Oxalato complexes, cobalt

Spectra of a variety of chromium compoimds in the +3 valence state are included in Fig. 16. The principal peak is centered at about 22-25 ev. in all cases. The CrjOa spectrum is almost identical to that of Mn02 of Fig. 5. Spectra of the oxalato complex and the ammonia complex are almost identical to spectra of the corresponding cobalt compounds of Figs. 13 and 11. 

Reaction of Cytochrome cIinn with Bis(ferrozine)copper(II) Knowledge of the redox properties of cytochrome c was an encouragement to initiate a kinetics investigation of the reduction of an unusual copper(II) complex species by cyt c11. Ferrozine (5,6-bis(4-sulphonatophenyl)-3-(2-pyridyl)-1,2.4-triazine)286 (see Scheme 7.1), a ligand that had come to prominence as a sensitive spectrophotometric probe for the presence of aqua-Fe(II),19c,287 forms a bis complex with Cu(II) that is square pyramidal, with a water molecule in a fifth axial position, whereas the bis-ferrozine complex of Cu(I) is tetrahedral.286 These geometries are based primarily upon analysis of the UV/visible spectrum. Both complexes are anionic, as for the tris-oxalato complex of cobalt in reaction with cytochrome c (Section 7.3.3.4), the expectation is that the two partners will bind sufficiently strongly in the precursor complex to allow separation of the precursor formation constant from the electron transfer rate constant, from the empirical kinetic data. 

Hexamminecobalt(III) salt cannot be used as a precipitant in the oxalato complex precipitation system because it precipitates as hexamminecobalt(III) oxalate. Besides the hexaureachro-mium(III) salt, hexamminechromium(III), tris(ethylenediamine)cobalt (III) or tris(trimethylenediamine)cobalt(III) salts can be used as precipitants. Hexamminechromium(III) and tris(ethylenediamine) cobalt (III) salts form precipitates with actinide(IV) or (VI) oxalato complex ions, whereas tris(trimethylenediamine)co-balt(III) salt forms precipitates with Th(IV) or U(VI) oxalato complex ions leaving Pu(IV) ion in the supernatant solution.Therefore, this reagent plays the role of both a separating agent and a precipitant and is applicable for the separation of Pu(IV) ion from Th(IV) or U(VI) ion. 

The oxalate system is not useful for the separation of Cm-(III) ion from Am(VI) ion because Am(VI) ion is reduced by oxalate ion and its oxalato complex precipitate like that of U(VI) ion with cobalt(III) complex ion cannot be obtained. 

Chromium(m) and Cobalt(iii) Complexes.—Just as copper(ii) catalyses aquation of oxalato-complexes of chromium(iii) (cf. Section 3, ref. 117), so do a variety of cations catalyse isomerisation of fra 5-[Cr(oxalate)2(OH2)2] . Rate constants are a function of the charge, radius, and structure of the... 

Cerium(iv).— The acid-promoted redox decomposition and cerium(iv) oxidation of the tris(oxalato)cobaltate(in) ion have been studied in aqueous acid media. In IM sulphuric acid, in the absence of oxidant, there occurs an induction period prior to the internal redox decomposition of the anion. On addition of the cerium(iv), however, there results an increased rate of reduction of the cobalt(ni) centre in contrast to the behaviour of this oxidant to M(C20 ) complexes where M = Cr, Rh, or Ir. The induction in the add-catalysed decomposition is consistent with the formation of a unidentate oxalato-complex-ion which may be the main route towards the stepwise reduction to yield Co and COg. From spectral studies on the total expected absorbance values on mixing, it would appear that the cerium(iv) ion is involved in the pre-equilibrium formation of a dinuclear species which might undergo internal electron transfer with reduction to cerium(in). A possible mechanism in this system may then be written as shown in Scheme 5 (ox = C2O4 -). The variations in rate of the one-electron redox reactions of this type are dependent on the nature of the activated complex, which may differ from one metal centre to another in respect of the number of protons and sulphate anions incorporated. 

