V, Nb, and Ta

Another type of more complex finite oxy-ion is formed by the bonding of XO ions to a metal atom, either through X or through one or more of the 0 atoms. Examples are noted in other chapters and include [Co(N02)6] and [Co(N03)4] -. 

There is much experimental evidence for the formation of complex oxy-ions in solutions of vanadates, niobates, and tantalates. We describe in Chapters 12 and 13 the structures of some crystalline vanadates here we note only certain finite complex ions which exist both in solution and in crystalline salts. The ion of Fig. 11.3(a) has been shown to exist in the salts Na7H(Nb60i9). 15 H20. Light scattering from an aqueous solution of Kg(Ta60i9). I6H2O indicates that the anion species contains 6 Ta atoms and is presumably similar to the ion in the crystal.  

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Other hydrides with interstitial or metallic properties are formed by V, Nb and Ta they are, however, very much less stable than the compounds we have been considering and have extensive ranges of composition. Chromium also forms a hydride, CrH, though this must be prepared electrolytically rather than by direct reaction of the metal with hydrogen. It has the anti-NiAs structure (p.. 555). Most other elements... 

The prototype hard metals are the compounds of six of the transition metals Ti, Zr, and Hf, as well as V, Nb, and Ta. Their carbides all have the NaCl crystal structure, as do their nitrides except for Ta. The NaCi structure consists of close-packed planes of metal atoms stacked in the fee pattern with the metalloids (C, N) located in the octahedral holes. The borides have the A1B2 structure in which close-packed planes of metal atoms are stacked in the simple hexagonal pattern with all of the trigonal prismatic holes occupied by boron atoms. Thus the structures are based on the highest possible atomic packing densities consistent with the atomic sizes. 

The trend of intermetallic reactivity and alloy stability of V, Nb and Ta with the different elements may be further discussed in terms of the melting points of the compounds as described in the following paragraphs. 

Unlike the related Na3[M (CO)5], where M = V, Nb, and Ta, which undergoes thermolysis below O0C (vide infra), these materials possess remarkable thermal stabilities for metal carbonyls and briefly survive without melting at temperatures as high as 300°C. By comparison, K2[Fe(CO)4], another metal carbonyl salt of high thermal stability, has been reported to melt at 270-273°C with decomposition (20). The related K[Co(CO)4] melts at about 203°C with decomposition (21). 

Of the homoleptic carbonylmetallates(l -) we have attempted to reduce, [Co(CO)4] appears to be the most difficult. Although the sodium salts of [M(CO)6] (M = V, Nb, and Ta) were quickly reduced in liquid ammonia by sodium metal to provide the corresponding trianions, [M(CO)5]3 (vide supra), it seems unlikely that we have ever effected complete reduction of Na[Co(CO)4] to Na3[Co(CO)3]. Even after 2 days of refluxing (at — 33°C) anhydrous ammonia solutions of Na[Co(CO)4] with excess Na, considerable amounts of the tetracarbonylcobaltate(l —) remained. Low yields of a heterogeneous-appearing brown to olive-brown insoluble solid were isolated this solid has been shown to contain Na3[Co(CO)3] (vide infra). As in the case of [Re(CO)s], we found that solutions of potassium in liquid ammonia were far more effective at reducing [Co(CO)4]-. However, unlike [Re(CO)s], [Rh(CO)4], or [Ir(CO)4] (vide infra), there was no evidence that [Co(CO)4] was reduced by sodium or potassium metal in hexa-methylphosphoric triamide. We observed that excess sodium naphthalenide slowly (over a period of 40-50 hr at room temperature) converted Na[Co(CO)4] in THF to an impure and insoluble brown powder that contained Na3[Co(CO)3], but this synthesis appeared to be of little or no utility. 

Acidic elements such as Mo, W, V, Nb and Ta, which are present as oxoanions... 

