(II) ions in various isodielectric water-cosolvent mixtures at 298 K. The rate law is in the form dFe(CN)₅(3-pzCO₂)^4?]/dt = ??_fFe(CN)₅H₂O^?3] pzCO₂^?]. Plots of log(??_f/dm³ mol^?1 s^?1) vs. D (where D_m is the bulk dielectric constant of the medium) and log(??_f/dm³ mol^?1 s^?1) vs. the Grunwald-Winstein parameter are nonlinear for some of the mixtures and to each mixture corresponds a different behavior in respect to the above parameters. The plots of log(??_f/dm³ mol^?1 s^?1) vs. the mol fraction of water are straight lines over the entire composition range studied, except for the water-methanol mixture. It is evident that the solvation phenomenon plays a dominant role and that the rate of formation is mediated by the dual solvent vectors, the overall basicity and acidity of the solvent mixtures.