Tantalum complexes oxidized

Tantalum and niobium are added, in the form of carbides, to cemented carbide compositions used in the production of cutting tools. Pure oxides are widely used in the optical industiy as additives and deposits, and in organic synthesis processes as catalysts and promoters [12, 13]. Binary and more complex oxide compounds based on tantalum and niobium form a huge family of ferroelectric materials that have high Curie temperatures, high dielectric permittivity, and piezoelectric, pyroelectric and non-linear optical properties [14-17]. Compounds of this class are used in the production of energy transformers, quantum electronics, piezoelectrics, acoustics, and so on. Two of... 

The processing of tantalum and niobium begins with the fluorination of the raw material, which always consists of complex oxide compounds containing tantalum and niobium. The main types of tantalum- and niobium-containing minerals are discussed in Chapter 1, and typical compositions of such minerals are presented in Table 2. 

In both cases, the fluorination of the complex oxides of tantalum and niobium leads to the formation of the water-soluble compounds (NH4)2TaF7 and (NH4)3NbOF6, the insoluble lithium fluoride and die gaseous components H20, NH3 and HF. 

Since the fluorination enables to separate components of complex oxide compounds containing tantalum and niobium, it seems that it is applicable for... 

TaCl593 reacted with metallic sodium in neat trimethylphosphine to give the phosphinocarbene tantalum complexes 100 and 101, respectively. These reactions are the first examples of double activation of coordinated trimethylphosphine via oxidative cleavage of a substituent methyl C-H bond. A similar process was also observed in the reduction of tantalum pentabrom-ide with magnesium turnings in the presence of dimethylphenylphosphine.94... 

Not many catalyzed processes involving free radicals are known with these metals. Some vanadium-catalyzed pinacol coupling reactions were developed (reviews [129, 171], [172, 173] and cited ref, [174]). Niobium and tantalum complexes were applied in pinacol coupling reactions [130]. Vanadium(IV) [175-179] and vanadium(V) ([129], reviews [180-186]) complexes are known to catalyze asymmetric oxidative dimerizations of phenols and naphthols in moderate to excellent ees applying oxygen as the terminal oxidant. Biaryls are accessible by intramolecular coupling of sodium tetraarylborates, catalyzed by EtOVOCl2 in the presence of air [187]. 

Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex shown in Fig. 2.4, are often referred to as Fischer carbenes. The other type of alkylidene complexes has the metal ion in a high oxidation state. The tantalum complex is one such example. For both the types of alkylidene complexes direct experimental evidence of the presence of double bonds between the metal and the carbon atom comes from X-ray measurements. Alkylidene complexes are also formed by a-hydride elimination. An interaction between the metal and the a-hydrogen atom of the alkyl group that only weakens the C-H bond but does not break it completely is called an agostic interaction (see Fig. 2.5). An important reaction of alkylidene complexes with alkenes is the formation of a metallocycle. 

In contrast, Schrock carbenes are electron deficient [10 to 16 valence electrons (VE)] early transition metal complexes with the metal atom in a high oxidation state and carbene substituents that are limited to alkyl groups and hydrogen [131]. Their bonding situation can be described in terms of the interaction of a triplet carbene with a triplet metal fragment resnlting in a covalent double bond [132], Tantalum complexes like [(np)3Ta=CHBu ] and [Cp2(Me)Ta=CH2] are representative of Schrock carbenes. 

Properties A complex oxide of sodium, calcium, and niobium. Tantalum, rare-earth metals, and other elements may be present. Color brown to black, light brown streak. Hardness 5-5.5, d 4.2-6.4. 

Another type of carbene complex is characterized by a high oxidation state of the central metal and an a-carbon atom that does not usually bear a hetero atom. It is called an alkylidene complex or Schrock-type complex, since R. R. Schrock first synthesized a tantalum complex of this type [14]. Formation of the tantalum carbon double bond is based on the a-elimination reaction of a neopentyl ligand as shown in eq.(2). 

Class 4 structures with octahedral anionic groups - complex oxides containing octahedrally coordinated titanium, niobium, tantalum. 

Samarskite-(Y) is a complex oxide of rare earths, uranium, calcium, niobium, tantalum and other elements it always occurs in the metamict state. This characteristic has been an obstruction to the study of the chemistry and crystal structure of this mineral. Although many chemical formulae and crystallographic data for samar-skite-(Y) have been proposed, no complete chemical or structural understanding of samarskite-(Y) has been achieved. 

The high Ni impurity content is significant and results from the dissolution of the inner current lead made from Ni which diffused through pores of the ceramic anode body. The increased content of Ca in tantalum produced in electrolysis of CaCl2 CaO melt is connected with the entrainment of complex oxides containing CaO. A great advantage of the electrolysis process with a ceramic anode over that with a carbon anode is the production of tantalum with a relatively low carbon content. 

Fig.2, this influence spreads to the character of subsequent change of the potential. Currentless transport is commonly due to the presence in the melt of two different valencies of the metal deposited [3]. According to the voltammetric studies of various authors [2,4-8], in melts of different compositions Ta(V) ions are discharged in one 5-electron stage, Le. only the pentavalent form is stable. However there is both indirect [7] and direct [9,10] evidence of the presence of tantalum ions in molten alkali metal halides in a lower oxidation state. Our findings also prove the presence of low-valent tantalum ions in the melts. The different stability of low-valent tantalum complexes in melts of varying compositions results in different concentrations of tantalum ions of the lower oxidation state. The fluoride ion... 

Panda et al. prepared an aqueous solution precursor for a ferroelectric tantalum com-poimd, SrBi2Ta209 (Panda etal., 2003). This solution was synthesized using tantalum(V)-tartarate complex, which were prepared by slowly dissolving the hydrous tantalum(V) oxide in an aqueous solution of tartaric acid on heating. Into the aqueous solution of the tantalmn-tartarate complex, the other components (Sr-EDTA and Bi-triethanolamine) and an excess amoimt of triethanolamine were added. The resultant solution was red colored and homogeneous. The solution was dried and then calcined, crystallization occurring at 700°C. 

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