Niobium/tantalum bonds

The octahedral metal clusters that have long been familiar features of the lower halide chemistry of niobium, tantalum, molybdenum, and tungsten represent a category of cluster different from those so far considered in that their metal-metal bonding is best treated as involving four AO s on each metal 49, 133,144,165,178). 

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). 

However, explanations for growth limitation based on repulsion of metal ions may be somewhat oversimplified. Elements other than vanadium, niobium, tantalum, molybdenum, and tungsten do not form isopoly anions. Other ions which have appropriate radii (e.g., Al,+, 67 pm Oa5+, 76 pm I7"1. 67 pm) for discrete isopoly anion formation instead form chains, sheets, or three-dimensional frameworks. Why does polymerization stop for isopoly anions An oxygen atom in a terminal position in an isopoly anion is strongly n bonded to a transition metal such as Mo(V ) or W (VI). These terminal oxygen atoms are never found trans to one another because they avoid... 

The chemistry of the niobium-tantalum pair is quite similar to that of the zirconium-hafnium pair. In aqueous media, all solvated species containing these elements have either metal-to-oxygen bonds, metal-to-fluorine bonds, or both. 

The group 5-7 supported transition metal oxides (of vanadium, niobium, tantalum, chromium, molybdenum, tungsten, and rhenium) are characterized by terminal oxo bonds (M =0) and bridging oxygen atoms binding the supported oxide to the cation of the support (M -0-MSUpport). The TOF values for ODH of butane or ethane on supported vanadia were found to depend strongly on the specific oxide support, varying by a factor of ca. 50 (titania > ceria > zirconia > niobia > alumina > silica). 

Caltx[4]arenes have been used as a preorganized set of O-donor atoms. Monomeric or dimeric species were obtained depending on the caUxarene substituents. They were used to develop the organometaUic chemistry of Nb and Ta in various oxidation states, the electron reservoir of the Nb=Nb bond is able to activate small molecules including N2 see Niobium Tantalum OrganometaUic Chemistry). 

Vanadium, Niobium, Tantalum and Zirconium Mixed sandwich complexes of vanadium containing arene rings have been described in two reports. The bisindenylbisvanadium complex (41) has been structurally characterised and contains a metal-metal bond length of 2.35A. The complex is formed in the reductive... 

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 ... 

Since niobates and tantalates belong to the octahedral ferroelectric family, fluorine-oxygen substitution has a particular importance in managing ferroelectric properties. Thus, the variation in the Curie temperature of such compounds with the fluorine-oxygen substitution rate depends strongly on the crystalline network, the ferroelectric type and the mutual orientation of the spontaneous polarization vector, metal displacement direction and covalent bond orientation [47]. Hence, complex tantalum and niobium fluoride compounds seem to have potential also as new materials for modem electronic and optical applications. 

IR absorption spectrum of the initial tantalum-saturated solution displays a weak band at about 880 cm 1, which corresponds to TaO bonds. The formation of the oxyfluorotantalate complex seems to be similar to the formation of oxyfluoroniobate in a niobium-saturated solution, but in the case of tantalum, the above effect is more emphasized. 

Precipitated, washed and filtrated hydroxides consisting of wet powder contain two kinds of water. The first is moisture, i.e. water remainders that include adsorbed water. This kind of water can be successfully removed by diying at 100-200°C. The second type is molecules of water that are incorporated with tantalum or niobium to form hydroxides. Because hydroxyl groups form relatively strong bonds with tantalum or niobium, the separation of the second kind of water requires thermal treatment at higher temperatures [501],... 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

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