Titanium coordination number

Stmctures are highly varied among the transition metals. The titanium atom in titanium tetraethoxide has the coordination number 6 (Fig. 1). The corresponding zirconium compound, with coordination number 8, has a different stmcture (Fig. 2). Metal alkoxides are colored when the corresponding metal ions are colored, otherwise they are not. 

Reactions with Alcohols. The tendency of titanium(IV) to reach coordination number six accounts for the rapid exchange of alkoxy groups with alcohols. Departure of an alkoxy group with the proton is the first step in the ultimate exchange of all four alkoxyls. The four-coordinated monomer is expected to react... 

The orange-red titanium acetylacetone chelates are soluble in common solvents. These compounds are coordinately saturated (coordination number equals 6) and thus much more resistant to hydrolysis than the parent alkoxides (coordination number 4). The alkoxy groups are the moieties removed by hydrolysis. The initial product of hydrolysis is beheved to be the bis-hydroxy bis-acetylacetone titanate, (HO)2Ti(acac)2, which oligomerizes to a... 

The 1-alkoxytitatranes can be synthesized by the reaction of equimolar amounts of tetraalkyl titanates and triethanolamine (105). X-ray crystallographic analysis of the soHd isolated from the reaction of one mole of triethanolamine and one mole of TYZOR TPT confirms the stmcture as a centrosymmetric dimer having a Ti isopropoxy nitrilotriethoxy ratio of 1 1 1. The titanium atoms have achieved a coordination number of six via a rather unsymmetrical titanium—oxygen bridge (106). 

The i5p-titanium(IV) atom is hard, ie, not very polarizable, and can be expected to form its most stable complexes with hard ligands, eg, fluoride, chloride, oxygen, and nitrogen. Soft or relatively polarizable ligands containing second- and third-row elements or multiple bonds should give less stable complexes. The stabihty depends on the coordination number of titanium, on whether the ligand is mono- or polydentate, and on the mechanism of the reaction used to measure stabihty. 

Titanium tetrachloride and tin tetrachloride can form complexes that are related in character to both those formed by metal ions and those formed by neutral Lewis acids. Complexation can occur with an increase in the coordination number at the Lewis acid or with displacement of a chloride from the metal coordination sphere. 

The anhydrous nitrates can be prepared by the action of N2O5 on MCI4. Ti(N03)4 is a white sublimable and highly reactive compound (mp 58°C) in which the bidentate nitrate ions are disposed tetrahedrally around the titanium which thereby attains a coordination number of 8 (Fig. 21.4). Infrared evidence suggests that Zr(N03)4 is isostmctural but hafnium nitrate... 

Another type of notation, introduced by P. Niggli, uses fractional numbers in the chemical formula. The formula Ti06/, for instance means that every titanium atom is surrounded by 6 O atoms, each of which is coordinated to 3 Ti atoms. Another example is NbOCl3 = NbC C C /i which has coordination number 6 for the niobium atom (= 2 -)- 2 + 2 = sum of the numerators), coordination number 2 for the O atom and coordination numbers 2 and 1 for the two different kinds of Cl atoms (cf. Fig. 16.11, p. 176). 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

Phosphorus compounds, 19 19-73 bond properties of, 19 26 chemical properties of, 19 20-31 chiral-centered, 19 25-26 economic aspects of, 19 67-69 as flame retardants, 19 51 inorganic, 11 487-488 oxidation states, coordination numbers, and geometries of, 19 20-26 as oxyacid derivatives, 19 20 reactive organic, 11 496 497 titanium in, 25 56-57 triply connected, 19 25 U.S. prices of, 19 68t U.S. production of, 19 67t... 

Rutile structure (Fig. 4-15). Titanium dioxide occurs naturally as ana-tase, brookite, and rutile, all of which contain octahedral TiC>68 units. The coordination number of the central Ti4+ is very obviously six, and a little thought confirms that the same is true of the Ti4+ ions at the corners. That the coordination number of the O2- ions is three is seen from the nearest... 

The most common coordination number of titanium is six (recognized for all oxidation states of the metal), although compounds are known in which the coordination number is four, five, seven or eight. The common oxidation states of titanium with the associated coordination numbers and stereochemistries are summarized in Table 3. The properties of these molecules will be discussed in the appropriate sections. In brief, however, titanium compounds in the +III or lower oxidation states are readily oxidized to the +IV state. Furthermore, titanium compounds can usually be hydrolyzed to compounds containing Ti—O linkages. 

Perhaps the best studied group of titanium(IV) complexes is the alkoxides. The metal alkoxides generally have received a great deal of attention because of their ease of hydrolysis and reactivity with hydroxylic molecules, and their tendency to increase the coordination number of the metal which is opposed by the steric effect of the alkyl group. These properties result in materials, the characteristics of which range from polymeric solids to volatile liquids. The definitive review of this area is that by Bradley. ... 

Table 11 Titanium-Chlorine Vibrations for some Selected Compounds—a criterion for Coordination Number...

Table 12 Charge-transfer Spectral Data for some Selected Titanium(III) and Titanium(IV) Complexes — a Possible Criterion for Coordination Numbers...

The rutile structure. Titanium dioxide crystallizes tn three crystal forms at utmospheric pressure anatase, brookite, and rutile (Fig. 4.4a). Only the last (tetragonal P42/mnin) will be considered here. The coordination numbers are 6 for the cation (six oxide anions arranged approximately octahedrally about the titanium ions) and 3 for the anion (three tiianium ions trigonally about the oxide ions). The rutile structure is also found in the dioxides of Cr, Mn, Ge, Ru, Rh, Sn, Os. Ir, Pt. and Pb. 

The chemistry of zirconium has some similarities to that of silicon and titanium, since it is in Group IV of the periodic table. Zirconium has a normal oxidation state of 4 with limited redox chemistry and a coordination number of up to 8. Zirconium compounds are normally colourless. 

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