Metal alkoxides such as V-, Nb- and Ta-(V) alkoxides, as well as oxides in various oxidation states-in particular Nb(II, III, IV, and V), V(III, IV, and V) and Ti (IV)-oxides-have been studied in transesterification reactions involving EC and... 

We discuss in this chapter the elements of the first transition series, titanium through copper. There are two main reasons for considering these elements apart from their heavier congeners of the second and third transition series (1) in each group (e.g., V, Nb, and Ta) the first-series element always differs appreciably from the heavier elements, and comparisons are of limited use, and (2) the aqueous chemistry of the first-series elements is much simpler, and the use of ligand field theory in explaining both the spectra and magnetic properties of compounds has been far more extensive. 

Titanium disulfide, like the disulfides of Zr, Hf, V, Nb, and Ta, has a layer structure two adjacent close-packed layers of S atoms have Ti atoms in octahedral interstices. These sandwiches are then stacked so that there are adjacent layers of S atoms. Lewis bases such as aliphatic amines can be intercalated between these adjacent sulfur layers similar intercalation compounds can be made with M and MSe2 compounds for M = Ti, Zr, Hf, V, Nb, and Ta. Many of these have potentially useful electrical properties, including use as cathode material for lithium batteries, and superconductivity, and may be compared with the intercalation compounds of... 

The nonstoichiometric monohydrides formed by V, Nb and Ta have structures that are determined by T and the H content . Order-disorder transitions involving the H atoms lead to structural complexity in the ordered phases the bcc metal lattice distorts to tetragonal, orthorhombic or monoclinic. In addition, V and Nb, but not Ta, at >0.1 MPa Hj form dihydrides that have the fluorite structure. Hence, the phase diagrams of these elements with H are complex. 

The reader may remember that the 77-complex structure for the adducts formed from acetylene and mercury dichloride, or of related complexes of non-transitional-metal halides, was disproved by our stereochemical investigations. The true 77-complexes, however, were also obtained by us from tolan and the derivatives of such transition metals as V, Nb, and Ta (451-455). 77-Cyclopentadienylniobium tetracarbonyl reacted with tolan according to the scheme shown on page 57. 

Bismuth Molybdate Catalysts. The Raman spectra of the bismuth molybdates, with Bi/Mo stoichiometric ratios between 0.67 and 14, have been examined using the FLS approach (see Section 3.2). " The bismuth molybdates fall into an unusual class of compounds, the ternary bismuth oxide systems Bi-M-0 (where M = Mo, W, V, Nb, and Ta) which exhibit a variety of interesting physical and chemical properties. Of commercial importance, the bismuth molybdates are heterogeneous catalysts for selective oxidations and ammoxidations (the Sohio process), for example, propylene ( 311 ) to acrolein (C3H4O) by oxidation or to acrylonitrile (C3H3N) by arrunoxidation. ... 

The hydrides of V, Nb, and Ta probably come closest to the idea of an interstitial compound and are discussed later. In other cases where the arrangement of metal atoms in the hydride is the same as in (one form of) the metal there may be a discontinuous increase in lattice parameter when the hydride is formed (Pd) or there may be an intermediate hydride with a different metal arrangement. For example, the h.c.p. 4f metals Gd-Tm form hexagonal trihydrides, but the intermediate dihydrides have the fluorite structure with c.c.p metal atoms. 

The metals V, Nb, and Ta have b.c.c. structures. The a solid solution of H in this kructure extends to VHq.os, NbH(,., and TaHo.2. The next distinct phase is the/3 hydride, a non-stoichiometric phase which is a (tetragonal) distorted version of the b.c.c. solid solution and is stable over a wide range of composition (for example. 

We saw in Chapter 12 that from the structural standpoint many transition metal-oxygen systems are surprisingly complex. This is also true of many metal-sulphur systems, as we shall show later for the sulphides of Cr, Ti, V, Nb, and Ta. Before doing this we shall note some of the simpler binary sulphide structures, taking them in the order M2S, MS, MS2, M2S3 and M3S4. The chapter concludes with a short account of thio-salts and complex sulphides. 

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