Chromium divalent state

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. 

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). 

Except for compounds with ir-add ligands, there is not a great deal of similarity to chromium. The divalent state, well defined for Cr, is not well known for Mo and W except in strongly M—M bonded compounds and the abundance of highly stable Crm complexes has no counterpart in Mo or W chemistry. For the heavier elements, the higher oxidation states are more common and more stable against reduction. 

On CO-reduced diluted sample chromium is isolated and prevalently in divalent state. The average Cr(II)-Cr(II) distance, in the case of a 1 wt. % Cr(II)/Si02 system, is about 10 A. 

This conclusion is consistent with other data. Cr(VI) /alumina catalyst can be treated with CO at 350 °C to yield a catalyst with a substantial portion of the chromium coordinatively unsaturated and in the divalent state. The reduction is not as clean on alumina as it is on silica, and considerable Cr(III) is also obtained [104]. Nevertheless, the reduced Cr/alumina catalyst exhibits chemiluminescence upon exposure to air, and the wavelength of light emitted, characteristic of oxygen, is similar to that of light emitted by Cr(II)/silica [274]. An example is shown in Figures 13 and 14. 

Studies (90, 91) with Cr02/Si02 catalyst have shown that formation of a surface chromate takes place by reaction of Cr02 and surface silanol groups on silica (Reaction 17). Reaction of this chemisorbed chromate with ethylene results in an oxidation-reduction reaction (90-95) with formation of a low-valent chromium center (Reaction 18). Proposals for Cr(II) as the active site are based on studies of the catalyst after reduction by ethylene, carbon monoxide, or hydrogen. One study (93. 94) showed that the polymerization rate increased with the fraction of Cr(II) in the catalyst. Another study (92) showed by polarography that the chromium is reduced to a divalent state by ethylene. 

With this element, the trends already noted in the relative stabilities of oxidation states continue, except that there is now no compound or chemically important circumstance in which the oxidation state is equal to the total number of valence-shell electrons, which in this case is eight. The highest oxidation state known is VI, and it is rare and of little importance. Even the trivalent state, which rose to a peak of importance at chromium, now loses ground to the divalent state. We shall see below that this trend continues, with the sole exception of Co111 which is stable in a host of complexes. 

Chromium has divalent, trivalent and hexavalent ions. The divalent state is unstable in water, producing hydrogen whilst being oxidised to a higher valency state (Baes and Mesmer, 1976). Chromium(lll) has a large range of stability, whereas chromium(VI), unlike vanadium(V), only forms anionic species and, as such, will... 

As stated in the introduction, hexamethyldisilazyl can be used to coordinate electropositive elements in a very efficient way. Nevertheless, with divalent metallic elements it is often found that further organic bases are coordinated to the metal thus, in [(Me3Si)2N]2Cr(THF)2 the chromium atom is linked to two nitrogen and two oxygen atoms [9]. Such compounds tend to dissociate in the gas phase which makes them less appropriate for reactions controlled by pressure and temperature, as in MO-CVD processes. 

Oxygen is adsorbed by the divalent catalyst with a brilliant flash of chemiluminescence, converting the chromium back to its original orange hexavalent state (15,17,41,42). The ease with which this reversal occurs suggests that there is little rearrangement during reduction at 350°C. 

In keeping with its 4d%5s electron configuration, molybdenum forms many compounds in which its oxidation state is 6+. to an even greater extent than chromium. Also, like chromium, it forms compounds in which II is divalent and those in which it is trivalenl unlike chromium, il forms a number of pentavalenl compounds, and a few more tetravalent compounds, especially complexes. 

The most stable state of chromium is the +3 state compounds of hexavalent chromium are almost as good oxidizing agents as elemental chlorine, whereas compounds of Cr(II) ( chromous compounds) are potentiometrically more easily oxidized than cadmium metal. Divalent chromium, like Ag(II) and Au(III), may exist in equilibrium with aqueous media only as the cation of a relatively insoluble salt or in a slightly dissociated complex. However, solutions containing the blue Cr24 ion may be... 

Owing to the slow rates of diffusion of the cations, the direct solid-state reaction of the oxides Cr2 03 and MO at an elevated temperature is not a good preparation of divalent metal chromium(III) oxides. They can be prepared by more elaborate methods, such as controlled reduction of dichromates MCr207,1 reaction of dichromium tungsten oxide Cr2W06 with a molten divalent metal fluoride2 at 1400°, pyrolysis of complexes,3 and pulverization of slurries containing Cr2 03 and a divalent metal salt.4... 

Most modern laboratories are nowadays equipped with flame atomic spectrometers, which are routinely used for the quantitative determination of metals in solution. If such equipment is available, detection of metals can be carried out much faster than with the usual wet tests, especially if separations are involved. Usually 1-2 ml solution is consumed during one test however in most cases the tests are so sensitive, that a portion of the original sample solution can be diluted 10-100 fold, leaving enough material for separations. Note that flame atomic spectrometric tests do not provide information about the oxidation state of the metal (e.g. they cannot differentiate between divalent and trivalent iron, trivalent or hexavalent chromium, etc.). ... 

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