Vanadium stoichiometry

In addition to the linear oligomers, cyclic derivatives (V33, V44, V/ ) are also known (see Section 2.2). The latter two are readily formed and are ubiquitous components of aqueous solutions. These compounds are relatively inert to complex-ation reactions that preserve their cyclic structure rather, the equilibria shift towards products of other vanadium stoichiometry. Presumably, one way to generate complexes of these oligomers would be to protonate one or more oxygens so that the reactivity is increased. However, neither protonated V4 nor V5 has been identified in solution, nor has there yet been any indication from studies in alcoholic solution that an alkoxo group can replace an oxo ligand, as might be expected if protonation occurs. 

Butane-Based Fixed-Bed Process Technology. Maleic anhydride is produced by reaction of butane with oxygen using the vanadium phosphoms oxide heterogeneous catalyst discussed earlier. The butane oxidation reaction to produce maleic anhydride is very exothermic. The main reaction by-products are carbon monoxide and carbon dioxide. Stoichiometries and heats of reaction for the three principal reactions are as follows ... 

The incorporation of the trapping reaction changes the stoichiometry of the net reaction. The result is an induced reaction. For example, the oxidation of vanadium(IV)... 

The kinetics of the oxidation of chromium(II) by vanadium(ni) in acid perchlorate media have been studied spectrophotometrically between 0.2° and 35.0 °C over a range of 0.027-0.500 M HC104 . The oxygen-sensitivity of both reactants meant that the air had to be excluded in all kinetic runs. Also, since V(III) slowly reduces perchlorate ion, fresh solutions of V(III) were required for each experiment. In terms of stoichiometry the reaction conforms accurately to... 

Both vanadium and niobium metals form dihydrides only at high pressures(27), and numerous phases with hydrogen compositions less than one(28). Experiments were performed to saturate vanadium clusters with deuterium. Figure 4 is a plot of the number of deuteri urn molecules found in the products. The solid straight lines are for D V ratios of 1 and 2. The corresponding curved dashed lines include corrections for some bulk atoms(2c). The best fit to the data including only surface atoms indicate a stoichiometry of 1.5. It is likely that this high surface stoichiometry is an indication of bulk i ncorporation of deuterium. 

It is quite often possible to prepare hydroxypyridinone complexes directly by one-pot synthesis from the appropriate hydroxypyranone, amine, and metal salt 90-92). They can also be prepared by reacting complexes such as P-diketonates with hydroxypyridinones (see e.g., Ce, Mo later). Several maltolate complexes, of stoichiometry ML2, ML3, ML4, or MOL2, have been prepared by electrochemical oxidation of the appropriate metal anode, M — a first-row d-block metal (Ti, V, Cr, Mn, Fe, Co, Ni), In, Zr, or Hf, in a solution of maltol in organic solvent mixtures 92). Preparations of, e.g., manganese(III), vanadium(III), or vanadium(V) complexes generally involve oxidation... 

The stoichiometry (equation 2) requires eight electrons transferred from vanadium(II) ions to dinitrogen atoms (six electrons) and hydrogen atoms (two electrons) and this kinetic equation (3) was interpreted as evidence for a polynuclear structure of the transition state during the reduction of dinitrogen. An alternative mechanism was suggested.145... 

A similar preparative procedure was used to isolate tetrahedral complexes of bidentate donors (L = bipy or phen) [VL2](SCN)3. However, complexes of vanadium(lll) chloride or bromide with these same bidentate donors have stoichiometries depending on conditions.199 With excess of bipy or phen, complexes [VX JX were obtained. In EtOH or in MeCN two series of complexes were isolated. VC13LL (L = bipy or phen L = EtOH or MeCN) and [VC12L2]+ + [VCUL]-. In the reaction of vanadium(III) with phenanthroline202 203 or bipyridine204 there is evidence for extensive hydrolysis. Spectrophotometry was used to estimate the equilibrium constant (Kcq = 0.43) for the reaction in equation (7). 03... 

Aromatic polyalcohols act as strong coordinating agents and Table 17 summarizes reported formation constants. The complexes are quite stable this behaviour has been used for the qualitative and quantitative determination of vanadium (e.g. refs. 494 and 495). At pH 3-4, an initial vanadyl catechol complex slowly converts to a tris complex.496 In fact complexes with 1 3 metal-ligand stoichiometry have been isolated (see below), but since in the equilibrium (30) no protons are consumed or liberated, [VO(cat)2]2- and [V(cat)3]2 are not distinguishable by potentiometric studies. 

Both oxidations are highly exothermic and carried out almost exclusively in tubular reactors cooled by a molten salt.1024 Supported vanadium oxide with additives to improve activity, selectivity, and stability usually serves as the catalyst.970 990 1025 Because of its more favorable stoichiometry (no carbon is lost in oxidation), most new plants use o-xylene as the starting material. 

In amido derivatives used as precursors to nitrides, the presence (e.g. in V(NEt2)4) of alkyl groups that could self-eliminate to alkenes, was not sufficient to ensure nitrogen incorporation into the films, and addition of NH3 was necessary. More important was control of the molar fraction of V(NEt2)4 and NH3 in the gas phase, which allowed the formation of vanadium carbonitride films with variable and adjustable stoichiometry. 

Well-characterized vanadium carbide thin films can be prepared by exposing a clean V(110) surface to ethylene or 1,3-butadiene at 600 K. The formation of vanadium carbide, rather than other forms of carbon-containing species such as graphite or carbonaceous overlayers, was confirmed by the characteristic AES and NEXAFS spectra.4 The stoichiometry and average thickness of the thin carbide films can be estimated by... 

The stoichiometry of reaction (14) is reminiscent of the electroreduction of 02 to H20. It is of interest that oxygenation of vanadium(III) gives oxovanadi-um(V), whereas multiple one-electron redox centers have been considered to be essential for the incorporation of 02. An explanation for this can be found in... 

Although attempts have been made to prepare other fluorides with this stoichiometry, for example, by the interaction of the di- and trifluorides of iron and cobalt (54), these have been unsuccessful. For vanadium, the compound VF2, presumably contains vanadium(III) but the structure remains that of rutile, with a maximum 8 value of 0.04 (18 ). 

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