Titanium valence states

The type of cleavage of the alkyl titanium bond is certainly dependent upon the titanium valence state. Tetravalent compounds will tend to cleave homolytically, but heterolytic cleavage will become more favorable at the lower valence states because of higher bond polarity (290). Ethylene polymerization takes place more readily on alkyl vanadium compounds than on alkyl titanium compounds and yields higher molecular weight linear polymer (340). This is attributable to the fact that... 

Zirconium [7440-67-7] is classified ia subgroup IVB of the periodic table with its sister metallic elements titanium and hafnium. Zirconium forms a very stable oxide. The principal valence state of zirconium is +4, its only stable valence in aqueous solutions. The naturally occurring isotopes are given in Table 1. Zirconium compounds commonly exhibit coordinations of 6, 7, and 8. The aqueous chemistry of zirconium is characterized by the high degree of hydrolysis, the formation of polymeric species, and the multitude of complex ions that can be formed. 

Physical properties of binary or ternary Ru/Ir based mixed oxides with valve metal additions is still a field which deserves further research. The complexity of this matter has been demonstrated by Triggs [49] on (Ru,Ti)Ox who has shown, using XPS and other techniques (UPS, Mossbauer, Absorption, Conductivity), that Ru in TiOz (Ti rich phase) adopts different valence states depending on the environment. Possible donors or acceptors are compensated by Ru in the respective valence state. Trivalent donors are compensated by Ru5+, pentavalent acceptors will be compensated by Ru3+ or even Ru2+. In pure TiOz ruthenium adopts the tetravalent state. The surface composition of the titanium rich phase (2% Ru) was found to be identical to the nominal composition. 

The above techniques have a wide array of applications, including those that are both analytical and physicochemical (such as bonding) in nature. Typical examples of research include the surface chemistry of ferrite minerals (38) and the valence states of copper in a wide array of copper (39) minerals. Other areas of bonding that have been studied include the oxidation state of vanadium (40) in vanadium-bearing aegirities (also using x-ray photoelectron spectroscopy) and the. surface features of titanium perovskites (41). ... 

This reaction can be carried out with numerous variations to give a broad range of catalysts. It is a heterogeneous high-surface TiCIs material of which the active sites contain titanium in an unknown valence state. It is quite likely that alkyltitanium groups at the surface are responsible for the co-ordination polymerisation. In more recent catalysts titanium supported on magnesium salts are used [4,5],... 

An interesting deoxygenation of ketones takes place on treatment with low valence state titanium. Reagents prepared by treatment of titanium trichloride in tetrahydrofuran with lithium aluminum hydride [205], with potassium [206], with magnesium [207], or in dimethoxyethane with lithium [206] or zinc-copper couple [206,209] convert ketones to alkenes formed by coupling of the ketone carbon skeleton at the carbonyl carbon. Diisopropyl ketone thus gave tetraisopropylethylene (yield 37%) [206], and cyclic and aromatic ketones afforded much better yields of symmetrical or mixed coupled products [206,207,209]. The formation of the alkene may be preceded by pinacol coupling. In some cases a pinacol was actually isolated and reduced by low valence state titanium to the alkene [206] (p. 118). 

Titanium in a low valence state, as prepared by treatment of solutions of titanium trichloride with potassium [206] or magnesium [207] in tetrahydro-furan or with lithium in dimethoxyethane [206], deoxygenates ketones and effects coupling of two molecules at the carbonyl carbon to form alkenes, usually a mixture of both stereoisomers. If a mixture of acetone with other ketones is treated with titanium trichloride and lithium, the alkene formed by combination of acetone with the other ketone predominates over the symmetrical alkene produced from the other ketone [20(5] Procedure 39, p.215). 

For the reduction to pinacols aluminum amalgam [825] or low valence state titanium chloride [207] were used. Under different conditions the titanium... 

It should be noted in this connection that, for ethylene polymerisation in the presence of a non-supported catalyst such as TiCl4-AlR3 (R = Et, z -Bu), polymerisation proceeds at the maximum rate when the average valence state of titanium is 2 [304], However, while catalysts with divalent titanium compounds as precursors are known to be highly active for the polymerisation of ethylene, their activities for the polymerisation of a-olefins are generally low [51,240],... 

In titanium, at the left end of the transition series, the 3d states will be much more like. S and p valence states and will contribute to the bonding. Even at the far left of the 5/ series, the / levels will behave as valence states and will form bands in metallic thorium (Freeman and Koelling, 1974). These two trend.s—a decrease in the extent of localization as we move left in each series and as we move down in the periodic table constitute the two most important trends in the transition series. They will show up in different ways in different systems, but have the same origin. 

We turn now to a very important class of materials that have the formula /IBC3, with the C frequently oxygen. Strontium titanate is a familiar example and one we shall use for illustrative purposes. Titanium is in the D4 column of the Solid State Table, having four electrons beyond its argonlike core. Strontium has two electrons outside its kryptonlike core, so we may think of the six valence electrons as having been transferred to the three oxygen atoms to form a simple ionic system. As we shall see, however, the titanium d states form the lowest conduction band and are important in the bonding properties as well. 

