Fractional oxidation state salts

It has long been known that sulfur, selenium, and tellurium will dissolve in oleums to give blue, green, and red solutions, respectively, which are unstable and change in color when kept or warmed. The colored species are cyclic polycations in which the element is formally in a fractional oxidation state. It is difficult to isolate solids from oleum solutions, and crystalline salts can be much more easily obtained by selective oxidations of the elements with SbF5 or AsF5 in liquid HF or S02,7 or with 82( 2 in HSO3F. 

A large number of C [M(dmit)2] complexes (n = 2, 1, 0) have been prepared. " The dianion salts [R4N]2[M(dmit)2] can be obtained by treating a solution of Na2dmit in methanol with the appropriate M metal salt and the appropriate tetraalkylammonium bromide (Figure 4.18). The corresponding monoanionic salt [R4N][M(dmit)2] is obtained from the dianionic salt by iodine oxidation. Further oxidation may lead to the neutral complex or to fractional oxidation state compounds (FOSCs) or donor-acceptor (DA) compounds. These oxidation steps are described in Section 4.3.2. 

The conductivity of one-dimensional metal complexes has been reviewed. The influence of structure is emphasized, as are the interesting structural changes which occur when the fractional oxidation state is varied. Measurements of dielectric relaxation frequency have been used to obtain ac and dc conductivities, the latter of which lead to the rate of hopping ( site-transfer ) conductivity. In the double salt K3(Mn04)2, these data give the rate of the outer-sphere electron transfer reaction (35). A... 

Originally, general methods of separation were based on small differences in the solubilities of their salts, for examples the nitrates, and a laborious series of fractional crystallisations had to be carried out to obtain the pure salts. In a few cases, individual lanthanides could be separated because they yielded oxidation states other than three. Thus the commonest lanthanide, cerium, exhibits oxidation states of h-3 and -t-4 hence oxidation of a mixture of lanthanide salts in alkaline solution with chlorine yields the soluble chlorates(I) of all the -1-3 lanthanides (which are not oxidised) but gives a precipitate of cerium(IV) hydroxide, Ce(OH)4, since this is too weak a base to form a chlorate(I). In some cases also, preferential reduction to the metal by sodium amalgam could be used to separate out individual lanthanides. 

The classical methods used to separate the lanthanides from aqueous solutions depended on (i) differences in basicity, the less-basic hydroxides of the heavy lanthanides precipitating before those of the lighter ones on gradual addition of alkali (ii) differences in solubility of salts such as oxalates, double sulfates, and double nitrates and (iii) conversion, if possible, to an oxidation state other than -1-3, e g. Ce(IV), Eu(II). This latter process provided the cleanest method but was only occasionally applicable. Methods (i) and (ii) required much repetition to be effective, and fractional recrystallizations were sometimes repeated thousands of times. (In 1911 the American C. James performed 15 000 recrystallizations in order to obtain pure thulium bromate). 

In fact, one of the peculiar properties of the title class of compounds is the ability of the molecular entity to carry a charge which can vary considerably, also assuming fractional values in non-integral oxidation state (NIOS) salts. The different molecular oxidation states are reversibly accessible by chemical or electrochemical means. A good example is the case of fe(l,2-dithiolene) complexes of ds metal ions [such as Ni(II), Pd(II), Pt(II), Au(III)], whose charge can assume values typically ranging between —2 and 0 (see Scheme 4). 

Although the values of T igp are relatively large in water and in methanol, a finite amount of Cu(I) exists in any Cu(II) solution that is in contact with metallic copper. In fact, the molecularity associated with dictates that the fraction of copper in solution in the form of Cu(I) increases as the total concentration of solvated copper ion decreases. Thus, at micromolar levels in water, for example, the two oxidation states can be maintained in essentially equal amounts. In acetonitrile, the equilibrium for reaction 5 lies far to the left so that solvated Cu(I) is readily generated by placing copper metal in contact with a Cu(II) solution (conproportionation). As a consequence, the Cu(I) salt, [Cu(CH3CN)4]C104, is easily prepared [18] and is temporally stable. 

It is particularly significant that the distribution of vanadium among tissues or subcel-lular fractions is independent of whether the oxidation state of the vanadium salt administered is +3, +4, or +5175,179. This result implies that all metabolic vanadium is converted to a common oxidation state in the circulation, probably to the V02+ ion, before being assimilated by most tissues, the kidney perhaps being an exception. 

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). 

This result shows that the fraction of Cu ions at equilibrium is generally very small. When, in the absence of complexing agents, a salt of monovalent copper is dissolved in water, the Cu" " ions therefore disproportionate according to reaction (2.75). In a general way, we may conclude that if a metal can form two ions of different oxidation state in solution, the lower valence ion is thermodynamically unstable if its equilibrium potential is more noble than that of the higher valence ion. 

When XeFe is hydrolyzed in a strongly alkaline solution, part of the xenon is lost as gas, but a large fraction precipitates as a perxenate (XeOg ) salt in which xenon is in the -1-8 oxidation state. Among the salts... 

This has always been a challenge owing to similar properties in the usual +III oxidation state. Initial separations based on fractional crystallization of salts were time-consuming and tedious with many false dawns (Section 1.3.1). Current separation methods are based on (i) less usual oxidation states and (ii) variations in complex stability resulting from ionic radii differences. 

Acid soluble rare earth salt solution after the removal of cerium may be subjected to ion exchange, fractional crystalhzation or solvent extraction processes to separate individual rare earths. Europium is obtained commercially from rare earths mixture by the McCoy process. Solution containing Eu3+ is treated with Zn in the presence of barium and sulfate ions. The triva-lent europium is reduced to divalent state whereby it coprecipitates as europium sulfate, EuS04 with isomorphous barium sulfate, BaS04. Mixed europium(ll) barium sulfate is treated with nitric acid or hydrogen peroxide to oxidize Eu(ll) to Eu(lll) salt which is soluble. This separates Eu3+ from barium. The process is repeated several times to concentrate and upgrade europium content to about 50% of the total rare earth oxides in the mixture. Treatment with concentrated hydrochloric acid precipitates europium(ll) chloride dihydrate, EuCb 2H2O with a yield over 99%. 

Beryllium Sulfate. Beryllium sulfate tetrahydrate [7787-56-6], BeS04 4H20, is produced commercially in a highly purified state by fractional crystallization from a beryllium sulfate solution obtained by the reaction of beryllium hydroxide and sulfuric acid. The salt is used primarily for the production of beryllium oxide powder for ceramics. Beryllium sulfate dihydrate [14215-00-0], is obtained by heating the tetrahydrate at 92°C. Anhydrous beryllium sulfate [13510-49-1] results on heating the dihydrate in air to 400°C. Decomposition to BeO starts at about 650°C, the rate is accelerated by heating up to 1450°C. At 750°C the vapor pressure of S03 over BeS04 is 48.7 kPa (365 mm Hg). 

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