Vanadium trivalent state

In 1898, Cowper-Coles 2 claimed to have successfully effected the electrolytic reduction of an acid solution of vanadium pentoxide to metallic vanadium, but the product was subsequently shown by Fischer 3 to have been a deposit of platinum hydride. Fischer, in a series of over three hundred experiments, varied the temperature, current density, cathode material, concentration, electrolyte, addition agent, and construction of cell, but in not one instance was the formation of any metallic vanadium observed. In most cases reduction ceased at the tetravalent state (blue). At temperatures above 90° C. reduction appeared to proceed to the divalent state (lavender). The use of carbon electrodes led to the trivalent state (green), but only lead electrodes produced the trivalent state at temperatures below 90° C. Platinum electrodes reduced the electrolyte to the blue vanadyl salt below 90° C. using a divided cell and temperatures above 90° C. the lavender salt was obtained. 

Hydrogen also reduces pentavalent and tetravalent vanadium salts to the trivalent state in the presence of spongy platinum.1... 

The change in the oxidation state of the vanadium ion has also been observed in the ESR spectra of the soluble V(acac)3/A1(C2H5)2C1 catalyst at various temperatures. At temperatures below —40 °C no ESR signal could be detected, which suggests that the vanadium ions exsist in the trivalent state. A broad ESR signal (AH 20 mT) apperared at g — 1.98 at temperatures above —30 °C, and its intensity increased with temperature to reach a constant value at 20 °C. Thus, these spectral data indicate that the vanadium species active for the living polymerization of propylene are in the trivalent state. 

Vanadium entering water is generally converted from the less-soluble trivalent state to the more-soluble pentavalent state (Byerrum et al. 1974). The species of vanadium most likely to be found in sea water are (H2V4O13), HVO, and VO (Van Zinderen Bakker and Jaworski 1980). Vanadium is continuously precipitated from sea water by ferric hydroxides and organic matter (WHO 1988) and forms sediments on the seabed. 

Process selection. The processes just described recovered neptunium only partially and in variable yield because of the difficulty in controlling the distribution of neptunium valence between 5 and 6 in the primary extraction step with nitrite-catalyzed HNO3 and the incomplete reduction of neptunium from valence 5 to 4 in the partitioning step with feirous ion. This section describes a modified Purex process that could be used if more complete recovery of neptunium were required. It is based on process design studies by Tajik [Tl]. The principal process steps are shown in the material flow sheet Fig. 10.32. In the primary decontamination step, pentavalent vanadium oxidizes neptunium to the extractable hexavalent state. In the partitioning step, tetravalent uranium reduces plutonium to the inextractable trivalent state while converting neptunium to the still-extractable tetravalent state. 

In addition to the exclusion of carbon dioxide, it is necessary to exclude oxygen from reactions where cations of the metal hydroxide layer are easily oxidized under basic conditions. LDHs of this type include manganese, cobalt (131) and iron in the divalent oxidation state, and vanadium in the trivalent state. Reactions involving these metal ions have formed LDHs with, for instance, Co " /Co and Fe " /Fe metal content, with the latter, known as green rust, being discussed earlier in Section II.J. 

There has been no structural study made on the phases observed in these systems which often have complicated structures. For the trivalent state of titanium or vanadium some phases have been pointed out, but the description of the networks should be reexamined. LaTiXs where X = S, Se and LaVX3 where X = S, Se are probably monoclinic (Donohue, 1975). R6Ti4Si5 where R = La to Gd and R6V4S15 where R = La to Gd are probably also monoclinic (Patrie et al., 1969c). 

In the trivalent state vanadium forms vanadites of composition RVO3 with the rare earth oxides. These compounds can be prepared directly from the trivalent oxides in vacuum or in a reductive at high temperatures (Wold and Ward, 1954 Reuter and Wollnik, 1963 Sakai et al., 1976). Another method of preparation is to reduce orthovanadates with hydrogen (Palanisamy et al., 1975). 

