The Vanadates

Like the system Lai-xYxTiOs, the system Sri xCaxV03 contains one electron per formula unit in a narrow n band. However, the 0-orthorhombic distortion is smaller and decreases with decreasing x until SrV03, which appears to be cubic at room temperature. The redox couple is more 

The resistivity of a polycrystalline sample of CaVOs showed a typical metallic temperature dependence, p(T) T, but it became higher above room temperature than is calculated on the basis of itinerant-electron scattering with a mean-free-path as short as one V—0—V distance, which makes CaVOs a bad metal. The resistivity also showed an unusually strong decrease with pressure as might be expected if pressure transfers charge carriers from a lower Hubbard band to the Fermi surface of a Fermi liquid. Pressure would increase not only Wb, but also coq, of Eq. (3), thereby broadening W. However, resistivity data on a polycrystalline sample may not be considered definitive, and they do not provide a measure of m.  

Interpretation of Fig. 4 begins with a general expression for the thermoelectric power  

A band-structure calculation [16] indicates that the Fermi surface is not perturbed strongly enough by its interaction with the Brillouin-zone boundary to make [01n (r(e)/dln e= sensitive to volume changes. Consequently the pressure sensitivity of a for an electron gas varies as l/ p Electron-phonon interactions of coupling strength A introduce into a a factor (1 +. ) that is also pressure-dependent [17]. In a homogeneous electronic system, this factor gives rise to an effective mass m p that is calculated by a renormalization procedure similar to that used by Brinkman and Rice [ 14] to obtain m e/mb due to electron-electron interactions. Therefore, we assume that 

Broadening of the bandwidth W by the application of hydrostatic pressure reduces the curvature of e(k) so as to reduce (1 -i- A) as well as the volume V. 

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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. 

The target of vanadate-catalyzed photolysis is presumably an amino acid near the catalytic site of the Ca " -ATPase. The vanadate-catalyzed photocleavage at the V cleavage site in the absence of Ca " is 500 amino acids away from the C cleavage site which is attacked in the presence of Ca. Both sites are probably adjacent in the native structure to the catalytic site of the Ca -ATPase. 

Antibody A52 with its epitope at residues 657-672 [129,139,274,275] inhibited the vanadate-induced crystallization of Ca " -ATPase and decreased the stability of preformed Ca " -ATPase crystals [285]. The vanadate-induced crystals arise by the association of the ATPase monomers into dimers (type A interaction), the dimers into dimer chains (type B interaction), and the dimer chains into 2-dimensional arrays (type C interaction). It is suggested that antibody A52 interferes with type B interactions, preventing the formation of dimer chains, without exerting major effect on the concentration of Ca -ATPase dimers in the membrane. The simplest interpretation of the destabilization of Ca -ATPase crystals by mAb A52 is that binding of the antibody to its antigenic site physically blocks the interaction between ATPase molecules [285]. Considering the large bulk of the antibody, such interference is not unexpected, yet only a few of the antibodies that bind to the Ca -ATPase in native sarcoplasmic reticulum interfered with crystallization. 

The intramolecular distances measured at room temperature with the AEDANS FITC pair were similar in the Ca2Ei and E2V states [297]. Ca and lanthanides are expected to stabilize the Ej conformation of the Ca -ATPase, since they induce a similar crystal form of Ca -ATPase [119,157] and have similar effects on the tryptophan fluorescence [151] and on the trypsin sensitivity of Ca -ATPase [119,120]. It is also likely that the vanadate-stabilized E2V state is similar to the p2 P state stabilized by Pi [418]. Therefore the absence of significant difference in the resonance energy transfer distances between the two states implies that the structural differences between the two conformations at sites recorded by currently available probes, fall within the considerable error of resonance energy transfer measurements. Even if these distances would vary by as much as 5 A the difference between the two conformations could not be established reliably. 

