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  • Kinetics of Iron(lI) Reduction of Bis(malonato)- & Bis ...nopr.niscair.res.in/bitstream/123456789/49074/1/IJCA 23A(12) 1001 … · The complex [Co(en)(C3Hz04hJ - may undergo reduction
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Indian Journal of Chemistry Vol. 23A, December 1984, pp. 1001-1003 Kinetics of Iron(lI) Reduction of Bis(malonato)- & Bis(oxalato )-ethylenediamineco baltates(lII) R VISWANATHAN & V R VIJAYARAGHAVAN* Department of Physical Chemistry, University of Madras, A.C. College Campus, Madras 600 025 Received 19 Marcb 1984; revised and accepted 13 June 1984 The kinetics' of Fe(Il) reduction of bis(malonato)- and bis(oxalato)-ethylenediamine>cobaltates(1I1) have been studied in aqueous perchlorate medium at J = 1.0mol dm -3 (LiCI0 4 ) and 30CC in the [H +] range (1.6-600) x 10 -3 moldm -3 The reductions are found to be second order, the rate constants being in the order [Co(en)(C 2 0 4 h]> [Co(enXC 3 H 2 0.h] -. While the rate of reduction of [Co(en)(C 2 0 4 )2] - is [H +] independent. that of [Co(en)(C 3 H 2 0 4 )2] - is accelerated by increase in [H +] in the range 0.02moldm -3 < [H +] <0.47moldm -3. The activation parameters have been determined forthe reduction of [Co(en)(C 3 H 2 0 4 hJ -, the values being; !!.H1: 74.7±2.2kJmol-'; !!.S': -81.4±4.SJK -1 mol :". An inner sphere mechanism is proposed involving a protonated monodentate malonato complex and an unprotonated species of the bismalonato complex. The reduction by iron(II), of cobalt(III) complexes with potentially bridging ligands takes place by an inner sphere mechanism I -6. The [H +] dependence of rates of these reactions indicates diversity, depending on the nature of complexes. While complexes like [Co(NH 2 .CH 2 .C0 2 )(en)2F+ (ref. 7), [CO(C 2 0 4 )(NH 3 )4r (ref. 8) and [Co(enh(C 2 0 4 )]+ (ref. 9) undergo reduction by Fe(If) without any noticeable dependence of their rates on [H "I, complexes like [Co(NH 3 )s(C 3 H 2 0 4 )]2 + (ref. 5) and [Co(NH 3 )s(C 2 0 4 )]2T (ref. 7) show an inverse dependence on [H +]. However, the rates of Fe(II) reduction of [Co(NH2·CH2.C02)iC204)] - (ref. 7), [Co(C 2 0 4 h] 3 - (ref. 9) and [Co(C 3 H 2 0 4 h] 3 - (ref. 10) show a linear dependence on [H +]. In some cases this linear acid dependence, when small, has been shown to be due to medium effect ll . 12 . The effect of [H+] on the rates of the Fe(Il) reduction of cobaloximes has been studied extensively in this laboratory13 -15 and the title investigation is in continuation of our these studies. Materials and Methods d i-Sodi urn bis(malona to )ethylenediamine- cobaltate (III) monohydrate, NaCo(C 2 H s N 2 ): (C3H204h·H20 and di-sodium bistoxalato)- ethylenediaminecobaltate (III) monohydrate, NaCo(C2HsN2)(C3H204h·H20 were prepared by the literature method 16 and their purity was checked by UV-visible, IR, conductivity measure- ments and elemental analyses. Iron(I1) perchlorate solution was prepared by dissolving 99.9% pure iron powder (Electrolytic Grade, Sarabhai M Chemicals) in a slight excess of '" 2MHCIO 4 in the presence of nitrogen and was stored under nitrogen. The concentrations of Fe(II) and the free [H +] in this solution were determined by literature methods'P'!". Lithium perchlorate was prepared by the neutrali- sation of standard perchloric acid solution with anhydrous lithium carbonate followed by re- crystallisatiorr'". Perchloric acid (E Merck, reagent grade) was used as such. Doubly distilled water was used throughout the kinetic study. Kinetic measurements Pseudo-first order conditions were maintained in all kinetic runs with 20-fold excess of Fe(CI0 4 h Solutions of the complex and Fe(II) were thermo stated separately and mixed in a thermo stated spectropho- tometric cell. The reactions were followed spectropho- tometrically at 540 nm for both the complexes using a Carl Zeiss UV-vis spectrophotometer. Nitrogen atmosphere was maintained using serum caps to seal the cells. Rate constants were calculated from the slopes of the plots of [log(Ao-Aoo)-log(A,-Aoo)] versus time. All the reactions were studied upto at least three half-lives and the plots were linear. The reactions were studied in perchloric acid medium and the ionic strength was adjusted to 1.0mol dm -3 with lithium perchlorate. For the determination of activation parameters, the reaction was studied in the temperature range 30-70°C. The reactions at 0.0016 < [H+] <0.0063 mol dm -3 were carried out in glycine buffer. Stoichiometry The stoichiometry of the reaction was determined by estimating the amounts of Fe(III) and Co(II) present in the reaction mixture at the end of the reaction. Iron(III) was determined spectrophotometricady!? and cobalt(II) as CoCli - in the presence of excess of hydrochloric acid. The ratio of Fe(III) to Co(II) was 1001

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  • Indian Journal of ChemistryVol. 23A, December 1984, pp. 1001-1003

    Kinetics of Iron(lI) Reduction of Bis(malonato)- &Bis(oxalato )-ethylenediamineco baltates(lII)

    R VISWANATHAN & V R VIJAYARAGHAVAN*Department of Physical Chemistry, University of Madras, A.C. College Campus, Madras 600 025

    Received 19 Marcb 1984; revised and accepted 13 June 1984

    The kinetics' of Fe(Il) reduction of bis(malonato)- and bis(oxalato)-ethylenediamine>cobaltates(1I1) have been studied inaqueous perchlorate medium at J = 1.0mol dm -3 (LiCI04) and 30CC in the [H +] range (1.6-600) x 10-3 moldm -3 Thereductions are found to be second order, the rate constants being in the order [Co(en)(C204h]> [Co(enXC3H20.h] -. Whilethe rate of reduction of [Co(en)(C204)2] - is [H +] independent. that of [Co(en)(C3H204)2] - is accelerated by increase in[H +] in the range 0.02moldm -3 < [H +]

  • INDIAN J. CHEM., VOL. 23A, DECEMBER 1984

    found to be 1:1 confirming the stoichiometry as 1:1(see Eq.l).

    The complex [Co(en)(C3Hz04hJ - may undergoreduction in accordance with Eq.(l)[Co(en)(C3Hz04hJ -+Fe2+ +4H+-+

    Co 2+ +2C3H404 + Fe3+ +2(en) ... (1)

    ResultsTable 1presents the second order rate constants (k2)

    for the Fe(II) reduction of [Co(en)(C3H204hJ - and[Co(en)(C204hJ - at 1=1.0mol dm -3 in the range0.0016< [H+J

  • VISWANATHAN & VIJA YARAGHA VAN: KINETICS OF REDUCTION OF [Co(en)(C3H204),] & [Co(en)(C,04),]

    observation and the second order kinetics suggest thetransition state'? of the type(O

    The higher reactivity of the protonated form may bedue to a rapid 'ene-ended' dissociation of malonate,assisted by the proton, facilitating chelation to Fe(II) inthe transition state. An approximately ten-foldincrease in k2 values with [H +] in the range 1.6 x 10-3


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