Niobium pentachloride Preparation

Niobium Pent chloride. Niobium pentachloride can be prepared in a variety of ways but most easily by direct chlorination of niobium metal. The reaction takes place at 300—350°C. Chlorination of a niobium pentoxide—carbon mixture also yields the pentachloride however, generally the latter is contaminated with niobium oxide trichloride. The pentachloride is a lemon-yeUow crystalline soHd that melts to a red-orange Hquid and hydrolyzes readily to hydrochloric acid and niobic acid. It is soluble in concentrated hydrochloric and sulfuric acids, sulfur monochloride, and many organic solvents. 

Also, niobium pentafluroide can be prepared by the reaction of fluorine with niobium pentachloride ... 

Niobium Pentafluoride, NbFs, is the only known compound of niobium and fluorine, and even this cannot be obtained in the free state by a wet method because of the extreme readiness with which it hydrolyses. Niobium pentoxide dissolves readily in hydrofluoric acid, but evaporation of the solution leaves a residue of the unchanged oxide. Niobium pentafluoride has been prepared synthetically 1 by passing dry fluorine over the gently heated metal contained in a boat in a platinum tube. The product is freed from platinum tetrafluoride, a little of which is formed at the same time, by distillation in vacuo at 100° to 110° . An alternative method consists in treating niobium pentachloride with anhydrous hydrogen fluoride in a freezing mixture and purifying by redistillation.3... 

Niobium Trichloride, NbCl3, is prepared by leading the vapour of niobium pentachloride through a heated tube.4 It is also formed in small quantity by the action of carbon tetrachloride vapour on niobium pentoxide contained in a hard-glass tube, and has probably been prepared in solution by the electrolytic reduction of the pentachloride.4 It forms a black, crystalline crust with an almost metallic lustre, which closely resembles the appearance of a film of sublimed iodine. It is not decomposed by water or ammonia, but is readily oxidised by dilute nitric add to niobium pentoxide. On being heated to a red heat in an atmosphere of carbon dioxide, a sublimate of niobium oxytrichloride, NbOCl3, is produced, the carbon dioxide undergoing reduction to the monoxide. 

Niobium Pentachloride, NbCl5, is the most important of the chlorides of niobium, and is the material from which the other chlorides are prepared. It can be obtained by several methods ... 

Niobium oxytrichloride is prepared by the action of chlorine on a mixture of niobium pentoxide and carbon, or on the lower oxide, NbO 5 or by the action of the vapours of carbon tetrachloride on the pentoxide. Some niobium pentachloride is produced in the same reaction, and this is removed either by distilling it away at the lowest temperature possible in an atmosphere of carbon dioxide, or by subliming the product in a current of chlorine over the ignited oxide.7 The oxychloride has also been prepared by passing the vapours of niobium pentachloride over niobium pentoxide at a red heat.8... 

Delafontaine and Linebarger, J. Amer. Ohem. Soc., 1896, 18, 532 Ohem. News, 1896, 74, 33 oompare preparation of niobium pentachloride. 

In addition to the obvious preparative advantages inherent in the pentachloride preparation, e.g., the low temperature of reaction and the possibility of performing the reaction without precautions against atmospheric moisture because of the protection afforded by thionyl chloride, the product is obtained free from oxide chloride. The major losses occur during the hydrous oxide precipitation and the nitric acid washings. The latter are essential to remove adsorbed ammonium ion, since, if this is not done, the reaction products will be niobium (V) chloride, in solution in thionyl chloride, and the bright yellow insoluble ammonium hexachloroniobate(V). In fact, the high purity of these two products in instances where complete removal of ammonium ion is not achieved shows clearly that the reaction of hydrous niobium (V) oxide with thionyl chloride is virtually quantitative. 

Niobium pentachloride is available commercially from Alfa Inorganics, Ltd. The checkers found that sublimation of the commercial material in vacuo was necessary before use in the preparation. 

The full report of the preparation and properties of two octacyanoniobates has appeared (Vol. 4, p. 75). Impure K5[Nb(CN)8] can be obtained from the reaction of an excess of aqueous potassium cyanide with an electrolytically reduced solution of niobium pentachloride in methanol. Oxidation of the product by air or H2O2 gives orange K4[Nb(CN)8],2H20 (see p. 73). Solutions of this salt are stable in the dark but disproportionate photolytically to the Nb complex, Nb20s, and HCN. M5[Nb-... 

All solvents used in these reactions were purified by standard methods and purged with nitrogen immediately before use. The tributyltin hydride was either purchased from Aldrich Chemical Company and distilled prior to use (the checkers noted that they did not distill this material) or prepared from (Bu3Sn)20/polymethylhydrosiloxane. In the latter case we recommend that the tributyltin hydride be immediately redistilled (through a 2-cm x 30-cm Vigreux column) after the initial distillation from the reaction mixture. The niobium pentachloride (Aldrich or Cerac) and the niobium pentabromide (Cerac) were used as received. 

