Vanadyl ion

Divalent copper, cobalt, nickel, and vanadyl ions promote chemiluminescence from the luminol—hydrogen peroxide reaction, which can be used to determine these metals to concentrations of 1—10 ppb (272,273). The light intensity is generally linear with metal concentration of 10 to 10 M range (272). Manganese(II) can also be determined when an amine is added to increase its reduction potential by stabili2ing Mn (ITT) (272). Since all of these ions are active, ion exchange must be used for deterrnination of a particular metal in mixtures (274). 

The 4-4 oxidation state, which for Nb and Ta is best represented by their halides, is most notable for the uniquely stable VO + (vanadyl) ion which retains its identity throughout a wide variety of reactions and forms many complexes. Indeed it is probably the most stable diatomic ion known. The M ions have only slightly smaller radii... 

In the higher oxidation states partially hydrolysed species dominate the aqueous chemistry, the most important being the oxovanadium(IV), or vanadyl, ion This gives the sul-... 

The blue [(H20)5V( = 0)] + ion is the vanadyl ion which is usually depicted as VO. Actually, the vanadium center is still sufficiently polarizing that a third deprotonation equilibrium is established in aqueous media to generate the ion [(H20)4V(=0)(0H)]+, which contains water, hydroxy and oxo ligands (Eq. 9.19). 

V0)2+ ion in a square pyramidal coordination. On alumina only the latter configuration is stable. Tanaka and Matsumoto IBS) attributed a poorly resolved spectrum on silica gel to the formation of small crystallites of a V02+ salt. Upon dilution with an alkali salt the resolution increased. They also observed much better resolution on alumina, indicating that the vanadyl ion is more uniformly dispersed on this surface. 

Table 3 Redox potentials (V vs. SCE) for the one-electron oxidation of Schiff base complexes of vanadyl ion...

For the vanadyl ion adsorbed on the various substrates examined, the g-factors were found constant, however, the hfs intervals varied, being largest on sulfonic acid-type cation exchangers, slightly smaller on charcoal and considerably smaller on an amine-type anion exchanger. 

The hydroxo and olated species indicated would be expected to have little or no activity, so that the observed rate would be expected to correlate with the concentration of the aquo vanadyl ion. 

Correlation of the observed rates with the concentrations of the substrate species (10) indicates that the metal ion does not catalyze the hydrolysis of the monoanionic form of salicyl phosphate. Combination of the monoanionic form of the substrate with the vanadyl ion would result in an unreactive complex having a neutral carboxyl group. Shift of the proton to the phosphate group could not take place in accordance with the requirements of the general reaction mechanism illustrated in Figure 3. Thus the vanadyl ion would be expected to catalyze the hydrolysis of only the di- and trinegative forms of the substrate. 

The +4 oxidation state is the most stable under ordinary conditions and the majority of vanadium(IV) compounds contain the V02+ unit (vanadyl ion), which can persist through a variety of reactions. Its complexes typically have square pyramidal or bipyramidal geometry... 

Because the vanadyl ion is lopsided, its chelates have the possibility of forming a greater number of isomers than monatomic ions. For example, for an N,0-coordinating chelate (e.g. glycine), two enantiomers, (68a) and (69a), may form (S = solvent). 

The only i 1 species of importance is the vanadyl ion. VO2 which is the most stable foim of vjnadium in aqueous solution. 

There is now an appreciable collection of structural, spectral, and magnetic data available for the vanadyl ion and its complexes. Therefore it seems desirable to develop a theory of the electronic structure of V02+ Which will be consistent with its physical properties. It also might be hoped that an understanding of the principal features of the bonding in V02+ will be a hdpful guide in attempts to develop a general theory of the electronic structures of MO + and M03n+ complexes. 

Structure of the Vanadyl Ion Complexes.— The structures of at least three different compounds containing vanadyl ion have been determined by X-ray methods. It is significant that V03 crystallizes in a highly distorted rutile (TiC ) structure, in which there is one conspicuously short V-O bond (1.76 A.) in each VOe unit.8 Thus there seems to be a greater driving force to... 

The electrostatic modd for the hydrated vanadyl ion consists of V,+ situated in a tetragonal dectric fidd caused by the oxide ion and five water dipoles. The ctystal fidd energy levd diagram for such a situation is given in Fig. 1. 

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