Octahedron complexes

It is often difficult to represent inorganic compounds with the usual structure models because these structures are based on complex crystals space groups), aggregates, or metal lattices. Therefore, these compounds are represented by individual polyhedral coordination of the ligands such as the octahedron or tetrahedron Figure 2-124d). 

The common structural element in the crystal lattice of fluoroaluminates is the hexafluoroaluminate octahedron, AIF. The differing stmctural features of the fluoroaluminates confer distinct physical properties to the species as compared to aluminum trifluoride. For example, in A1F. all corners are shared and the crystal becomes a giant molecule of very high melting point (13). In KAIF, all four equatorial atoms of each octahedron are shared and a layer lattice results. When the ratio of fluorine to aluminum is 6, as in cryoHte, Na AlF, the AIFp ions are separate and bound in position by the balancing metal ions. Fluorine atoms may be shared between octahedrons. When opposite corners of each octahedron are shared with a corner of each neighboring octahedron, an infinite chain is formed as, for example, in TI AIF [33897-68-6]. More complex relations exist in chioUte, wherein one-third of the hexafluoroaluminate octahedra share four corners each and two-thirds share only two corners (14). 

Complex carbides are very numerous. Many newer compounds of this class have been discovered and their stmctures elucidated (20). The octahedron M C is typical where the metals arrange around a central carbon atom. The octahedra may be coimected via corners, edges, or faces. Trigonal prismatic polyhedra also occur. Defining T as transition metal and M as metal or main group nonmetal, the complex carbides can be classified as (/)... 

In fact the vast majority of 6-coordinate complexes are indeed octahedral or distorted octahedral. In addition to the twist distortion just considered distortions can be of two other types trigonal and tetragonal distortions which mean compression or elongation along a threefold and a fourfold axis of the octahedron respectively (Fig. 19.8). 

A similar type of isomerism occurs for [Ma3b3] octahedral complexes since each trio of donor atoms can occupy either adjacent positions at the comers of an octahedral face (/hcial) or positions around the meridian of the octahedron (meridional). (Fig. 19.12.) Geometrical isomers differ in a variety of physical properties, amongst which dipole moment and visible/ultraviolet spectra are often diagnostically important. 

In polymerizing these compounds, a reaction between a-TiCls and triethylaluminum produces a five coordinate titanium (111) complex arranged octahedrally. The catalyst surface has four Cl anions, an ethyl group, and a vacant catalytic site ( ) with the Ti(lll) ion in the center of the octahedron. A polymerized ligand, such as ethylene, occupies the vacant site ... 

We saw in Chapter 7 that octahedral geometry is characteristic of many molecules (e.g., SF6) in which a central atom is surrounded by six other atoms. (Remember, an octahedron has eight sides, which is irrelevant here it has six comers, which is important) All complex ions... 

The drawing at the left shows six ligands (represented by spheres) at the comers of an octahedron with a metal atom at the center. A simpler way to represent an octahedral complex is shown at the right... 

Taking position 1 in Figure 15.4 as a point of reference, you can see that groups at 2,3, 4, and 5 are equidistant from 1 6 is farther away. In other words, positions 1 and 2,1 and 3, 1 and 4,1 and 5 are cis to one another positions 1 and 6 are trans. Hence a complex ion like Co(NH3)4Cl2+ (Figure 15.5) can exist in two different isomeric forms. In the cis isomer, the two Cl- ions are at adjacent comers of the octahedron, as close together as possible. In the trans isomer they are at opposite corners, as far away from one another as possible. 

Octahedral Having the symmetry of a regular octahedron. In an octahedral species, a central atom is surrounded by six other atoms, one above, one below, and four at the comers of a square, 176 complex in transitional metals, 418-420 geometric isomerism, 415 Octane number, 584... 

According to crystal analysis performed by Stomberg [173], Na2NbOF5 is made up of sodium ions and isolated NbOF52 complex ions and is similar in structure to FeWC>6. NbOFs2" polyhedrons comprise slightly distorted octahedrons that are located in one of two equivalent positions. The niobium atom is shifted 0.234 A from the equatorial plane towards the oxygen atom. 

There are two main differences between the structure of the NH4NbOF4 chains and that of the Rb5Nb30Fi8 chains. The first difference is, that in the case ofRbjNbjOFu neighboring octahedrons along the chain are rotated by 7t/4 relative to one another (the rotation axis coincides with z-axis), as shown in Fig. 31. The second difference is that in the NbOF4 complexes, the niobium atoms are all shifted in the same direction, forming a polar structure. 

K2NbOF5 Isolated complexes NbOFs2 Li2NbOF5 is composed of (0,F) linked octahedrons. 

According to the above classification, the structures of LiNb(Ta)F6 and Li2Nb(Ta)OF5 should be composed of lithium cations and isolated octahedral complex ions, Nb(Ta)F6 or Nb(Ta)OF52, respectively. It is known, however, that the structure of these compounds consists only of octahedrons linked via their vertexes in the first case, and via their sides in the second case. The same behavior is observed in compounds containing bi- and trivalent metals. 

The validity of this approach can be demonstrated by the example of several complex fluoride compounds that exhibit ferroelectric properties, such as compounds that belong to the SrAlF5 family [402, 403]. The crystal structure of the compounds is made up of chains of fluoroaluminate octahedrons that are separated by another type of chains - ramified chains. Other examples are the compounds Sr3Fe2Fi2 and PbsWjOgFio. In this case, the chains of iron- or tungsten-containing octahedrons are separated from one another by isolated complexes with an octahedral configuration [423,424]. 

The crystal structure of MsM OF compounds, where M = NH4, K, Rb, is made up of infinite chains of oxyfluoroniobate octahedrons that are similar to MNbOF4 chain-type compounds. Infinite chains are separated by isolated complexes NbFy2, whose structure is similar to that found in the island-type compound K2NbF7. The structure of the M5Nb30Fi8 compounds was described and discussed in Chapter 3.2. Due to the separation of the chains, the displacement of the niobium ion is in the same direction in all chains. The above displacement leads to a spontaneous polarization value that is as high as 4-5 pC/cm2. 

The strongest mode observed near 800 cm 1 is polarized along c and is a totally symmetrical vibration mode (Ai) corresponding to the niobium-oxygen vibrations vs (NbO) of infinite chains (NbOF4 )n running along the c -axis. The mode observed at 615 cm 1 is polarized perpendicular to c and corresponds to the NbF vibrations of the octahedrons of the same chains. The mode at 626 cm 1 is attributed to NbF vibrations of isolated complex ions - NbF 2 . The lines at 377, 390 and 272 cm 1 correspond to deformation modes 8(FNbF) of the two polyhedrons. 

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 ... 

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