The preparation and characterization of the µ-amido-µ-oxalato-bis[tetra-amminecobalt(III)] complex, and the kinetics of reduction with Cr²⁺and V²⁺

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-

A Structural and Functional Model of Dinuclear Metallophosphatases

“…This parameter first has to be normalized before it is used to estimate the degree of bond cleavage at the transition state and choose between the two mechanisms. In these complexes ( 5 ), we estimate that the phosphate monoester coordinated to the Co(III) centers resembles an uncoordinated monoester monoanion; that is, the two Co(III) centers have approximately the same effect as a single proton . For equilibrium phenol transfer in monoanionic phosphate esters, β eq = 1.74, indicating that the aryl oxygen has a partial charge of +0.74 in the starting material (Scheme a).…”

.mu.-Oxalato-cobalt(III) complexes

“…The formation and spectrophotometric identification of a cobalt(III)chromium(III) binuclear intermediate is also evidence for an inner-sphere mechanism in the case of the Cr2+ reduction of (a). 13 It is noteworthy that although rate constants for the reduction of the tetranuclear complexes are slow, the reactions are appreciably faster than the Cr2+ reduction of Co(NH3)6…”

“…The chloride salt of the protonated form of (a), [(NH3)4Co^(NH2,C204H)-Co(NH3)4] Cl4 -H20, was prepared from [(NH3)4Co-M(NH2,Cl)-Co(NH3)4]Cl4-4.5H20 and then converted into the perchlorate salt, which is half-protonated, by the procedure already described. 13 The unprotonated form of (a) was obtained as the bromide salt by neutralizing a solution of the perchlorate salt with 0.1 M NaOH and then adding a saturated solution of NaBr.…”

Kinetics and mechanism of the amine exchange reactions of 2‐aminopyridine derived Schiff‐base ligands and their copper(II) complexes

“…If the formation of the intermediate klo were to be the ratedetermining step, a first-order behavior on [R'NH2] and either single-stage or a two-stage kinetics would be expected. On the other hand a mechanism involving a rapid preequilibrium to form (C) followed by a rate-limiting elimination k l l of 2 mol of 2-aminopyridine is entirely consistent with the rate law (6) and (9). Further if the rate-determining step were to involve a bimolecular attack of R'NHB on the azomethine carbon, the coordination of -CH=N-to Cu(I1) will be expected to favor rather than disfavor the amine exchange reaction.…”

“…By carrying out a consecutive reaction treatment by standard methods [9], the rate constants for the first and the second stages were obtained. The pseudo-first-order rate constants for the first and second steps are listed in Table IV as k i b , and k&, respectively.…”