Niobium, and Tantalum

Vanadium, Niobium, and Tantalum.—Photochemical reactions of CpV(CO)4 with Group Vb donor ligands have given products of the types CpV(CO)3XRs (X = As, Sb, or Bi R = alkyl, Ph, OEt, etc.f and CpV(CO)2L (L = bidentate diphosphine, diarsine, or arsine-phosphine ligand). The dianion [CpV(C0)3] , prepared by reduction of CpV(CO)4 with Na in THF, has been converted into the isolable salt [(Ph3P)2N]+[CpV(CO)3H], which effects hydro-genolysis of various alkyl, aryl, and acyl halides apparently by a radical-chain mechanism.  

Sequential treatment of CpNbCl4 in THF with PR3, then Mg in the presence of CO (1 atm), gives CpNb(CO) (PR3)2Cl2. The carbene complex CpTa(CHBu )-CI2 has been shown to be an effective catalyst for selective dimerization of alkenes, and has been converted into the carbyne complex CpTa(CBu )- 

Even lower formal oxidation states can be found in compounds prepared by reacting ZrCl4(THF)2 and six equivalents of K(naphthalenide) 74 

Following similar reaction strategies, compounds containing anions such as 7-coordinate [Zr(CO)5(SnMe3)2]2 and [Zr(CO)4(dppe)SnMe3] or 8-coordinate [Zr(CO)4(SnMe3)4]2 can be synthesized.76 The formal oxidation number of the Zr atom is 2 for the latter compound and 0 for the others. 

Catalysis Today, 1994,20,185-320. (An issue of the journal dedicated to Zirconium in Catalysis ). 

Ziegler Catalysts, G. Fink et al., Eds., Springer-Verlag, Berlin, 1995 (several chapters discuss various aspects of olefin polymerization catalysis by zirconium and hafnium complexes). 

These two elements have very similar chemistries, though not so nearly identical as in the case of zirconium and hafnium. They have very little cationic behavior, but they form many complexes in oxidation states II, III, IV, and V. In oxidation states II and III M—M bonds are fairly common and in addition there are numerous compounds in lower oxidation states where metal atom clusters exist. An overview of oxidation states and stereochemistry (excluding the cluster compounds) is presented in Table 18-B-l. In discussing these elements it will be convenient to discuss some aspects (e.g., oxygen compounds, halides, and clusters) as classes without regard to oxidation state, while the complexes are more conveniently treated according to oxidation state. 

The tris-tiron [tiron (20)] complex of vanadium(IV) is formed slowly, with a half-life of several minutes, from aq, both in acidic and in basic solution. The rate-determining step, here as in the production of the earlier-characterized octahedral tris-catechol-vanadium(IV) complex, is believed to be the removal of the vanadyl-oxygen needed for the addition of the third catecholate ligand. Repeat scan spectra indicate a half-life of about 10 minutes for ligand hydrolysis in the binuclear oxo-bridged complex [ V 0(salamhp) 20], salamhp = (21).  

Kinetics and mechanisms of hydrazine-forming reactions, involving coupled redox and substitution processes, from dinitrogen-bridged dinuclear diethyldithiocarbamate complexes of niobium and tantalum have been reported and discussed.  

Niobium-93 NMR data other than chemical shifts are even more scarce. The information on temperature gradients tg, coupling constants J and isotope effects A available to date is sununarized in Table 4. The temperature behaviour of Nb shielding and linewidth compares with that discussed for in Section 2.1.3. The trinuclear complex Cp3Nb3(CO)7 (20), with one of the 

10) is a rare example of an NMR spectrum of the X nucleus providing the complete coupling information the C(CO) signal consists of ten lines and the C(Cp) signal shows an unresolved 10-line pattern [2c]. If the X and M ( Nb 

Rehder, Bull. Magn. Reson. 4, 33 (1982). (b) D. Rehder, Magn. Reson. Rev. 9, 125 (1984). (c) D. Rehder, Chimia 40, 186 (1986). (d) D. Rehder, in Multinuclear NMR, edited by J. Mason, Plenum Press, New York, 1987, Chapter 19. (e) O. W. Howarth, Prog. Nucl. Magn. Reson. Spectrosc. 22, 453 (1990). (0 D. Rehder, in Transition Metal Nuclear Magnetic Resonance, edited by P. S. F gosin, Elsevier, Amsterdam, 1991. (g) D. Rehder, Coord. Chem. Rev. 110, 161 (1991). 

