Aluminum octahedron

Figure 3.2. A silicon tetrahedron (left), an aluminum octahedron (middle) as a central layer in a 2 1 clay, and an aluminum octahedron (right) as a surface layer in a 1 1 clay (right). The oxygen atoms are bonded to other silicon and aluminum atoms in the clay (bonds are not intended to be shown at the correct angles). Below is a water molecule showing partially positive hydrogen atoms and partially negative oxygen atoms. Also shown are the two lone pairs of electrons on all the oxygen atoms.

Figure 3.4. Schematic of an aluminum octahedron made up of six oxygen atoms forming an octahedral structure around one aluminum atom. The six oxygens of the octahedron satisfy the three valence bonds of the central aluminum, leaving each oxygen with one and one-half unbalanced valence bonds, which are satisfied by a second aluminum, a silicon, or one hydrogen (from Taylor and Ashcroft, 1972, with permission).

Figure 10. Comparison of As(V)-metai distances obtained for different binding modes of As(V) with (a) corner-sharing Al octahedra as found in gibbsite or in the gibbsite sheet of kaolinite (b) corner-sharing Fe octahedra as found in goethite (c) a Si tetrahedron (d) an aluminum octahedron and (e) an aluminum tetrahedron.

The structure of spinel, MgAl204, showing how the aluminum octahedrons (shown in blue) are edge-sharing and the magnesium tetrahedrons (shown in red) don t touch each other in the crystalline iattice. 

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). 

In the solid state, aluminum chloride exists in a crystalline lattice. Each aluminum atom is surrounded by six chlorine atoms arranged around the metal atoms at the comers of an octahedron. Aluminum bromide and aluminum iodide form AI2 Xj molecules in all three phases. 

Nonpromoted catalysts rapidly lose their activity, particularly under severe conditions of operation. This is in no way contradicted by reports stating that reduced magnetites are good catalysts without addition of promoters. The author had made a spectrographic analysis of an octahedron crystal of Ural magnetite, and the examination disclosed a content of calcium, not less than 1 %, of titanium between 0.5 and 1 % and smaller contents of magnesium and aluminum, which proves that well-known promoters were present in the magnetite matrix. 

Silicon and aluminum, considered the central ion of the tetrahedrons or octahedrons, respectively. 

These differences may result, in part, from the higher coordination number of aluminum in AIF3. In it, each aluminum atom is surrounded by a distorted octahedron of six fluorine atoms. Each fluorine is two-coordinate, being comer-shared by pairs of octahedra. Thus, the stmcture is similar to ReOs and has a relatively open lattice. As a result, numerous sites exist for water molecules, leading to the occurrence of a wide range of nonstoichiometric and stoichiometric hydrates (AIF3 (H20) n = 1, 3, 9). Quite curiously, no hexahydrate corresponding to [Al(H20)6]Cl3 is known. 

In alkali fluoroaluminate solid compounds, aluminum is present only in octahedral coordination with fluorine. According to Spearing et al. (1994) and Smith and Van Eck (1999), their Al chemical shifts range between -13 and -1.4 ppm and are typically more shielded than the AlOe octahedrons in oxide compounds. Only a few studies report lower coordination numbers for Al in fluorides. Kohn et al. (1991) have described the Al MAS NMR spectra of glasses of jadeite mixed with cryolite in terms of the 5-fold and 6-fold coordination of aluminum at 22 and -5 ppm, respectively. Herron et al. (1993) reported a Al chemical shift at 49 ppm for the tetrahedral anion AIFJ in a [l,8-bis-(dimethylamino) naphtathalene H ] [AIFJ] saturated solution. 

Looking at the amount of material on this topic should lead to a useful didactical reduction if only the shown complexes of copper, silver and aluminum are offered to students, they have the chance for successful learning. The complexity can also be reduced with the use of model material like 3D-octahedron and model drawing, and by developing mental models of these complex particles. If acid-base reactions and redox reactions are taught to students, the basic idea of equilibria can be taken to develop the idea of complex reactions. Finally, convincing experiments can be demonstrated or done by students themselves, as will be shown below. 

Aluminum has 3d orbitals relatively accessible, and not only may the valency of aluminum rise above four, but some d character may be present in the bonds of the tetravalent and also in the bonds of the trivalent aluminum compounds. At present only few organic aluminum compounds with five- and six-coordinated aluminum are known (sp3d and sp3d2 hybrids see Sections III,D and IV,C). The differences between the behavior of aluminum and boron compounds can partially be explained by the possibility of formation of these structures (trigonal bipyramid, octahedron). 

FIGURE 5.1. Diagram of (a) a silica tetrahedron, and (b) an octahedron of aluminum, magnesium. or iron. 

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