Tantalum lower oxidation states

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 ... 

The heavier metal tantalum is distinctly less inclined than niobium to form oxides in lower oxidation states. The rutile phase TaOz is known but has not been studied, and a cubic rock-salt-type phase TaO with a narrow homogeneity range has also been reported but not yet fully characterized. TazOs has two well-established polymorphs which have a reversible transition temperature at 1355°C but the detailed structure of these phases is too complex to be discussed here. 

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. 

Clearly, U is the biggest number in the cycle and is the main driving force for the formation of ionic compounds. Nevertheless, the other factors can tip the balance one way or another. For example, AHSub is particularly large for the transition metals niobium, tantalum, molybdenum, tungsten, and rhenium, with the result that, in their lower oxidation states, they do not form simple ionic compounds such as ReCl3 but rather form compounds that contain clusters of bonded metal atoms (in this example, Re3 clusters are involved, so the formula is better written ResClg). 

Compared to vanadium, the higher oxidation states are much more frequently and the lower ones much less frequently encountered in niobium and tantalum. Niobium and tantalum are also much more prone to extended metal-metal bonding in their lower oxidation states V6-based clusters, for example, remain undiscovered. Little resemblance is found to the group VB elements phosphorus and arsenic. 

The chemistry of niobium and tantalum in their lower oxidation states is expanding rapidly. The first structurally characterized molecular Nb111 derivative was reported in 1970,525 while Nb111 and Tam halide adducts were described in 1973580 and 1978, respectively.581... 

The atoms of the vanadium group metals have five valence electrons. In vanadium (Z — 23) and niobium (columbium, Z = 41), these valence electrons lie beyond ra re-gas cores, whereas in tantalum (Z = 73), they lie beyond the xenon core which has been augmented by fourteen 4/ electrons. The +5 oxidation state is characteristic of this family for niobium and tantalum it is the only oxidation state of importance. Oxidation is often regarded as removal of five valence electrons, followed by coordination of the pentapositive ion (which cannot exist for appreciable time in chemical systems) to basic groups which are present (H2O, OH, Cl, etc.). Although such a description almost certainly has very little resemblance to the actual path of oxidation of these metals, it is clerically convenient and may be used if not taken literally. In the same way, the lower oxidation states of vanadium may be considered vanadium atoms with the two 4s electrons removed, and with additional removal of one or two 3d electrons. 

For lanthanide metals other than Eu, Sm, and Yb, compounds in lower oxidation states generally involve metal—metal bonding rather than discrete Ln2+ ions. In general they are formed by heating LnX3 with Ln in tantalum containers. 

The development of the chemistry of niobium and tantalum in their lower oxidation states, three or less, has long been precluded by the lack of convenient starting materials. Indeed, stoichiometric trihalides MX3 (X = Cl, Br) are only incidental compositions. They are obtained by reduction of the pentahalides or disproportionation of the tetrahalides, and are polymeric and rather inert. 

The lower oxidation states of niobium and tantalum are unimportant compared to the -1-5 state. Because of the general insolubility of the oxides and (he lack of stable lower oxidation slates, (here is little solution redox chemistry. Niobium(III) does appear to form upon the reduction of niobium(V) with zinc, and is stable in the cold in the absence of air, but if the solution is heated, decomposition occurs with precipita-lion of mixed oxides. 

Niobium and tantalum, though metallic in many respects, have chemistries in the V oxidation state that are very similar to those of typical non-metals. They have virtually no cationic chemistry but form numerous anionic species. Their halides and oxide halides, which are their most important simple compounds, are mostly volatile and are readily hydrolyzed. In their lower oxidation states they form an extraordinarily large number of metal-atom cluster compounds. Only niobium forms lower states in aqueous solution. The oxidation states and stereochemistries (excluding those in the cluster compounds) are summarized in Table 26-B-l. 

The most studied of the Group 5 porphyrins have been compounds with vanadyl and niobium metal ions no electrochemical data have yet been reported for tantalum porphyrins. Vanadium porphyrins have been synthesized as both V(IV) and V(II) complexes. The vanadyl derivatives, represented as (P)VO, generally undergo well-defined reduction and oxidation reactions involving the porphyrin, macrocyle the lower oxidation state V(II) complexes have been prepared as (P)V (L) and (P)V (L)2 derivatives, where P = T(p-Me)PP or OEP and L = THE or PPhMe2, but these species have not been examined as to their electrochemical properties [7]. 

Fig.2, this influence spreads to the character of subsequent change of the potential. Currentless transport is commonly due to the presence in the melt of two different valencies of the metal deposited [3]. According to the voltammetric studies of various authors [2,4-8], in melts of different compositions Ta(V) ions are discharged in one 5-electron stage, Le. only the pentavalent form is stable. However there is both indirect [7] and direct [9,10] evidence of the presence of tantalum ions in molten alkali metal halides in a lower oxidation state. Our findings also prove the presence of low-valent tantalum ions in the melts. The different stability of low-valent tantalum complexes in melts of varying compositions results in different concentrations of tantalum ions of the lower oxidation state. The fluoride ion... 

Aspects of borole complex reactivity have been studied in detail, including the behavior of tantalum sandwiches bearing alkyl ligands on the metal.15-17 Complexes such as 6 are best regarded as resonance hybrids where strong B-N 7t-overlap lowers the formal oxidation state of the metal 15 16... 

The crystal structures of both (PPN)[M(CO)6] derivatives have been determined.717 The coordination polyhedron is octahedral (Nb—C 2.098(5) A CNbC 89.2(2)°). The PPN moiety is constrained to be centrosymmetric, and thus linear. These compounds correspond to the lowest oxidation state of niobium and tantalum for which structural data are available. A single v(CO) is found in the IR (1854 and 1852 cm-1 for Nb and Ta respectively). Comparable spectra are observed for Na[M(CO)6] in pyridine, but in solvents of lower dielectric constants such as tetrahydrofuran, additional bands attributed to distortion of the anion by the countercation are observed. [Nb(CO)6] appears to be the most labile carbonyl of the group VA analogs. 

The number of niobium and tantalum coordination compounds in oxidation states lower than II is limited, and they often are organometallic derivatives involving carbonyl, cyclopentadienyl, or alkenic ligands. 

Quite recently attention was paid to the role of oxides, either as electro-active species, as impurities or as additives in the electro-deposition of transition metals. This may be demonstrated, e.g. in the case of electro-deposition of molybdenum, where the electrolysis of neither pure K2M0O4, nor the KF-K2M0O4 mixture yields a molybdenum deposit. However, introducing small amounts of boron oxide, or silicon dioxide to the basic melts, smooth and adherent molybdenum deposits may be obtained. Also, in the case of niobium and tantalum deposition, the presence of oxygen either from the moisture or added on purpose leads to the formation of oxohalo-complexes, which due to their lowered symmetry and thus lower energetic state, decompose easier at the cathode yielding pure metal. 

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). 

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