Herberhold, G. Frohmader, and W. Milius, Phosphorus Sulfur Silicon 93/94,205 (1994). 

Ihmels, D. Rehder, and V. Pank, Inorg. Chim. Acta 96, L69 (1985). 

Eight-co-ordination is confirmed for the vanadium atom in the tetrakis-(dithiophenyiacetate), V(S2CCH2Ph 4. The geometry is dodecahedral, with V-Sa 2.524 A and V-Sb 2.470 A. 

In curienite, [Pb(U02)(V04)j,5H20], the vanadium is present as the dinuclear [VaOj] species, which contains two basal-edge-shared, distorted square pyramids V=0 is normal, at 1.62 A, and V-O distances range from 1.79 to 2.04 A. 

In other vanadates, octahedral co-ordination is adopted, but again with 

Introduction.—The compilation of the inorganic chemistry of niobium and tantalum has been revised.341 Reviews have appeared on the physicochemical properties of tantalum compounds and alloys,498,499 on the extraction, properties, and uses of niobium and tantalum,500 and on the structures of their compounds determined by diffraction methods.339 The half-life of 182Ta has been determined as 114.74 + 0.08 days.501 

Chemical shift data have been obtained by photoelectron spectroscopy501 for the core electrons of niobium and tantalum compounds. The similarity in chemical shifts between structurally similar compounds of these two elements is in accord with the similarity in their ionic radii. 

Introduction.—A text describing the analytical chemistry of niobium and tantalum has been published.  

Absorption spectra have been recorded for tantalum atoms isolated in an argon matrix at 4.2 K. A complex spectrum was observed between 228.8 and 504.1 nm which, with only a few exceptions, correlates with the transitions within the ground state observed for tantalum atoms in the gas phase. The 

Gibalo, Analytical Chemistry of Niobium and Tantalum , Ann Arbor-Humphrey, Ann Arbor, Mich., 1970. 

Several studies have been concerned with the chemistry of the + ni oxidation state of these elements, and the characterization of the first tantalum(iii) compounds has been claimed. The diamagnetic dimer [TaCl3(MeCN)2]2 has been prepared and used to obtain [TaClafphen)], [TaCljfbipy)], and tris-(dibenzoylmethanato)tantalum(ni). NbFa has been characterized as the product of the reaction of Nb and NbF (1 1) at 750 °C under pressure. Electrolytic reduction of niobium(v) in ethanol,formamide, and dimethylformamide can afford preparative concentrations of niobium(iii) and the new compound niobium(iii) trilactate has been obtained from ethanol. 

Carbonyl Complexes.—The first direct substitution of donor molecules into the co-ordination sphere of metal carbonyl anions has been reported for [M(C0)5] (M = Nb or Ta). Photolytically induced substitution of uni-and bi-dentate phosphines has been used to prepare (Et4N)[M(CO)5PPh3] and (Et4N)[M(CO)4(diphos)]. 

Introduction.—The m.p. of 99.7% pure niobium has been measured as 2469 + 9°C. The crystal structure of p-tantalum appears to be isomorphous with that of P-uranium, and its X-ray powder diffraction pattern has been indexed in terms of a tetragonal unit cell. The absorption spectra of niobium atoms isolated in Ar, Kr, and Xe matrices at 14 K have been recorded. The absorptions observed in the u.v. region correlate well with those expected from known gas-phase data, but this was not the case for the visible and i.r. spectral regions. Spectral features observed upon annealing an Ar matrix containing Nb 

The stereochemistry of tetrakis (dipivaloylmethanato)niobium(iv) has been shown by X-ray analysis to be the first example of a M(bidentate)4 complex whose stereochemistry is based on an idealized square antiprism (38). The first Ta pure n.q.r. spectra have been detected in Ta2X o (X = Q or Br) as part of a detailed n.q.r. study of such dimers.  

