Tantalum complexes structure

Two kinds of tantalum-containing initial solutions were chosen according to their ionic complex structure. The first one contained mostly TaF6 ions (Ta F = 1 18) while the second was characterized predominantly by TaF72 ions (Ta F = 1 6.5). The ionic composition of the solutions was determined by Raman spectroscopy. 

NMR, Raman and IR spectroscopy are most frequently used to investigate the complex structures of fluoride solutions containing tantalum and niobium. Most investigations of such solutions were performed on the liquid-liquid extraction of tantalum and niobium, with the objective of describing the mechanism of the process. These publications will be discussed separately. 

Tantalum powder is produced by reduction of potassium heptafluoro-tantalate, K2TaF7, dissolved in a molten mixture of alkali halides. The reduction is performed at high temperatures using molten sodium. The process and product performance are very sensitive to the melt composition. There is no doubt that effective process control and development of powders with improved properties require an understanding of the complex fluoride chemistry of the melts. For instance, it is very important to take into account that changes both in the concentration of potassium heptafluorotantalate and in the composition of the background melt (molten alkali halides) can initiate cardinal changes in the complex structure of the melt itself. 

Agulyansky et al. [492, 493] investigated the complex structure and composition of solid phases precipitated by ammonia solution from experimental and industrial niobium and tantalum strip solutions. Fig. 136 shows isotherms (20°C) of Nb205 content versus pH for solutions prepared by the dissolution of (NH4)3NbOF6 and (NH4)2NbOF5 in water and of Nb metal in... 

Nevertheless, the precipitation process can be specified based on data available on the complex structure of the strip solutions. In the case of tantalum strip solutions, the interaction can be written as follows ... 

The uniformity of tantalum powder is also a veiy important parameter of capacitor-grade tantalum powder. The loss of powder uniformity can initiate during the regular reduction process due to varying conditions at the beginning and end of the reduction process. At the end of the process, the concentration of tantalum in the melt is very low, while the sodium content increases. Based on the complex structure model of melts, it should be noted that the desired particle size of the powder is formed at the veiy beginning of the process, while the very fine fraction forms at the end of the process, independent of the initial content of the melt. The use of special equipment enables to perform a continuous reduction process with simultaneous loading of K TaFy and sodium, which can influence the improved uniformity of the primary powder [592,603,604],... 

Probably the first isolated tungsten alkylidene complex active in metathesis and completely characterised is the one shown in Figure 16.10 reported by Wengrovius and Schrock the analysis included an X-ray structure determination by Churchill and co-workers [18], The alkylidene was transferred from a tantalum complex to yield the hexacoordinate tungsten complex containing two PEt3 ligands. One of these can be removed by the addition of half an equivalent of palladium chloride. The total turnover number of these catalysts with Lewis acids added was 50 in 24 hours. 

Low-valent niobium and tantalum react with imines to give the corresponding imine-niobium or -tantalum complexes. Some of these are isolated, and structures of the complexes are confirmed by X-ray analyses. These imine complexes react with aldehydes to give /3-hydroxyamines in good to excellent yields (Equation... 

Alkilidine tantalum complexes have also been utilized for the preparation of ni-trido Tav. Reaction of ammonia with 31 or 32 afforded nitride 33 in moderate yields [14]. The structure of 33 has been determined by single-crystal X-ray crystallography and shown to consist of a pentameric complex, as depicted in Eq. (9). All of the Ta-N bond lengths are equivalent (2.00 A), suggesting a structure with extensive delocalization of the multiple bonds. 

Several borollide complexes and cluster complexes have been prepared and structurally characterized by X-ray crystallography. These include borollide-tantalum complexes <1995JA2671, 1995POL93, 1996JA10317,... 

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