Vanadium coordination number

Although much of the V NMR has been performed on model systems or catalytic materials containing vanadium, 29 >30 compounds such as V2O5 or VOPO4 are used in both the catalysis and lithium battery fields, and many of the results can be used to help elucidate the structures of vanadium-containing cathode materials. V NMR spectra are sensitive to changes in the vanadium coordination number and distortions of the vanadium local environments from regular tetrahedra or octahedra. >33 5>V isotropic chemical shifts of between —400 and —800 ppm are seen for vanadium oxides, and unfortunately, unlike... 

The analysis of structural rearrangement could be essential to explain the properties that characterize VPG systems. The main features that could affect the electronic behaviour are the number of V" +-0-V + linkages, the vanadium coordination number and the V-V distance. In our knowledge, a unique description of VPGs structure has not been already reached. For instance, Jordan et al. [36, 91] supposed that a... 

Coordination compounds of vanadium are mainly based on six coordination, in which vanadium has a pseudooctahedral stmcture. Coordination number four is typical of many vanadates. Coordination numbers five and eight also are known for vanadium compounds, but numbers less than four have not been reported. The coordination chemistry of vanadium has been extensively reviewed (8—12) (see Coordination compounds). 

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

In Rb2V308 the Rlr ions have coordination number 10 there are two kinds of vanadium ions, V4+ with c.n. 5 and V5+ with c.n. 4, and four kinds of O2- ions. The mutual coordination of these... 

It has been suggested that an increase in the coordination number of vanadium from 4 to 5 already takes place in the second protonation step, i.e. when [H2V04] is formed (21). For reactions (1) and (2), however, the protonation constants and thermodynamic parameters are comparable with those reported for P04 and As04 , providing firm evidence that reaction (2) is not accompanied by incorporation of water in the vanadate ion (15, 17). Further, the estimated thermodynamic quantities for reaction (6), AH° = -39 kJ/mol and AS0 = —51 J/(mol K), obtained by extrapolation from the experimental values for reactions (1) and (2) and those for the three protonation steps of P04 and As04 , are not typical of a simple protonation reaction (17). For such a reaction the entropy change is normally a positive quantity often amounting to 100 50 J/(mol K) and the enthalpy... 

Protonation of a carbonyl oxygen rather than the metal may be encouraged in this case by the high coordination number of vanadium. This would then promote halide attack on the carbonyl carbon to yield an intermediate hydroxyhalocarbene, which reacts further to yield the indicated products. This system represents a potential photoassisted water-gas shift catalyst system since H3V(CO)3(diars) upon photolysis with a mercury vapor lamp yields H2, and in the presence of CO regenerates the starting complex HV(CO)4(diars). The feasibility of coupling these two reactions in the same reaction solution remains to be demonstrated. 

Applications of NMR spectroscopy to structural, thermodynamic, and dynamic processes have been described. A brief discussion of the types of problems appropriate for study by this technique has been included. H and 13C NMR spectroscopy has been applied to define the ligand coordination in complexes. These experiments, combined with 170-labeling experiments, allowed deduction of the coordination number of the vanadium atom. Integration of NMR spectra allowed measurement of the formation constants and equilibrium constants. 2D 13C and 51V EXSY experiments were used in a qualitative and quantitative manner to examine intra- and intermolecular dynamic processes, of which several examples are discussed. The interpretation of the rate matrix and its relationship to the chemical processes under examination were also described. 2D EXSY spectroscopy has great potential as a tool with which to probe mechanisms in complex reactions however, such uses often requires estimation of errors. The major source of error in 2D 51V EXSY NMR studies on a two- and four-site vanadate system were found to be baseline distortion and the errors were estimated. Our results suggest... 

A range of coordination complexes of has been prepared in which the vanadyl group is absent, a number of these compounds being synthesized as part of an effort to understand the nature of biological vanadium sites, such as amavadin," which contains an octadentate, hydroxyiminodipropionate ligand bound to vanadium A number of model complexes of this type have been prepared and discussion of this area can be found in the article Vanadium in Biology Further discussion in this section follows the main ligand donor atom to vanadium. 

Vanadium compounds exist in a variety of formal oxidation states from -3 to +5. Their coordination number varies from 3 to 8. The stereochemistry depends on their oxidation state V(II) 6-coordinate octahedral, V(III) 3-7-coordinate (V[CH(SiMe3)2]3 3-coordinate planar), V(rV) 4-6 or 8-coordinate (V(CH2SiMe3)4 4-coordinate tetrahedral, VO(acac)2 5-coordinate tetragonal), V(V) 4-7-coordinate (VOCI3 4-coordinate tetrahedral). Standard electrode potential in acidic aqueous solution is as follows V + -F 2e- = V-1.19V, V + -F e =... 

The complex nature of vanadium species in catalytic materials means that no single technique is ideally suited to their study. NMR techniques can, however, provide information on the local environment of vanadium nuclei, including the geometry and coordination number of species, the number of non-equivalent... 

Fig. 7 Crystal structure of orthorhombic V2O5 with netplane stacking along (010). Vanadium (oxygen) centers are shown by large (small) balls. Inequivalent oxygen centers, 0(1), 0(2), 0(3), with 1, 2 and 3 coordination number, respectively, are labelled accordingly. (From Ref. 229).

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