Multidentate Leaving Groups.—The hydrolysis of [Co(ox)a] - and of [Co(ox)2(OH2)2], which ultimately produces cobalt(n) and carbon dioxide, involves the formation of an intermediate containing a unidentate oxalate ligand previous to the rate-determining step. Free radical intermediates are thought unlikely in the decomposition of these oxalato-complexes, but malonate ion-radicals are thought to be intermediates both in the thermal and photochemical hydrolysis of the [Co(mal)3] anion. Kinetics are reported for a third example of these aquation-redox processes, [Co(acac)2] in acidic solution. ... 

The rapid rates of reduction of the oxalato (10) (k = 450 + 1,000 (H+)) and of the pyruvate (2) complexes (2A x 103at 25°C. and (H+) = 0.1) can hardly be understood as caused by chelation. Binoxalate does not chelate unless the proton is lost, and the rate law for the reduction of the complex shows that it brings a proton into the activated complex. Pyruvate almost certainly is not chelated in the product. Both groups are rapidly reduced by Craq.+2 when they are feee from the cobalt center. (The reduction of H2C2O4 by Craq+2 was explored by R. Milburn and the present author (29). The observations on pyruvate were made by R. Butler (2)). The complexes of pyridine-2-carboxylate and pyridine-4-carboxylate are rapidly reduced by Cr+2 at least in the forms which present the nitrogen without associated protons. Radical ion intermediates for these structures are not unreasonable. In fact, a stable free radical derived from AT-ethyl-4-carbethoxypyridinyl has been... 

After the resolution of 1-2-chloro-ammino-diethylenediamino-cobaltie chloride many analogous resolutions of optically active compounds of octahedral symmetry were carried out, and active isomers of substances containing central cobalt, chromium, platinum, rhodium, iron atoms are known. The asymmetry is not confined to ammines alone, but is found in salts of complex type for example, potassium tri-oxalato-chromium, [Cr(Ca04)3]K3, exists in two optically active forms. These forms were separated by Werner2 by means of the base strychnine. More than forty series of compounds possessing octahedral symmetry have been proved to exist in optically active forms, so that the spatial configuration for co-ordination number six is firmly established. 

It must be concluded, therefore, that the ligands do not become completely detached from the metal ion in isomerization reactions. Comparable results have been observed in the isomerization95 of potassium diaquodioxalatochromium(III) and the racemization96 of optically active potassium tris(oxalato)chromium(III) when no exchange with free ligand in solution occurs. Thus, although it is not practicable to take advantage of the desirable properties of individual isomers of 2 1 chromium and cobalt complexes of tridentate azo compounds because of the facility with which such compounds isomerize in solution, the technically important unsymmetrical 2 1 complexes are capable of practical application because they show little or no tendency to disproportionate in solution. 

The catalytic action of CDTA complexes of Fe(III), Ni(II), Cu(II), Cr(in), and Mn(II) on the oxidation of L-ascorbic acid with tris(oxalato)cobaltate has been investigated. The rate is proportional to the concentration of the complex. Fe(III)-CDTA is the best catalyst for the reaction.76... 

Reaction of Cytochrome cimu with Tris(oxalato)cobalt(III) The cytochrome c protein was also used as reductant in a study of the redox reaction with tris (oxalato)cobalt(III).284 Selection of the anionic cobalt(III) species, [Conl(ox)3]3 was prompted, in part, because it was surmised that it would form a sufficiently stable precursor complex with the positively charged cyt c so that the equilibrium constant for precursor complex formation (K) would be of a magnitude that would permit it to be separated in the kinetic analysis of an intermolecular electron transfer process from the actual electron transfer kinetic step (kET).2S5 The reaction scheme for oxidation of cyt c11 may be outlined ... 

Because the optically active forms of the (ethylenediamine)bis-(oxalato)cobaltate(III) ion have been, and will continue to be, excellent resolving agents for many cationic cobalt(III) complexes, it is only proper that a detailed resolution procedure be developed to produce both enantiomorphic forms in useful quantities. 

The tris(oxalato)chroinate(III) ion, [Cr(Cj04)3P , possesses the double historical distinction of being both the first resolved complex anion and the first resolved complex that did not contain nitrogen. Since it racemizes rapidly in aqueous solution, more so than the corresponding cobalt(III) ion, virtually all of this labile complex can be separated as a single en-... 

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