Tantalum zinc compounds

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 ... 

Tantalum compounds in solution do not give a colour change on bring reduced with zinc, and this test also serves to establish niobium in the presence of titanium, which produces a violet coloration.2 Vanadium, molybdenum, and tungsten solutions, however, behave similarly to niobium, and these metals must, therefore, be previously removed. 

The combined action of zinc and tantalum(V) chloride on alkynes generates low-valent tantalum-alkyne complexes, which react with carbonyl compounds to yield ( )-allylic alcohols stereoselectively. The tantalum-6-dodecyne complex, for instance, and j -phenyl-propionaldehyde afford the alcohol 370 in 96% yield. Mixtures of structurally isomeric a,/ -unsaturated ( )-amides 371 and 372 (R, = alkyl or Mc3Si) are obtained from the... 

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Schrock carbenes are typically generated by a-hydrogen elimination from a high-valent, early metal, dialkyl complex concomitant with or followed by elimination of alkane. The first S5mthesis of an alkylidene by Schrock is summarized in Equation 13.13. Addition of neopentyl zinc to tantalum pentachloride generated tris(neopentyl)dichlorotantalum. Subsequent addition of neopentyllithium did not generate the homoleptic neopentyltantalum compound. Instead, this reaction generated the first alkylidene complex, tris(neopentyl) tantalum neopentylidene. ... 

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