Titanium anionic complexes

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 ... 

A chiral titanium(IV) complex has also been used by Wada et al. for the intermole-cular cycloaddition of ( )-2-oxo-l-phenylsulfonyl-3-alkenes 45 with enol ethers 46 using the TADDOL-TiX2 (X=C1, Br) complexes 48 as catalysts in an enantioselective reaction giving the dihydropyrans 47 as shown in Scheme 4.32 [47]. The reaction depends on the anion of the catalyst and the best yield and enantioselectivity were found for the TADDOL-TiBr2 up to 97% ee of the dihydropyrans 47 was obtained. 

In polymerizing these compounds, a reaction between a-TiCls and triethylaluminum produces a five coordinate titanium (111) complex arranged octahedrally. The catalyst surface has four Cl anions, an ethyl group, and a vacant catalytic site ( ) with the Ti(lll) ion in the center of the octahedron. A polymerized ligand, such as ethylene, occupies the vacant site ... 

Titanium imido complexes supported by amidinate ligands form an interesting and well-investigated class of early transition metal amidinato complexes. Metathetical reactions between the readily accessible titanium imide precursors Ti( = NR)Cl2(py)3 with lithium amidinates according to Scheme 84 afforded either terminal or bridging imido complexes depending on the steiic bulk of the amidinate anion. In solution, the mononuclear bis(pyridine) adducts exist in temperature-dependent, dynamic equilibrium with their mono(pyiidine) homologs and free pyridine. 

Aminopyridinato ligands form a special class of anionic ligands in which an aromatic ring is part of an amidinate system. These ligands have frequently been employed in early transition metal and lanthanide coordination chemistry. Their diverse and interesting chemistry has been described in detail by Kempe et al. ° and will thus be covered here only briefly. Typical reaction pathways leading to titanium aminopyridinato complexes are outlined in Scheme 169. Metathetical as well as salt-free routes have been developed. 

The anionic polymerization of isocyanates using NaCN as an initiator was first reported in I960.998 The living coordinative polymerization of n-hexylisocyanate has been described using the titanium(IV) complexes (345) (348).999-1001 A trifunctional initiator has also been used to prepare star polyisocyanates.1002... 

If it is necessary to extract the rare metal from an acid or alkaline leach liquor, or similar solution, a resin should be selected which is known to be stable in the leaching reagent. For extraction of the rare metal ion itself, the resin would normally be of the cation-exchange type, unless the rare metal ion could be converted to an anionic complex. However, the possibility of designing a process in which the impurities are absorbed by the resin and the rare metal remains unextracted, should not be neglected. An example of this type has been developed by Ayres,i > in which iron, titanium, lanthanum and beryllium impurities are extracted from a zirconium nitrate solution by operation at a pH where the zirconium is converted to a non-ionic hydrated oxide sol. 

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