Chromium(II) and vanadium(II) reductions of hexa-aquocobalt(III) in aqueous perchlorate media

Abstract

A full kinetic study has been made of the reaction of Cr2+ with hexa-aquocobalt(lll), [H+] = 0.10-3.0~. p = 3 . 0 ~ (LiCIO,). The rate law is of the form -d[CoI1IJ/dr = (k, + ka[H+]-l) [Cr2+] [ColIJ] and a t 25 "C : kl = 1-25 x lo* I mol-l s-l, AH1% = 9.5 f 0.4 kcal mol-l. AS,% = -7.8 f 1.5 e.u.; k, = 6.59 x lo3 s-l, AH,$ = 12.7 f 0.6 kcal mol-l, AS,$ = 1.7 f 2-0 e.u. Chloride ions catalyse the reaction and the products CrCI2+ and CP+ were determined quantitatively after ion-exchange separation. The formation of CoCI2+ proceeds relatively slowly and it was possible to identify two chloride-dependent terms , k3[Cr2+] [CoCI2+] and k,[Cr2+] [Co3+] [CI-I.The reaction of V2+ with hexa-aquocobalt(ti1) was too fast to study directly. Competition studies indicated a rate constant kv ca. 8.8 x lo6 I mol-1 s-l a t 25 "C and p = 3 . 0 ~ (HCIO,).VARIOUS aspects of the solution chemistry of hexaaquocobalt (111) [hereafter cobalt (III)] have been considered in a recent review1 and much information is available for redox processes in which cobalt(II1) is the ~x i d a n t . ~~~ In a recent study,, the reactions with manganese(I1) and iron( 11) have been re-investigated with ionic strength p = 3 . 0 ~ (LiClO,) and p = 3. 0111 (NaC10,). Despite the extensive information available no studies have yet been made of the chromium(I1) and vanadium-(11) reductions of cobalt (111)-Both reactions have very favourable free-energy changes, and the differences in standard reduction potentials for the relevant couples are 2-25 V (Co2+/Co3+ and Cr2+/Cr3+) and 2-10 V (Co2+/Co3+ and V2+/V3+) .5 Studies on both these reactions are now reported. RESULTSStoicheiornetry .-Preliminary experiments confirmed that both reactions are rapid and that the stopped-flow technique was required. The initial products of the oxidations are chromium(n1) and vanadium(II1) respectively. Subsequent oxidations of the chromium (111) to chromium (VI) ,6 and vanadium(II1) to vanadium(1v) are slow. Since the concentration of reductant used was always in excess of that of the oxidant, the reactions were assumed to be as in equations (1) and (2) respectively. Kinetic data are consistent with 1 : 1 stoicheiometries.

Studies of ammineaquocobalt(III) chemistry. Kinetics and mechanism of some electron-transfer reactions of cis-diammineaquocobalt(III) in acid perchlorate solution

“…The mechanism shown by eq 9-11 is consistent with the fast Co3+aq * -CoOH2+aq + H* Kh (9) fc0 Co3+aq + Red -+ Co2+aq + Ox (10) CoOHJ+aq + Red ^4 Co2+ + Ox (11) observed rate law for the one-electron reactions 1,2,4, and Under these conditions A = ko and B = k\Kh for reactions the stoichiometric and kinetic data for reactions 3 and 6.…”

Computational Investigation and Design of Cobalt Aqua Complexes for Homogeneous Water Oxidation

“…It is known that DFT can fail to correctly predict the ground state spin state , and energies between different spin configurations depend, among others, on the exchange–correlation functional employed, especially on the amount of exact exchange. − Moreover, no relativistic effects such as spin–orbit coupling are considered in the calculations. Nevertheless, a d 7 high spin configuration is found to have the lowest free energy for the initial state S 0 whereas the low spin case is significantly higher in free energy (see Supporting Information Table S1 for spin populations and free energy differences (Δ G ) for the various structures of each state of the catalytic cycle) . This was found for all exchange–correlation density functionals used (see Supporting Information Table S3).…”

Theory of electron transfer in donor–acceptor pairs

“…6 and 7 the theory of electron transfer must take into account the electronic and vibrational characteristics of the donor and acceptor. This conclusion follows from the analysis of experimental data [8]. Evaluation of the contribution of such characteristics is the aim of this paper.…”