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.
The kinetics of the redox reaction between [Et,N] [NO,] and [MoOC13(0PPh,),] have been studied in CH,CI2 solution at temperatures from 3.2 to 25 "C. This reaction proceeds in three observable steps when [NO,-] = .[Mov]. The first step involves co-ordination of the nitrate ion via an S , 1 (limiting) mechanism involving loss of a triphenylphosphine oxide molecule; at 25 "C, kobs. = 40 f 1 s-l and the corresponding activation parameters are AH$ = 9.7 f 0.5 kcal rn0l-l and ASS = -18.4 f 1.7 cal K -l mol-l. The data obtained for the second and third steps strongly suggest that they involve intramolecular substitution a t molybdenum(v) and intermolecular substitution at molybdenum(v1) centres, respectively. Rapid non-rate-determining electron transfer from molybdenum(v) to nitrate is proposed to occur between these two latter steps, ke.t. >, 1 s-1 at 25 "C, giving a dioxomolybdenum(vt) complex and nitrogen dioxide. The kinetic results imply that a specific orientation of the nitrato-group with respect to the oxomolybdenum(v) centre is necessary prior to rapid electron transfer.INTERACTION between a molybdenum(v) centre and a nitrate ion has been proposed to occur immediately prior to the final step in the electron-transfer sequence of the reduction path of the nitrate reductase enzymes.1.2 These enzymes, which require molybdenum both for their formation and a ~t i v i t y , ~ are responsible for the first step in nitrate assimilation and for nitrate respiration, converting nitrate to nitrite with the molybdenum apparently changing between the oxidation states MoV andAs part of a study of the chemistry of molybdenum related to that of the molybdenumcontaining enzymes, we report details of a kinetic investigation of the reaction between nitrate ions and the molybdenum(v) complex [MoOCl,(OPPh,),] .t
EXPERIMENTAL
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