Niobium Valency

The compound K0 3NbF3 has an average niobium valency of 2.7 and forms a crystal structure that is referred to as hexagonal tungsten bronze [239]. 

Refractory metals and particularly niobium and tantalum are obtained by molten salt electrolysis. Several authors 71-6/ investigated the electrochemical reduction of niobium ions in chloride or chloride + fluoride melts. However the current efficiency during electrolysis is strongly dependent on the average valence of the bath. Different reduction steps are proposed with niobium valences ranging from V to I. This is supported by the existence of many stoichiometric compounds. However, very few if not nothing is known on thermodynamics. 

Perchloric acid (HCIO4 Ho —13.0), fluorosulfuric acid (HSO3F Ho — 15.1), and trifluoromethanesulfonic acid (CF3SO3H Ho —14.1) are considered to be superacids, as is truly anhydrous hydrogen fluoride. Complexing with Lewis acidic metal fluorides of higher valence, such as antimony, tantalum, or niobium pentafluoride, greatly enhances the acidity of all these acids. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Physical Properties. Molybdenum has many unique properties, leading to its importance as a refractory metal (see Refractories). Molybdenum, atomic no. 42, is in Group 6 (VIB) of the Periodic Table between chromium and tungsten vertically and niobium and technetium horizontally. It has a silvery gray appearance. The most stable valence states are +6, +4, and 0 lower, less stable valence states are +5, +3, and +2. 

Uranium Purification. Subsequent uranium cycles provide additional separation from residual plutonium and fission products, particularly zirconium— niobium and mthenium (30). This is accompHshed by repeating the extraction/stripping cycle. Decontamination factors greater than 10 at losses of less than 0.1 wt % are routinely attainable. However, mthenium can exist in several valence states simultaneously and can form several nitrosyl—nitrate complexes, some for which are extracted readily by TBP. Under certain conditions, the nitrates of zirconium and niobium form soluble compounds or hydrous coUoids that compHcate the Hquid—Hquid extraction. SiUca-gel adsorption or one of the similar Hquid—soHd techniques may also be used to further purify the product streams. 

Table 6.1 Increase in the extent of metal-metal bonding in niobium chlorides as the number of valence electrons per niobium atom (n) increases.

The number of valence electrons in niobium oxychloride clusters decreases as the number of oxide hgands increases (Table 6.4). The compound Cs2LuNbi Cli70 has the VEC of 16 as found in most chloride clusters, suggesting that the presence of one oxide ligand per cluster does not tip the balance between the Nb-Nb bonding and Nb-L anti-bonding contributions to the a2u state. The VEC of most clusters... 

Table 6.4 Number of valence electrons per cluster in niobium oxychlorides.

Let us now explain how the formal MI X and bonding assignments are obtained, using Cp2Nb(CO)Cl as an example. A niobium atom has five valence electrons. A cyclopentadienyl (Cp) ligand has formal Lewis structure... 

Although niobic and tantalic acids are extremely difficult to separate, Marignac finally succeeded, not only in separating them, but also in showing that niobium is both tri- and pentavalent, whereas tantalum always has a valence of five. The separation is based on the insolubility of potassium fluotantalate in comparison with potassium fluo-oxyniobate (12, 20). In the United States the element discovered by Hatchett used to be known as columbium, but in Europe most chemists prefer to use the name niobium which Heinrich Rose gave it. 

The lower halides of niobium and tantalum consist of tightly bound clusters of metal atoms, with metal-metal distances close to those found in the metal. They contain ions with average oxidation numbers between +III and +1 (Table 43). Their size depends on the valence electron concentrations (VEC) that are available on the metal atoms for M—M bonding, and on the halide-metal ratio.644 Several reviews have been devoted to the clusters of early transition metals.3,643... 

The only well-established, structurally characterized mixed valence compounds are the niobium selenide derivatives Nb3Se5Ciy/IV and [(MeCN)2Br2Nb( U-Se)2( U-Se)NbBr2-(MeCN)2]IV/m (Section 34.3.4). 

Nickel and chromium mixed-phase rutile pigments are industrially important. When nickel and chromium are substituted into the rutile lattice, higher valency metals (e.g., antimony, niobium, or tungsten) must also be substituted to maintain an average valency of four. 

As is usual with transitional elements, vanadium possesses considerable freedom in its valencies with niobium and tantalum, however, this freedom is less marked. This can be seen in the variation of the readiness displayed by the compounds of these elements to undergo reduction ... 

Niobium Oxytrichloride, NbOCls.—This compound is of historical interest in that it was one of the oxy-halogen compounds prepared originally by Rose in which the oxygen atom was overlooked. Rose called this substance hyponiobium chloride its correct formula, and hence the true valency of niobium, were first established by Blomstrand.4... 

Group r In order of increasing atomic number, ihese are vanadium, niobium Isomelimes culled columbiunti. and tantalum. Vanadium and tanlalum have two electrons in an ouler shell niobium has one electron in ils ouler shell. Although niobium and vanadium also have other valences, all of ihe elements in this group have a 5+ valence in common... 

The ternary iron oxides, as exemplified by the iron-niobium system, offer an opportunity to obtain single-phase, conducting n-type iron oxides in which the conductivity can be controlled by means of chemical substitution. At first glance, FeNbO and FeNb Og might appear to be very different materials. Yet as MM O and MM Og they merely represent superstructures of the basic a-PbO. structure obtained under the conditions of preparation (7 ). Consequently, they form a solid solution in which the two valence states of iron are uniformly distributed throughout a single homogeneous phase (j3). 

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