Vanadium divalent 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. 

Aqueous pentavalent vanadium is readily reduced to the tetravalent state by iron powder or by S02 gas. A stronger reducing agent, eg, zinc amalgam, is needed to yield divalent vanadium. Divalent and tfivalent vanadium compounds are reducing agents and require storage under an inert atmosphere to avoid oxidation by air. 

Phases in which titanium or vanadium are in the divalent state have well-known crystal types. There are, for example, R4VS7 of the Ho4FeS type (table... 

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... 

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... 

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. 

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. 

Vanadium has divalent, trivalent, tetravalent and pentavalent cation states and hydrolysis species form with respect to all of these four states. The tetravalent state exists as the oxoanion, and similarly the pentavalent state as V02 . ... 

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