Electrostriction and counter ion effects in an outer-sphere electron transfer reaction. Kinetics of the reduction of hexachloroiridate(IV) by iodide ion†

Abstract

The kinetics of the reaction between the hexachloroiridate() ion and iodide ion in aqueous acidic media has been monitored spectrophotometrically in the temperature range 20 to 40 ЊC, at ambient and at elevated pressures (up to 125 MPa). With suitable selection of reactant concentrations the reaction can be studied as simple pseudo first-order (iodide ion in excess) yielding a second-order rate constant of 1.42 × 10 3 M Ϫ1 s Ϫ1 at 25 ЊC and atmospheric pressure. This compares favourably with a value of 1.38 × 10 3 M Ϫ1 s Ϫ1 for the same parameter, determined previously under the same conditions. The reaction rate varies to a limited degree with variation in the conjugate base of the acids employed, which may be the result of a secondary medium effect. Potassium ions exert a significant catalytic effect, a finding that parallels those reported for other anion-anion redox reactions. The reaction is moderately sensitive to temperature: ∆H ‡ ranges from 18 to 34 kJ mol Ϫ1 depending on the reaction medium. All ∆S ‡ values are distinctly negative, suggesting a degree of molecular ordering and/or increasing electrostriction in the rate limiting step. Under all conditions employed pressure accelerates the reaction rate, resulting in large negative volumes of activation, typically Ϫ20 to Ϫ25 cm 3 mol Ϫ1 . How the thermal and activation parameters relate to the mechanism is discussed.

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

“…The values of the ion-pair constant K ip (dm )3 mol )1 ) obtained for the chloro, bromo and iodo complexes are 5.19 Â 10 4 , 3.00 Â 10 2 and 4.02 Â 10 4 , respectively. It is not unusual that these values are large, as larger values have been reported in the literature [7,8] for ion-pair formation constants of the reactions between IrCl 6 2) and I ) . Moreover, the large values of K ip are not surprising as larger values have been reported in the literature for moderately charged reacting species [7,8].…”

“…Ionpairing has since then been investigated between metal complexes by many workers [7,8]. The redox pairs Fe(Phen) 2þ 3 and Co(CN) 5 X 3À studied in this work are significantly oppositely charged and hence coulombic forces of attraction between the pair may facilitate ion-pair formation in the system.…”

Mechanistic Investigation of the Oxidation of Glyoxylic and Pyruvic Acids by Tris(biguanide)manganese(IV) in Weakly Acidic Aqueous Media

“…The estimated maximum value for the pre-equilibrium formation of a 1:1 adduct between Mn IV and RCOCOO Ϫ (R ϭ H for glyoxylate and CH 3 for pyruvate) lies between 1 and 10 and this does not confirm a strong inner-sphere attachment. However, we note that among the three s 2 metal-ion centres studied for the reduction of Mn IV , only the reaction of In I with [Mn(LH 2 ) 3 ] 4ϩ is subject to kinetic saturation for which an association constant of 6 ϫ 10 3  Ϫ1 for the formation of this adduct (believed to be due to strong hydrogen bonding) was estimated by Gould et al [5] The estimated low value for the formation of adducts in the present study may be due to electrostriction [15] or a process aided by hydrogen bonding [16] involving the reductants and the NH or NH 2 fragments of the ligand biguanide moiety of the title complex. The presence of a strong chelating ligand can be expected to stabilise Mn IV to a great extent and render the complex a mild oxidant.…”

Mechanistic Studies on the Oxidation of Hydrazine by Tris(biguanide)manganese(IV) in Aqueous Acidic Media

“…However, for both the pathways, k 1 and k 2 , a pre-equilibrium weak adduct formation through H-bonding involving NH or NH 2 fragments of the biguanide ligand with the reducing agent or any kind of it aided by electrostriction [21] [22] is possible. Such interactions were proposed for the reduction of this Mn IV complex with Fe 2 [2] [3], In [3], and a-keto acids [4].…”