Titanium ionic radii

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

Zirconium, with a larger ionic radius (0.79 A), did not substitute as easily as titanium in the USb OiQ lattice. In only one case, x 1.0, was a pure USb2 Zr 0y phase obtained. The other catalysts contained USbO, Sb2O4, Sb205, and Zr02 type phases. The X-ray diffraction pattern of USb2ZrOy is compared in Table II with the unsubstituted and titanium-substituted phases. As with the titanium catalyst the d-spacing for the 004 reflection was Increased. 

For some period of time zirconium, like titanium(IV), was thought to exist as the zirconyl (ZrO ) ion. However, X-ray crystallographic data of zirconium solids and X-ray scattering data of zirconium solutions have shown conclusively that zirconium binds as the tetravalent ion (Clearfield and Vaughan, 1956 Muha and Vaughan, 1960). Moreover, there is no unequivocal identification of the zirconyl ion in either the solid state or aqueous solution (Brown, Curti and Grambow, 2005). The ionic radius for the Zr ion has been shown to be 0.84 A (Shannon, 1976). 

Titanium has both trivalent and tetravalent states in aqueous solution. The tetravalent data have been discussed in Chapter 10. The ionic radius of the titanium(III) ion is 0.670 A (Shannon, 1976). 

As a result of the high ionic charge to radius ratio of titanium(IV), normal salts of titanium(IV) are difficult to prepare from aqueous solutions these often yield basic, hydrolyzed species. A tris-catechol species, [Ti(cat)3], prepared by Raymond etal. is one exception it is stable in aqueous solution up to pH 12. The catechol ligand is so stabilizing to Ti that the Ti ATi reduction potential is shifted from the value of -1-0.1V cited as the standard potential in acid in Scheme 1 to a value for [Ti(cat)3] of -1.14 V vs. NHE, affording a powerful example of ligand tuning of metal redox potential. 

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