Gmelin s Handbook Inorganic Chemistry, System No. 49, Niobium Pt B, Sect 4, Gmelins 

R R King, D. M. Braitsch, and R.N. Kapoor, J.C.S. Chem. Comm, 197Z 107Z 

Niobium (Nb, at. mass 92.91) hydrolyses (in the absence of complexing anions) over the pH range 0-14. Polymerized forms of Nb(V) give pseudo-solutions or they separate as a white precipitate. When fused with NaOH, Nb20s forms the niobate, which is soluble in NaOH solutions. Niobium(V) forms stable fluoride, tartrate, oxalate, and peroxide complexes. The niobium complexes are more stable than the corresponding Ta complexes. A niobium chloride complex is formed in 5 M HCl solutions. Niobium(V) can be reduced to coloured species of Nb(III) and Nb(IV). In an acid medium, zinc metal reduces Nb(V), but not Ta(V). 

Tantalum (Ta, at. mass 180.95) is, in its chemical properties, similar to Nb. The Ta(V) complexes are less stable than the corresponding Nb complexes. Tantalum(V) is harder to reduce to the lower, coloured, oxidation states, than is Nb(V). 

On being heated in acid solutions (or pseudo-solutions), niobium and tantalum hydrolyse and coagulate to form hydrous oxides. The following compounds may be used as collectors for traces of niobium or tantalum Zr(OH)4 and Mn02aq. in acid solutions, and Fe(OH)3 and Mg(OH)2 in alkaline solutions. When an alkaline melt (Na2C03, NaOH) is leached, Nb and Ta remain in the solid phase, while W, Mo, V, and Re pass into the aqueous solution [1]. 

From solutions containing not too much oxalic or tartaric acid, or EDTA, Nb and Ta may be precipitated by cupferron [2,3] or phenylarsonic acid [4] zirconium is often used as collector. 

When boric acid is added to a solution of Nb-, Ta-, W-, Mo-, or Zr fluoride complexes, the boron displaces the fluoride (formation of BFT) from the Nb, Ta, and W complexes, causing the precipitation of these metals, while Zr and Mo remain in solution, since their fluoride complexes are more stable [5]. 

LiCl + TaCl4(CH2SiMe3) TaCl3(CH2SiMe3)2 + 2LiCl 

Note that there is a preferred direction of reaction with the original complex. The reaction is prevented from developing into a chain reaction by the propensity of the [Ta]=CH2 complex to undergo the homologation reaction (McLain 1977 Schrock 1980 Rocklage 1981) see Abbenhuis (1994) for another example of a tantalum carbene complex which will undergo only stoichiometric metathesis. However, the complexes 2 and 3 (Table 2.2) are good initiators of ROMP as also is the tantalacyclobutane complex derived from 2 by the addition of norbomene. The complexes 1 and 2 are both effective for the metathesis of ds-pent-2-ene. 

Reaction of Ta(=CHCMe3)(Cl)(OCMe3)2(PMe3) with but-l-ene (Rocklage 

Tantalum and niobium halides also initiate the polymerization of acetylenes see Ch. 10. 

A large amount of work has been carried out, primarily by Shilov, on the chemical fixation of dinitrogen using various vanadium-containing systems. However, no stable dinitrogen complexes of vanadium have been reported or in situ nitrogen fixation observed with phosphines present. 

An impure dinitrogen complex of niobium has been reported by Leigh et Complex 4 (see Table 1) is formulated as a dinuclear complex with a bridging dinitrogen. Treatment with acid yields no ammonia, but ca. 20% of the nitrogen is converted to hydrazine, the remainder being evolved as N2. 

In 1980, Schrock et al. reported the first stable dinitrogen complexes of a group 5 metal. Of the more than ten complexes synthesized, two are prepared directly using dinitrogen (Table 1, structures 5, 7), while the remainder are prepared by a metathesis-like reaction of an alkylidene complex with a diimine (Equation 9)  

A -Ray analysis of the acetylene bridged dimer, [Ta2Cl4(iU- CCMe3 2)(iW-Cl)2], is consistent with the presence of a Ta=Ta.  