The ionic substances containing niobium(V) that exist in these red solutions have not all been identified. In addition to the ion indicated, species of the form [(i/-CjH,)2Nb(X)Y] (X, Y = Cl, Br, OH, or O—are probably present in an unknown equilibrium with each other, t Niobium pentachloride is available commercially from Alfa Inorganics, Ltd. The chedcers found that sublimation of the commercial material in vacuo was necessary before use in the preparation. 

A number of pentavalent trispyrazolylborate derivatives (see Tris(pyrazolyl)borates) have been reported, especially for niobium. Most of them are monomeric Tp MCl2X (X = O, NR) derivatives, jj and jj -pyrazolato (16) or 1,2,4-triazolato derivatives have been prepared as potential sources of nitrides for CVD. The reactions between dilithium l,4-diaza-l,3-diene and pentachlorides have been investigated. ... 

Numerous methods have been described for the preparation of niobium(V) chloride, among them the reaction of niobium(V) oxide with thionyl chloride in a sealed system. In such a procedure some niobium(V) oxide trichloride, NbOCls, is almost always formed, and it is difficult to obtain the pentachloride completely free from this impurity, even by repeated sublimation. The simple, efficient method described here consists in allowing hydrous niobium(V) oxide to react with thionyl chloride at room temperature. Almost quantitative conversion is observed, the pentachloride dissolving in the thionyl chloride, from which it may be recovered, free of oxide trichloride, by vacuum evaporation... 

This procedure has been used to prepare many other chlorides, e.g., tungsten hexachloride, vanadium tetrachloride, and the pentachlorides of niobium and tantalum. However, it is normally much easier to prepare chlorides than fluorides. This method will therefore find application only where this generalization is not applicable. In particular, it will find application where the chloride is thermally unstable under normal chlorination conditions and must be prepared at or below room temperature. This method has been used to advantage for the preparation of ReCle and the new compound osmium pentachloride. ... 

Tetrachloro(cyclopentadienyl) complexes of niobium and tantalum, and their ring-substituted derivatives can be prepared by two general, methods. One method uses tin reagents of cyclopentadienes and the other uses trimethylsilyl reagents, which are reacted with niobium or tantulum pentachloride. Here the former method (Method 1) is described for the synthesis of Ta(ii -C5Me5)Cl4, and the latter (Method 2) for the synthesis of Nb(T -C5Me5)Cl4. 

Tantalum carbide is very similar structurally and chemically to niobium carbide and is used as a protective coating for tantalum metal. It can be prepared by the CVD of a mixture of elemental Ta metal and methane or a mixture of tantalum pentachloride, methylchloride and hydrogen [10]. 

An analogous reaction carried out on niobium and tantalum pentachlorides is reported to give the carbonyl derivatives [Na(diglyme)2][M(CO)6] (M = Nb, Ta) (63). Similar formation of a sodium derivative of a metal carbonyl as an intermediate may also occur in the following preparations of chromium (42) and manganese (64) carbonyls with elemental sodium as the reducing agent... 

Information on the carbonyl chemistry of niobium and tantalum is, to date, very meager. The main difficulty appears to be the reduction of the usual pentavalent derivatives of these metals to the very low formal oxidation states of metal carbonyl derivatives. Nevertheless, the yellow anions [M(C0)6] (M = Nb, Ta) have been obtained by a method analogous to, but more difficult than, one of the preparations of the [VCCO) ]" anion. The method involves reduction of the pentachlorides with sodium metal in diglyme in the presence of high pressures of carbon monoxide (63). The niobium and tantalum derivatives are much more air-sensitive than the analogous vanadium derivative. The niobium derivative has not yet been obtained analytically pure (63). No chemistry of the [Nb(CO)J and the [Ta(CO)6] ions has been reported, even conversion to the neutral carbonyl derivatives [M(CO) (M = Nb or Ta = 1 or 2) or to the carbonyl hydride derivatives HM(CO)6 (M = Nb, Ta) still presenting unsolved problems. 

A range of other cationic niobium and tantalum tetrakis(dithiocarbamate) complexes, [M(S2CNR R )4]X (M = Nb, Ta R = R = Me, f-Bu R = Me, R = I -Pr, Ph, Cy, Bz), have been prepared from the metal pentachlorides and anhydrous dithiocarbamate salts (719). They are fluxional on the NMR time scale, a polytopal mechanism being proposed via a cubic or square antiprismatic transition state. The kinetics of this process has been studied by total line shape analysis, giving AG values of 42-50 kJ moP. ... 

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