Vanadium oxide, electron paramagnetic

The spectrum for LaY impregnated with vanadyl naphthenate shows a characteristic band at 365 nm that loses most of its intensity after calcination, Figs. 5a, 5b. This is not surprising since Pompe et al (30), using TGA/DTA data,have shown that the oxidative decomposition of the vanadyl naphthenate is complete at 500°C. Electron paramagnetic resonance (EPR) studies have shown that vanadium (after calcination) is stabilized mainly in the form of vanadyl (V02+) cations in the zeolite supercages (29). 

Analysis of vanadium-loaded model materials (such as EuY, amorphous aluminosilicate gels and EuY-gel mixtures) by electron paramagnetic resonance (EPR) has provided information concerning metal oxidation state and stereochemistry (67). EPR data has indicated that when vanadyl cations are introduced in the form of vanadyl naphthenate, they were stabilized in a zeolite with the faujasite structure as pseudo-octahedral V02+ even after calcination at 540°C. Upon steaming, these V02+ cations were then converted almost entirely to V+5 species (67). The formation of EuV04 was verified but the concentration of this vanadate was never proportional to the total rare-earth content of the zeolite. In EuY-gel mixtures the gel preferentially sorbed vanadium where it was stabilized mainly in the form of V205. 

Components of fluidized cracking catalysts (FCC), such as an aluminosilicate gel and a rare-earth (RE) exchanged zeolite Y, have been contaminated with vanadyl naphthenate and the V thus deposited passivated with organotin complexes. Luminescence, electron paramagnetic resonance (EPR) and Mossbauer spectroscopy have been used to monitor V-support interactions. Luminescence results have indicated that the naphthenate decomposes during calcination in air with generation of (V 0)+i ions. After steam-aging, V Og and REVO- formation occurred. In the presence of Sn, Tormation Of vanadium-tin oxide species enhance the zeolite stability in the presence of V-contaminants. 

By using non-invasive techniques, it is possible to determine the concentration and oxidation state of vanadium in living cells. In A. ceratodes, EXAFS (X-ray absorption fine structure) techniques showed that 90% of the vanadium is in the form of V(III), 10% is V(IV)84. In A. nigra, EPR (electron paramagnetic resonance) spectroscopy showed that 95% of the vanadium is in the V(III) form, and 5% is V(IV)85. ... 

The O oxidation state is known in vanadium hexacarbonyl. V(CO)(,. a blue-green, sublimable solid. In the molecule VfCO), if each CO molecule is assumed to donate two electrons to the vanadium atom, the latter is still one electron short of the next noble gas configuration (krypton) the compound is therefore paramagnetic, and is easily reduced to form [VfCO, )]. giving it the... 

Vanadium, a typical transition element, displays weU-cliaractetized valence states of 2—5 in solid compounds and in solutions. Valence states of —1 and 0 may occur in solid compounds, eg, the carbonyl and certain complexes. In oxidation state 5, vanadium is diamagnetic and forms colorless, pale yeUow, or red compounds. In lower oxidation states, the presence of one or more 3d electrons, usually unpaired, results in paramagnetic and colored compounds. All compounds of vanadium having unpaired electrons are colored, but because the absorption spectra may be complex, a specific color does not necessarily correspond to a particular oxidation state. As an illustration, vanadium(IV) oxy salts are generally blue, whereas vanadium(IV) chloride is deep red. Differences over the valence range of 2—5 are shown in Table 2. The stmcture of vanadium compounds has been discussed (6,7). 

Complexes containing vanadium in low oxidation states, apart from organometallic compounds, are known with ligands such as bipy, phen, nitric oxide, and tertiary phosphines, which stabihze such oxidation states. Depending on their electronic structure, V and V complexes may be diamagnetic, which permits study by NMR spectroscopy, and EPR spectroscopy has been used to study paramagnetic V complexes. 

Perhaps the most important area of biochemistry in which ESR is used is the study of metalloproteins. Transition metals in certain oxidation and spin states have unpaired electrons, are paramagnetic, and in many cases are amenable to ESR spectroscopy. The most commonly found transition metals in biological systems are iron, copper, molybdenum, cobalt, and manganese. The remainder, including metals such as vanadium and... 

With the cydopentadienyl ligand, vanadium forms the simple sandwich compound, "vanadocene , [V( - -C5Hj)2] which is dark violet, paramagnetic (3 unpaired electrons) and extremely air-sensitive. Oxidative addition reactions are possible and provide compounds such as [V()9 -CjH5)2C1 ] (n = 1,2,3) and LV(rj -C5H5)2R2], while it.s reaction with dithioacetic acid produces the dark-brown tetramer [V4(fj -C H5)4( 3-S4), Fig. 22.11. " With four V " atoms, eight electrons are available for six V-V bonds and the implied bond order of 2/3... 

Many industrially important selective oxidation reactions are catalyzed by transition metal oxides. The activity of such catalysts is related to the reducibility of the transition metal ion, which enables the bulk oxide lattice to participate actively in the redox processes present in the Mars van Krevelen mechanism. Unfortunately, NMR spectroscopic investigations are severely limited by the occurrence of paramagnetic oxidation states. As a general rule, NMR signals from atoms bearing unpaired electron spins cannot be detected by conventional methtxls, and the spectra of atoms nearby are often severely broadened. For this reason, most of the work published in this area has dealt with diamagnetic vanadium(V) oxide-based catalysts. 

Cr(CO)sI is isoelectronic with V(CO)6 and like the latter exhibits the expected paramagnetism for a species containing one unpaired electron. Chromium pentacarbonyl iodide appears to be more stable to oxidation but less stable to thermal decomposition than vanadium hexacarbonyl. This higher oxidative stability of Cr(CO)5l as compared with that of V(CO)j may be due to the higher formal oxidation state of the central metal atom of Cr(CO)5l. 

Although the trinuclear core formally contains vanadium(lll) and vanadium (IV) ions, all are equivalent suggesting that the unpaired electron(s) are delocalized. The paramagnetic trimetallic center also has an unusual effect on the chemical shifts of a-protons of the dithiocarbamate ligands in the NMR spectrum, which can be shifted downfield as far as 6 10.7 ppm. The clusters are redox active and undergo reversible one-electron reduction and oxidation processes (697). 

It can be seen also that the 18-electron rule will tend to break down when the first-row transition metals are in a high oxidation state. There may also be steric limitations to the formation of 18-electron complexes in high oxidation states - consider for example the hypothetical cation [Cr(CO)9] . Steric limitations may also explain why vanadium forms the monomeric, paramagnetic and 17-electron hexacarbonyl, V(CO)6, rather than the dimeric complex [V(CO)6]2 which would be diamagnetic and would obey the 18-electron rule. 

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