Reagents such as magnesium, or low-valency states of titanium dissolving in acid, function by donating an electron to the carbonyl group to form a radical anion. The reductive process may be completed by the dimerization of these radicals to form 1,2-diols (pinacols) in the case of magnesium or alkenes in the case of titanium. 

Transition metal oxides exhibit a number of properties that are conducive to catalytic applications, including thermal and mechanical stability needed to survive severe reaction conditions. More importantly, transition metal cations can typically exist in several different valence states. Titanium dioxide has a bulk band gap energy of about 3.2 eV, but electrons can be placed in (3d) gap states... 

The catalysts are usually prepared in hydrocarbon solvents, essentially in the absence of air or moisture and are mixtures of ill-defined composition. In many instances dark-coloured precipitates are formed of variable stoichiometry containing complexes of the organo-metal compound with the transition metal in a lower valence state. Natta [5] showed that pure lower valence transition metal compounds, such as titanium or vanadium trichloride, when treated with organo-metal compounds were effective catalysts, and were particularly suitable for the preparation of crystalline high melting point polyolefins. The close identity of these two classes of catalyst has led to their description as... 

Titanium tetrachloride and aluminium triethyl form a hydrocarbon soluble complex at low temperatures which decomposes at —30°C to give the trichloride as a major product [32]. Complexes containing tetravalent titanium stabilized by adsorption on titanium trichloride apparently persist in catalysts prepared at Al/Ti ratios below 1.0 [33], but at higher ratios there are some Ti(II) sites present in the catalyst [34]. Analysis shows that at Al/Ti ratios above 1.0 the solid precipitate contains divalent titanium or even lower valency states of the metal [35]. Reduction of TiCl4 with AlEt2 Cl is less rapid and extensive than with AlEts and even at high Al/Ti ratios [36] reduction does not proceed much below the trivalent state. Aluminium alkyl dihalides are still less reactive and reduction to TiClj is slow and incomplete except at high Al/Ti ratios or elevated temperatures [37]. 

The incorporation on a cation site of an impurity cation of higher charge than the host cation can stabilize a lower valence state of the host cation. The addition of titanium (as Ti02> to the spinel, zinc ferrite, leads to an B-type semi-conductor of this type of formula, Zn (Ti + Fe t28 Fe ) O. The defect centre may be described as an impurity cation of higher relative charge on a cation site plus a quasi-free electron bound on a neighbouring host cation. A necessary condition for the application of the prindple of controlled valency is that the impurity cation should be of much the same size as the host cation. 

For its application as a cold conductor, small amounts of barium or titanium are substituted by higher valency state ions such as antimony, niobium or the rare earths together with traces of copper, iron or manganese. Such cold conductors exhibit a strong increase in specific resistance (10 to 100 Q-cm) with temperature e.g. an increase of a factor of lO-" to 10 between 50 and 100°C. 

In the chloride process natural rutile ore or synthetic rutile with a Ti02-content of up to 96% is chlorinated in a fluidized bed reactor with oil-coke and chlorine. The raw TiCl4 produced is mixed with reducing agents to convert impurities, such as vanadium oxychloride, to lower valency state vanadium compounds. The titanium tetrachloride formed is then distilled yielding titanium tetrachloride in... 

The fate of the alkyl fragment [R ] remains uncertain. There is still controversy concerning the precise extent of reduction that is reached with trialkylaluminum compounds at different ratios of aluminum to titanium. However, scientists generally agree that the more active organoraetallic compounds such as trialkylaluminum provide more extensive alkylation, and reduction of the intermediate 6-form of TiCl3 may occur (Reactions 7-10). This reduction process leads to lower valence states presumable by reactions of the following type ... 

The basic feature of proposals for the monometallic mechanism is that propagation occurs entirely at one metal center. A monometallic mechanism involving titanium in a lower valence state, for example, RTiCl, has been proposed (63) to be an active site for ethylene polymerization with propagation occurring by coordination and insertion into the titanium-carbon bond (Reaction 12). 

Some of the steps in the above sequence of reactions are reduction steps in which the transition metal is reduced to a low valency state possessing unfilled ligand sites. The reduction steps are very important as the low-valency transition metal species are believed to be the real catalysts or precursors of real catalysts. For heterogeneous catalysts, the reactions are, in fact, more complicated than those shown above. Radicals formed in these reactions may be removed by different processes such as combination, disproportionation, or reaction with solvent. Unlike heterogeneous catalysts, the soluble catalysts appear to have well defined structures. For example, the soluble catalyst system that is obtained by the reaction of triethyl aluminum and bis(cyclopentadienyl)titanium dichloride is known by elemental and X-ray analysis to have a halogen-bridged structure (I) ... 

Compounds containing CO or NO are poorly established, presumably because of the lack of re-bonding electron density on titanium. The dicarbonyl (/25-C5H5)2Ti(CO)226 is reasonably stable like [Ci0H10Ti]2 it undergoes, with loss of CO, a number of oxidative addition (Chapter 24) reactions. The lower valence states for titanium of 0 and — 1 are known only in the bipyridine complexes such as TTbipy3.27... 

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