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

Abstract

The kinetics of the reactions between Fe(phen) 3 2+ [phen = tris-(1,10) phenanthroline] and Co(CN) 5 X 3À (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 Â 10 4 , 3.00 Â 10 2 and 4.02 Â 10 4 mol )1 dm )3 for X = Cl, Br and I, respectively. Activation parameters measured for these systems were as follows: DH* (kJ K )1 mol )1 ) = 94.3 ± 0.6, 97.3 ± 1.0 and 109.1 ± 0.4; DS* (J K )1 ) = 69.1 ± 1.9, 74.9 ± 3.2 and 112.3 ± 1.3; DG* (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) Â 10 16 , (1.413 ± 0.011) Â 10 17 , and (9.772 ± 0.027) Â 10 20 for X = Cl, Br, and I respectively. An outer -sphere mechanism is proposed for all the reactions.

Intermediate and ion-pair formation in the outer-sphere reactions between azido-pentacyanocobaltate(III) and iron(II) polypyridyl complexes in aqueous medium

“…]. The higher reactivity of the protonated cobalt complex is in conformity with our study on similar systems [7,32]. The constant k c is the forward rate constant for Eq.…”

“…In our earlier papers [6,7], we reported the formation of ion-pairs in the reactions of Co(CN) 5 X 3-(X = Cl, Br, I) with Fe(phen) 3 2? in aqueous acidic medium and the protonation of these halo-pentacyanocobaltate(III) complexes.…”

“…in aqueous acidic medium and the protonation of these halo-pentacyanocobaltate(III) complexes. So far, our investigations on the reactions of these halo-pentacyanocobaltate(III) complexes have presented very interesting results [6,7] because of the relatively high protonation constants and ion-pairing constants of the cobalt complexes with the polypyridyl iron(II) complexes. The chemical reactions of the pentaamine complex, Co(NH 3 ) 5 N 3 2?…”