Acid catalyzed dissociation of 4-aminopyridine complex of pentacyanoferrate (II)

Fe(CN)(5)ampy(3-) (ampy = 4-aminopyridine) complex was unstable at pH less than or equal to 4 with respect to the dissociation due to the protonations of the complex. The first protonation on the 4-aminopyridine ligand at pK( a) = 3.2 resulted in a congruent to 150-fold increase in rate of dissociati on as compared to that of the complex in the neutral solution. However, fur ther protonation of the complex on the cyanide ligand at pK(a) = 1.5 increa sed the rate only slightly. The large increase in the rate constant of diss ociation for Fe(CN)(5)ampyH(2-) complex suggests that in the transition sta te the proton is likely shifted from the amino group and becomes coordinate d to a pair of d pi electrons directed at the face of the coordinated octah edron.