The phosphorus(V) oxide written in simplest form is P205, so it should be expected that there would be considerable similarity between the various "phosphates" and the "vanadates." This is precisely the case, and the various forms of "vanadate" include V043-, V2074, V3()<,3, HV042, ll2V()4, H3V04, V ()0286, and others. These species illustrate the fact that vanadium is similar in some ways to phosphorus, which is also a group V element. The numerous vanadate species can be seen to result from reactions such as the hydrolysis reaction... 

The vanadate (1, 2), molybdate (1-5), and tungstate (1-3) systems have been described in previous reviews. Although the focus in this chapter is on more recent developments, earlier well-established knowledge is included where needed for perspective and also to present a coherent picture of the hydrolysis behavior of these oxyanions. Equilibria of mono- and polynuclear species are described and information about known structures are given. Some recent work about mixed polyoxoanions is briefly reviewed. 

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... 

Analogously, when a colourless aqueous solution of quinolinium chloride (Q+ CP) is mixed with an almost colourless aqueous solution of Na+V(CO)p the well-formed dark green crystals of the vanadate salt precipitate immediately. In each case, the spontaneous separation of the highly coloured salts is made even more dramatic by the absence of colour in the aqueous mother liquors throughout the course of precipitation. The quantitative effects in these coloured salts are observed as broad absorption bands in the spectral region between 350 to beyond 700 nm when they are dissolved in dichloromethane, or in the diffuse reflectance spectra of the crystalline salts. The correspondence of the band maximum (Act) and... 

The vanadate equilibria are given in Table 7.7. The V02(0H)2 and V03(0H) anions are sorbed on positively charged sites on oxides and silicates at low pH, but sorption decreases with pH as the surface positive charge decreases. Consequently V is quite soluble at high pH and less soluble at low pH. 

In later sections of this paper the influence of other c.t. transitions will show to be of importance too. These may be of two types, viz. ligand-to-central-metal transitions in the surrounding groups and metal-to-metal transitions. The former type has been discussed and reviewed extensively [13,28). An example of relevance within this type is the UV absorption transition in the vanadate (VO4 ) group Examples of the second type are absorption bands ascribed to electron transfer between Pb2+ and W + [29) and Tb + and V + [30) in oxides. 

Finally it may be mentioned that other c. t. states can also play a role in the temperature quenching. The absence of Tb + emission in YVO4 has been ascribed to the presence of a low-lying metal-metal c.t. state in which one of the Tb + electrons is transferred to the vanadate group (formally written as Tb4+-1-V4+). (30, 83). Assuming that this c.t. state has a large Franck-Condon shift it is easy to explain the absence of Tb + luminescence. Because one of the 4/ electrons of Pr + is also easily excitable, similar phenomena are expected for Pr +. In fact Pr + in YVO4 luminesces only very weakly. 

Our second example concerns Eu3+ in YPO4. In this lattice Eu3+ absorbs 254 nm only to a minor cimount. If part of the phosphorus is replaced by vanadium [(Y,Eu)Pi a Va 04] the radiation is absorbed by the vanadate group and transferred to the Eu3+ ion. 

Both mechanisms discussed above involve an acidic species, and differ primarily in whether that species behaves as a Bronsted or Lewis acid. The vanadic acid approach treats the Y species as a proton donor whereas the oxygen abstraction implies electrophilic attack by V. In either case, it is easy to see how oxygen rich, basic oxides such as MgO function as Y passivators. 

The vanadate(V) anions give rise to numerous salts. Among the most common are the orthovanadates (V04 ), metavanadates and... 

The portion of this work on the vanadates has been supported by the Department of Energy, Basic Energy Sciences, Division of Chemical Sciences, and that on the heats of reoxidation by the National Science Foundation. PM acknowledges support by the Battelle s NASA Advanced Materials Center for the Commercial Development of Space, and LO acknowledges fellowship support from the 3M Company and a Faculty Minority Internship from the Monsanto Company. 

In May, 1830, a careful comparison of vanadium and uranium was made in Berzelius laboratory. It was found that vanadium forms two series of compounds, the vanadic and the vanadous, but Berzelius and Sefstrom did not succeed in isolating the metal. 

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