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Gr. Tantalos, mythological character, father of Niobe) Discovered in 1802 by Ekeberg, but many chemists thought niobium and tantalum were identical elements until Rowe in 1844, and Marignac, in 1866, showed that niobic and tantalic acids were two different acids. The early investigators only isolated the impure metal. The first relatively pure ductile tantalum was produced by von Bolton in 1903. Tantalum occurs principally in the mineral columbite-tantalite. 

E. Eairbrother, The Chemistry of Niobium and Tantalum, Elsevier Scientific Publishing Co., London, 1967. 

Nitrogen and carbon are the most potent solutes to obtain high strength in refractory metals (55). Particulady effective ate carbides and carbonitrides of hafnium in tungsten, niobium, and tantalum alloys, and carbides of titanium and zirconium in molybdenum alloys. 

A large deposit of loparite occurs ia the Kola Peninsula, Russia. The production of REO reaches 6500 t/yr. Loparite contains over 30% of rare-earth oxides from the cerium group. In addition, loparite contains up to 40% titanium oxide and up to 12% niobium and tantalum oxides. 

Occurrence. Niobium and tantalum usually occur together. Niobium never occurs as the metal, ie, ia the free state. Sometimes it occurs as a hydroxide, siUcate, or borate most often it is combiaed with oxygen and another metal, forming a niobate or tantalate ia which the niobium and tantalum isomorphously replace one another with Htde change ia physical properties except density. Ore concentrations of niobium usually occur as carbonatites and are associated with tantalum ia pegmatites and alluvial deposits. Principal niobium-beariag minerals can be divided iato two groups, the titano- and tantalo-niobates. 

Another method of purifying niobium is by distillation of the anhydrous mixed chlorides (29). Niobium and tantalum pentachlorides boil within about 15°C of one another which makes control of the process difficult. Additionally, process materials must withstand the corrosion effects of the chloride. The system must be kept meticulously anhydrous and air-free to avoid plugging resulting from the formation of niobium oxide trichloride, NbOQ. Distillation has been used commercially in the past. 

W. R. Scboellei, The Mnalytical Chemisty of Niobium and Tantalum, Chapman and Hall, London, 1937. 

R. J. H. Clark and D. Brown, The Chemisty of Vanadium, Niobium and Tantalum, Pergamon Press, Ehnsford, N. Y., 1975. 

F. Fairbrother, The Chemisty of Niobium and Tantalum Elsevier Publishing Company, New York, p. 16,1967. 

Columbium (Niobium) and Tantalum in 1994 U.S. Bureau of Mines, Mineral Industry Survey, 1995. 

One was Ekeberg s tantalum and the other he called niobium (Niobe was the daughter of Tantalus). Despite the chronological precedence of the name columbium, lUPAC adopted niobium in 1950, though columbium is still sometimes used in US industry. Impure niobium metal was first isolated by C. W. Blomstrand in 1866 by the reduction of the chloride with hydrogen, but the first pure samples of metallic niobium and tantalum were not prepared until 1907 when W. von Bolton reduced the fluorometallates with sodium. 

Table 22.2 Oxidation states and stereochemistries of compounds of vanadium, niobium and tantalum...

Niobium and tantalum provide no counterpart to the cationic chemistry of vanadium in the -t-3 and -t-2 oxidation states. Instead, they form a series of cluster compounds based... 

Niobium and tantalum also form various oxide phases but they are not so extensive or well characterized as those of vanadium. Their pentoxides are relatively much more stable and difficult to reduce. As they are attacked by cone HF and will dissolve in fused alkali, they may perhaps... 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

V. Boas and L. C. Pessoa, Vanadium, Chap. 33, pp. 453-583, and L. G. Hubert-Pfalzgraf, M. Postel and J. G. Riess, Niobium and Tantalum, Chap. 34, pp. 585-697 in Comprehensive Coordination Chemistry, Vol. 3, Pergamon Press, Oxford, 1987. 

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