Ion-pair formation in the outer-sphere electron transfer reactions between tris-(1, 10) phenanthroline iron(II) and the halopentacyanocobaltate(III) complexes in aqueous acidic solutions

摘要

The kinetics of the reactions between Fe(phen)_3~(2+) [phen = tris-(1,10) phenanthroline] and Co(CN)5X~(3-) (X = Cl, Br or I) have been investigated in aqueous acidic solutions at I = 0.1 mol dm~(-3) (NaCl/HCl). The reactions were carried out at a fixed acid concentration ([H~+] = 0.01 mol dm~(-3)) and the second-order rate constants for the reactions at 25 °C were within the range of (0.151-1.117) dm~3 mol~(-1) s~(-1). Ion-pair constants K_(ip) for these reactions, taking into consideration the protonation of the cobalt complexes, were 5.19 x10~4, 3.00 x 10~2 and 4.02 x 10~4 mol~(-1) dm~(-3) for X = Cl, Br and I, respectively. Activation parameters measured for these systems were as follows: ΔH~* (kJ K~(-1) mol~(-1)) = 94.3 ± 0.6, 97.3 ± 1.0 and 109.1 ± 0.4; ΔS~* (J K~(-1)) = 69.1 ± 1.9, 74.9 ± 3.2 and 112.3 ± 1.3; ΔG~* (kJ) = 73.7 ± 0.6, 75.0 ± 1.0 and 75.7 ± 0.4; E_a (kJ) = 96.9 ± 0.3, 99.8 ± 0.4, and 122.9 ± 0.3; A (dm~3 mol~(-1) s~(-1)) = (7.079 ± 0.035) x 10~(16), (1.413 ± 0.011) x 10~(17), and (9.772 ± 0.027) x 10~(20) for X = Cl, Br, and I respectively. An outer - sphere mechanism is proposed for all the reactions.