Divalent states oxides

Divalent manganese compounds are stable in acidic solutions but are readily oxidized under alkaline conditions. Most soluble forms of manganese that occur in nature are of the divalent state. Manganese(Il) compounds are characteristically pink to colorless, with the exception of MnO and MnS which are green, and Mn(OH)2, which is white. The physical properties of selected manganese(Il) compounds are given in Table 6. 

The only oxide formed by any of these metals in the divalent state is CoO this is prepared as an olive-green powder by strongly heating the metal in air or steam, or alternatively by heating... 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

After the loss of potential control, the Pb UPD layer may react with residual electrolytic solution to generate H2 and the Pb may be oxidized to the divalent state yielding a layer of PbO, Pb(0H)2 or PbF2. This is more likely to occur with UPD of Pb on Ag than for example on Au since the UPD potentials are more cathodic on Ag. Thus, for AEp = 0.15 V (see Figure 2a) and E°(Pb/PbF2) — -0.344 V in the reaction ... 

Thermal adsorption of NO on hematite took place after the surface atoms of the oxide were reduced to the divalent state by treatment with CO (Rethwisch and Du-mescic, 1986). Photoadsorption of NO which was several orders of magnitude greater than that due to thermal adsorption could be induced by treating powdered hematite with aqueous NH4CI followed by calcination at ca. 300 °C (Blomiley and Seebauer, 1999, 1999 a). The reaction appears to be a complex one with water, CI2 and Fe" and Fe being involved. 

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%. 

The vast majority of electrochemical data on americium ions has heen obtained in aqueous solutions. Americium can exist in aqueous solutions in the oxidation states III, IV, V, and VI. The divalent state is difficult to attain in aqueous solutions because of the proximity of the standard potential of the Am(III)/Am(II) couple to the solvent/supporting electrolyte breakdown potential. Previous reviews have presented the formal and standard potentials for the various americium couples and these reviews should be consulted by the interested reader for more detailed discussion [133, 134]. Table 3 contains a summary of selected formal potentials Ef from these reviews in 1 M HCIO4 for convenience. AU values are calculated from various measurement techniques except for the Am(VI)/Am(V) couple (Am02 /Am02" "), which was determined directly. 

CuO, the mineral tenorite, and AgO are well known but their structures are quite different. More importantly the valence states in these two compounds are quite different. In CuO, the copper is formally in the divalent state, whereas in AgO, there exist two types of silver atoms, one in formal oxidation state 1+, the other in 3+. These two silver ions also possess strong covalent character. PdO and CuO, however, have similar crystal structures based on chains of opposite edged-shared, square-planar M04 groups. 

Cyanide complexes of platinum occur most commonly in the divalent state, although there has been increasing interest in the complexes formed with platinum in a higher oxidation state. Among the complexes most recently studied have been the mixed valent complexes where platinum cyanides in the divalent state are partially oxidized. These complexes form one-dimensional stacks with Pt-Pt interactions. In the solid state these materials show interesting electrical conductivity properties, and these compounds are discussed by Underhill in Chapter 60. In this section the preparative procedures and spectroscopy of the complexes will be covered, but for solid state properties the reader is referred to Chapter 60. 

The rare earth chlorides can be separated through sublimation but a very high temperature and good vacuum are required. Recently [46] Eu2+ has been obtained pure by the distillation of its halides using the fact that Eu2+-halides are less volatile than the halides of trivalent rare earths. Sm, Eu and Yb oxides can be reduced to the divalent state by carbon and volatilized selectively from a mixture with other rare earth oxides [47]. 

Glasner et al. [399], however, showed that Eu2(Ox)3 behaves somewhat differently from other rare earth oxalates, in being easily reduced to the divalent state. According to these investigators, the first step of the thermal decomposition of Eu2(Ox)3 at 320° C in a CO2 atmosphere involves the formation of Eu(Ox) with a weight loss of 28.6 per cent. In an oxidizing atmosphere, however, reoxidation takes place, and the final... 

Inorganic systems featuring linkage isomerism induced by changing the oxidation state of a metal are also known.145-561 For instance, a sulfoxide is O-bonded to ruthenium n) in its stable form (Ru-OSR2). On reduction of the metal to the divalent state, the initially obtained O-bonded species rearranges to afford the stable S-... 

Fig. 19 A three-configuration Cu(i) catenate, the general molecular shape of which can be dramatically modified by oxidizing the central metal [Cu(i) to Cu(n)] or by reducing it back to the monovalent state. Each ring of the [2]-cate-nate now incorporates two different coordinating units a bidentate unit and a terdentate fragment. Starting from the tetracoordinated monovalent Cu complex (Cu(i)Nj top left) and oxidizing it to the divalent state (Cu(ii) N4), a thermodynamically unstable species is obtained, which should first...

It is based on the addition of Mn2+ solution, followed by die addition of a strong alkali to die sample in a glass-stoppered bottle. Dissolved 02 rapidly oxidizes an equivalent amount of the dispersed divalent manganous hydroxide precipitate to hydroxides of higher valence states. In die presence of iodide ions in an acidic solution, the oxidized manganese reverts to die divalent state, with die liberation of a quantity of iodine equivalent to die original dissolved 02 content. The iodine is then titrated with a standard solution of thiosulfate. The titration end point can be detected visually with a starch indicator, or by potentiometric techniques. The liberated iodine can be determined colorimetrically. 

The solid is not affected in the presence of air for short periods, but the solution is easily oxidized. However, in the absence of air, the solution is stable indefinitely. The observed magnetic moment (fiet = 1.85 fiB) and four equally spaced, equally intense lines in the ESR spectrum clearly establish the divalent state for Au. The conductivity, electronic spectra, and cyclovoltammetry results corroborate this conclusion. The structure is suggested to be square planar (2). The same compound is also formed (121) by reacting equimolar amounts of Au(I) and Au(III) with the ligand (mnt)2- [Eq. (5)]. 

Silicon in the lower oxidation states is discussed by Burger673. Silicon compounds in the divalent state also exist with formation of Si—Si bonds. In the carbon group the tendency to divalent state increases with higher atomic numbers while the tendency to tetravalence declines. Since silicon atoms are relatively small, the tetra valent state is strictly preferred. 

The important valence states of chromium are II, III, and VI. Elemental chromium, chromium(O), does not occur naturally. The divalent state (II or chromous) is relatively unstable and is readily oxidized to the trivalent (III or chromic) state. Chromium compounds are stable in the trivalent state and occur in nature in this state in ores, such as ferrochromite (FeCr204). The hexavalent (VI or chromate) is the second most stable state. However, hexavalent chromium rarely occurs naturally, but is produced from anthropogenic sources (EPA 1984a). Chromium in the hexavalent state occurs naturally in the rare mineral crocoite (PbCr04) (Hurlburt 1971). 

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