Electron Transfer Reactions Between Peroxodisulfate-and Iron(II) and Nickel(II)-oxime-imine Complexes--a Kinetic Study

The kinetics of the oxidation of complexes, [Fe(H 2 L) 2+ and [Ni(HL')] 2+ (H 2 L u = u 3,14-dimethyl-4,7,10,13-tetraazadeca-3,13-diene-2,5-dione-dioxime, (I)) and HL'=15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-ene-2-one-oxime, (II)) by peroxodisulfate ion was studied spectrophotometrically under pseudo-first-order conditions keeping the complexes as minor components, within the pH range 3.0-7.93 at I =0.20 u mol u dm m 3 (NaClO 4 ) for the former and I =0.25 u mol u dm m 3 for the latter complex at 303 u K. At a particular pH, plots of k obs vs. [S 2 O 8 2 m ] yield ascending curves with decreasing slope values for both the reactions, and the general rate law: d/d t [complex]=2 kQ [S 2 O 8 2 m ]/(1+ Q [S 2 O 8 2 m ]) is followed. A pre-equilibrium ion-pair formation between the reactants prior to electron transfer is proposed. A plot of m [log( k 12 / W 12 ) m 1/2 u log u k 22 ] vs. 1/2 u log( K 12 f 12 ) for such reactions yields a straight line with slope m (0.73 - 0.10) which deviates significantly from the ideal slope value of 1.0. On the other hand, for the direct electron transfer reactions (points 6-11 in Table II), such a plot is applicable. The reason for the large deviation might be due to the fact that intramolecular electron transfer occurs within the ion-pair, and in that event the Marcus equations might not be strictly valid.