Tantalum octahedral

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. 

Since hydrofluoride synthesis is based on thermal treatment at relatively high temperatures, the possibility of obtaining certain fluorotantalates can be predicted according to thermal stability of the compounds. In the case of compounds whose crystal structure is made up of an octahedral complex of ions, the most important parameter is the anion-cation ratio. Therefore, it is very important to take in to account the ionic radius of the second cation in relation to the ionic radius of tantalum. Large cations, are not included in the... 

The ratio between the anionic and cationic radii leads to coordination numbers, the lowest of which is 6, which correspond to a octahedral polyhedron of anions around a central cation [135]. In this case, the compound structure type depends on the ratio of total number of anions and cations. The total number of anions (X) is calculated by summing up the number of oxygen (O) ions and of fluorine (F) ions X=0+F, while the total number of cations (Me) is the number of tantalum ions, niobium ions and other similar cations. 

The lowest coordination number of tantalum or niobium permitted by crystal chemistry formalism is 6, which corresponds to an octahedral configuration. X Me ratios that equal 3, 2 or 1 can, therefore, be obtained by corresponding substitutions in the cationic sub-lattice. A condition for such substitution is no doubt steric similarity between the second cation and the tantalum or niobium ion so as to enable its replacement in the octahedral polyhedron. In such cases, the structure of the compound consists of oxyfluoride octahedrons that are linked by their vertexes, sides or faces, according to the compound type, MeX3, MeX2 or MeX respectively. Table 37 lists compounds that have a coordination-type structure [259-261]. 

The type of crystal structure depends on the ratio X Me, where X is the total number of anions (oxygen and fluorine) and Me is the total number of all cations that can fit into/occupy octahedral voids (tantalum, niobium, lithium and other metals with similar ionic radii). 

X Me Second cation present, with coordination number greater than 6 (second cation s ionic radius > tantalum/niobium ionic radius) Only cations that can fit into/occupy octahedral voids are present (second cation s ionic radius tantalum/niobium ionic radius)... 

S Tantalum and niobium are present in the crystal structure in the form of complex ions. The lowest coordination number, 6, corresponds to the formation of slightly distorted octahedrons. The linking and packaging of the octahedrons depends on the X Me ratio, where X is the total number of oxygen and fluorine atoms, and Me is the total number of tantalum or niobium ions as well as other metals that can replace tantalum or niobium in the octahedral polyhedron. The crystal structure type can be defined based on the X Me ratio, as follows ... 

Attempts to prepare related hpp complexes of tantalum were only partially successful. Reaction of 2 equivalents of either (hpplSiMes or in situ generated Li[hpp] with Tads afforded a solid that analyzed correctly for the compound (hpp)2TaCl3. X-ray structural analysis of the crystallized product identified the coordination isomer [Ta(hpp)4][TaCl6], with a distorted dodecahedral cation and a regular octahedral anion (Figure 28). °... 

Fig. 1.3 Elements forming layered sulfides or selenides with the metal in octahedral or trigonal prismatic coordination (niobium and tantalum are found in both). (Adapted from [35])...

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). 

Systematic studies of the structural properties of TaS2 by Jellinek10 revealed the presence of several polymorphic forms of the compound as a function of temperature. Within a layer, the tantalum atom sits in the holes formed between two layers of sulfur atoms in the S—Ta—S sandwich. The coordination of the tantalum is trigonal prismatic or octahedral, depending on whether the two sulfur layers lie one on top of the other or are rotated by 60°. Thus the various poly-... 

Fig. 1. The a notation indicates that the Tantalum atoms are aligned along the crystallographic c-axis. R indicates rhombohedral symmetry. The 3R phase has three trigonal prismatic slabs per unit cell, whereas the 6R phase has six slabs that are alternately trigonal prismatic and octahedral. F. Jellinek, J. Less-Common Met., 4, 9-15 (1962) F. J. DiSalvo et al., J. Phys. Chem. Solids, 34, 1357 (1973).

Tals consists of iodine atoms in a hexagonal-close-packed arrangement in which 2/5 of the octahedral holes are randomly occupied by tantalum atoms.21 The Ta2Ii0 molecules are arranged in layers (Ta—Ib 2.932 A Ta—Iax 2.704 A Ta—I 2.619 A). Triclinic Nbl5 is isostructural with /3-UCl5.22... 

Tantalum pyrazolylborate derivatives [TaClMe3 HB(pz)3 ] (20) and [TaClMe3 H2B(3,5-Me2pz)2 l (21) (pz = pyrazolyl) have been reported and characterized by X-ray and NMR studies.337 Complex (21) adopts a capped octahedral structure in the solid with a close contact between tantalum and one hydrogen, which was described as a three-centered B—H—Ta bond, filling the seventh coordination site (Figure 12). The Ta—B distance is short (2.90(1) A ... 

Disporportionation of MOCl3 at 300-400 °C yielded M02C1 and MC15.3 Nb307Cl has been prepared from Nb2Os and NbOCl3 at 600 °C its structure shows octahedrally coordinated metal atoms. A tantalum analog has been mentioned. 

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. 

Rupprecht, G.A., Messerle, L.W., Fellmann, J.D. and Schrock, R.R. (1980) Multiple metal-carbon bonds. 15. Octahedral alkylidene complexes of niobium and tantalum by ligand-promoted a abstraction. J. Am. Chem. Soc., 102, 6236. 

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