Other studies investigated marine animals such as ascidia (subtype of Ttini-cata ) and holothurians (type Echinodermata ). Peterson [8] is only one of the many authors that have pointed out the high vanadium concentrations common in ascidians [7]. These animals contain 10,000 times the vanadium that is present in the water in which they live [5]. In ascidians, special green blood cells called vanadocytes concentrate the element. Vanadium in these cells is in the trivalent state, is complexed to pyrrole rings, and is associated with unusually high concentrations of sulfuric acid [4]. In other ascidians, vanadium is concentrated in the blood plasma (hemovanadium) rather than in individual cells. Although the vana-... 

The most widely studied transition metal is titanium. At various times, all oxidation states of titanium (II, III, IV) have been proposed for the active site of titanium-based initiators. Most of the evidence points to titanium (HI) as the most stereoselective oxidation state, although not necessarily the most active nor the only one [Chien et al., 1982]. (Data for vanadium systems indicate that trivalent vanadium sites are the syndioselective sites [Lehr, 1968].) Initiators based on the a-, y-, and 8-titanium trihalides are much more stereoselective (iso-selective) than those based on the tetrahalide or dihalide. By itself, TiCl2 is inactive as an initiator but is activated by ball milling due to disproportionation to TiCl3 and Ti [Werber et al., 1968]. The overall stereoselectivity is usually a-, y-, 8-TiCl , > TiCL > TiCLj P-TiCl3 [Natta et al., 1957b,c],... 

Reduction of add solutions of vanadium pentoxide to the tetravalent state also takes place with bismuth amalgam 5 magnesium gives the trivalent salts of vanadium, while by using zinc, zinc coated with cadmium, electrolytically deposited cadmium, or sodium amalgam in the absence of air, divalent vanadium salts are obtained in solution.7 Vanadous salts and hypovanadous salts are, however, much more conveniently prepared by electrolytic reduction of acid solutions of vanadium pentoxide in an atmosphere of carbon dioxide.8... 

Electron exchange between trivalent and tetravalent vanadium is also slow. Here, the two oxidation states form different types of ions (probably VOH 2 and VO 2, respectively) and conversion from one state to another involves the making or breaking of one or more bonds ... 

Now let us consider some particular cases. [V(H,0)o] + is a d ion in an octahedral environment and the pertinent qualitative correlation diagram. Fig. 12-7.2, shows that there should be three spin-allowed transitions from the T,(F) ground state to the excited states 7, (P) and j4,(F) the symbol in brackets, in each case, denotes the parent state of the free ion. Experimentally, aqueous solutions of trivalent vanadium salts show two absorption bands, one at 17 200 cm and the other at 25 600 these give rise to the green colour of such... 

New important information about the structure of the intermediate complex was obtained from the X-ray studies of V complex with di-ferf-butylcatecholate [18]. The complex, the structure of which is presented in Figure 4, contains four vanadium ions two are divalent and two trivalent, but they are indistinguishable, thus their oxidation state is 2.5 and the complex can be regarded as existing in a semi-reduced state, similar to the s-r state of FeMoco in the presence of dithionite, which is not sufficiently reduced to activate dinitrogen. 

The oxidation state of the active vanadium species is under discussion [13] some authors propose the trivalent V(III) state and describe the deactivation to occur by reduction to V others favor V" to be the active form. Nevertheless it is generally assumed that the active site contains alkylvanadium halide entities such as VCI2R. Chain initiation is believed to arise from the sequence of reactions shown in eq. (1). 

Apparently, soluble catalysts are obtained by reaction of Ti(OR)4 with AIR3 [144]. High-molecular-weight polyethene is obtained in variable amounts, with Al/Ti ratios ranging between 10 and 50. Similar results are attained by replacing titanium alkoxide by Ti(NR2)4 [145]. Soluble catalytic systems are also obtained by reaction of Ti(acac)3 [146] and Cr(acac)3 [147] with AlEts as well as by reaction of Cr(acac)3 and VO(acac)2 with AlEt2Cl in the presence of triethyl phosphite [121]. With vanadium catalysts the activity reaches its maximum at Al/V ratio = 50. Under these conditions up to 67% vanadium is in the bivalent oxidation state. Bivalent and trivalent compounds will be active. 

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