Heptavalent oxidation states

The oxidation-reduction behavior of plutonium is described by the redox potentials shown in Table I. (For the purposes of this paper, the unstable and environmentally unimportant heptavalent oxidation state will be ignored.) These values are of a high degree of accuracy, but are valid only for the media in which they are measured. In more strongly complexing media, the potentials will change. In weakly complexing media such as 1 M HClOq, all of the couples have potentials very nearly the same as a result, ionic plutonium in such solutions tends to disproportionate. Plutonium is unique in its ability to exist in all four oxidation states simultaneously in the same solution. Its behavior is in contrast to that of uranium, which is commonly present in aqueous media as the uranyl(VI) ion, and the transplutonium actinide elements, which normally occur in solution as trlvalent... 

Technetium compounds of the oxidation states 0 to -1-7 have hitherto been isolated, pertechnetates being the most stable. Also, compounds of quadrivalent technetium are fairly stable. Oxidation states lower than -1-4 can easily be oxidized to the quadrivalent or heptavalent state, whereas penta- and hexavalent technetium readily disproportionate into the hepta- and quadrivalent states. 

Rhenium has a 5d56. electron configuration and all oxidation states from 0 to 7+ are known, although the heptavalent is the most stable state. 

The solubilities of heptavalent Np and Pu in alkaline solutions are significantly higher than those of these hydroxides in the (III)-(VI) oxidation states. Thus, real IM NaOH solutions containing 5 10 M of Np(VII) or Pu(Vll), and 2M LiOH solutions containing 0.2M of Np(VII) were obtained [88]. 

In aqueous solutions neptunium exists in the five oxidation states Np(III), Np(IV), Np(V), Np(VI), and Np(VII), although the heptavalent Np(Vn) is stable only in alkaline solutions. In the absence of complexing agents the first four oxidation states exist as Np, Np, NpOj, and NpOj , usually in the hydrated form, whereas in strong alkaline solutions the heptavalent state isNp05 [K2]. 

With the metals of the first transition series, the maximum coordination number of higher oxidation states is six, and this is so firmly fixed that in their fluoride complexes the oxidation state of the metal in question can be fixed by controlling the mol fraction of alkali metal present 26). Thus, the fluorination of a vanadium salt in the presence of a one, two, or three mol ratio of potassium ion, yields KV F6, KgWVFe, or KsV Fe. The same tendency is shown, but to a lesser degree, by metals of the second transition series, as exemplified by KRuFe and KgRuFe. For an unusual example in the third series, note that OsFe is known, OsFt is not stable, but heptavalent osmium is found as six-coordinated OsOFs (27). 

All of the isotopes of technetium are radioactive. The longest-lived isotope is Tc with a half-life of 2.6 million years. Technetium has six oxidation states from divalent to heptavalent, with the first three forming cations. However, hydrolysis data are only available for tetravalent technetium that exists in solution as the oxo-cation TcO. ... 

Ballestra et al. [32] described a radiochemical measurement for determination of "technetium in rain, river, and seawater, which involved reduction to technetium (IV), followed by iron hydroxide precipitation and oxidation to the heptavalent state. Technetium (VII) was extracted with xylene and electrode-posited in sodium hydroxide solution. The radiochemical yield was determined by gamma counting on an anticoincidence shield GM-gas flow counter. The radiochemical yield of 50 to 150 litre water samples was 20-60%. 

Satisfactory separation of pertechnetate from perrhenate can be achieved by reducing Tc (VII) with hydrochloric acid and co-precipitating technetium with Fe(OH)j Technetium can then be oxidized to the heptavalent state by oxidizing the precipitate in cone, nitric acid and precipitating Fe(OH)j with ammonia. 

Only one reaction was reported in which FClOg was oxidized from the penta- to the heptavalent state (49, 52, 69). The powerful oxidizer PtFg was required to obtain the following reaction ... 

Tc is a more complex isotope that is normally found in a quadrivalent state, but tends to oxidize to its heptavalent state as pertechnetate, which is easily leachable. As seen from Fig. 17.2, its oxidation potential in acidic and neutral environments is small hence. 

Heptavalent neptunium and plutonium can be prepared in highly alkaline aqueous media via electrochemical or chemical oxidation of An, Arr, or An species, with Np being more easily obtained and isolated than Pu. The complexes formed have been characterized in solution primarily by optical absorbance and vibrational spectroscopy, and more recently by NMR and EXAFS, and in the solid state by EXAFS and X-ray diffraction. Most research in this area was conducted at the Russian Academy of Sciences. 

Green solutions believed to be Am are prepared by oxidation of Am in concentrated aqueous basic solution by either ozone or the O radical. In contrast to Np, and similar to Pu, Am is unstable and reduces to the hexavalent state within minutes. A review on the chemistry of heptavalent transplutonium elements can be found in the Handbook of the Physics and Chemistry of the Actinides. ... 

The heptavalent state Pu(VII) is observed when Pu(VI) in a basic solution is oxidized by ozone, persulfate, or electrolytically. The resulting Pu02 is highly unstable and readily reverts to Pu(VI) in a less oxidizing environment or in acidic solutions. 

The kinetics of oxidation of bivalent manganese to the heptavalent state by peroxomonophosphoric acid has been investigated/ " In perchloric acid, the reaction rate is negligible without the addition of an appropriate catalyst. Trace quantities of Co(II) increase the observed reaction rate significantly the resulting rate law and mechanism are shown in equations (46)-(53). The oxidation was... 

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