Protonation of chloro-, bromo- and iodo-pentacyanocobaltate(III) complexes

The reactions between Fe(Phen)(3)(2+) [phen= tris-( 1,10) phenanthroline] and Co( CN)(5)X3- (X= Cl, Br or I) have been studied in aqueous acidic solutions at 25 degrees C and ionic strength in the range I= 0.001 - 0.02 mol dm(-3) (NaCl/HCl). Plots of k(2) versus root I, applying Debye - Huckel Theory, gave the values -1.79 +/- 0.18, -1.65 +/- 0.18 and 1.81 +/- 0.10 as the product of charges (Z(A)Z(B)) for the reactions of Fe(Phen)(3)(2+) with the chloro-, bromo- and iodo-complexes respectively. Z(A)Z(B) of approximate to - 2 suggests that the charge on these Co-III complexes cannot be -3 but is -1. This suggests the possibility of protonation of these Co-III complexes. Protonation was investigated over the range [H+]= 0.0001 -0.06 mol dm(-3) and the protonation constants K-a obtained are 1.22 x 10(3), 7.31 x 10(3) and 9.90 x 10(2) dm(6) mol(-3) for X= Cl, Br and I, respectively.