Figure 6. CD spectra of Co(II)-deprotonated hydroxy acid anion complexes with similar configurations on a and ß carbons (multipliers to nominal AA scale in parentheses): (a) a-D-saccharine (XO.OIO); (b) -D-glucoheptonate (XO.OIO); (c) D-arabonate (XO.OIO); (d) D-mannonate (X0.004); (e) L-erythronate (X0.020).certain. Neither the absorption spectra nor the CD spectra for these complexes are like those for the cobalt(II)-dipeptide systems recorded for pH 9 and above.16'17

No abstract

No abstract

The chloride salt of the protonated form of the p-amido-p-oxalato-bis[tetra-amminecobalt(lll)] complex, has been prepared and characterized. The perchlorate salt has also been prepared and it would appear that the oxalate bridge is only 50% protonated in this solid. From spectrophotometric measurements on the complex in acidic solutions a t 15-45', p = 2 . 0 ~ (LiCIO,), values of the protonation constant K = 18 f 2 I mol-l have been determined. Reduction with chromium(l1). 2Cr2+ + (COIII)~-2Coz+ + (Cr111)2, proceeds in two stages, each involving attack of the chromium(1l) and reduction of cobalt(iii). Both protonated (k,) and unprotonated (kb) forms participate in the first stage of the reaction (kl), and the rate law (CP+ in large excess) is as in (i) At 25" and p = 2 . 0 ~ (LiCIOJ. k, = 1 a45 f 0.07 I mol-l s-l, and kb = 35.2 f 0.4 I mol-1 s-l. The second stage of the reduction is independent of [H+] as in (ii) = k2 --1 d(ln[CoI1I . CrIII]) [Cra+] dr (ii) and k2 = 1 -88 f 0.05 I mol-1 s -l at 25". Information regarding the spectrum of the intermediate and final product has been obtained. The reduction with V8+ also proceeds in two stages, the first of which is directly measurable. The rate law observed is of the same form as (i). and again both the protonated (k,) and unprotonated (kd) forms are reactive. At 25" and p = 2 . 0 ~ (LiCIO,), k, = 3.51 f 0.04 I mol-l s -l and kd = 22.7 f 0.4 I mol-1 s-l. Activation parameters have been obtained for k,, kb, kz, kc, and kd. KINETIC studies on the chromium(I1) and vanadium(I1) reductions of binuclear cobalt (111) complexes (NHJ4Co*-V(NH,,X)*CO(NH,),~-~ with Xn-= OH-(ref. l), SOq2-(refs. 2 and 3), Se042-(ref. 2 and 3), and HP0,2-(ref. 4)have been reported previously. The Cr2+ reductions with Xn-= S042-and HP042proceed via binuclear cobalt (III)-chromium(III) intermediates, the final product being a dichromium(II1) species. A similar reaction path has also been invoked with SeOq2as the bridging ligand.The corresponding V2+ reductions yield kinetic data which suggest that the rate determining process is not electron transfer, but is connected with substitution into the V2+ co-ordination sphere. With Xn-= OHan outer-

which subsequently decomposes, as reported el~ewhere.~ Plots of log (ODt -OD,) against time were linear to only ca. 10% reaction, values of OD, being calculated on the assumption that the p-amido-p-hydroxo-complex is formed. Rate constants k, (Table 5), obtained from these plots, are seen to fit a dependence of the form (2). -At 50 "C, a = K, = a + b[H+] (5-9 f 1.7) x lO-'s-landb = (9-4 & 1.2) x 10-71mol-1s-1, (2) [2/440 Received, 28th February, 19721 Alternative p.ocedure.