Octahedron model

Figure 10 The effect of particle size on the change in the normalized Pt J-band vacancies d-band vacancies/% surface atoms) (...) in going from 0.54 to 0.0 V. The change in the Pt-Pt coordination number as determined from the Pt L3 edge EXAFS analysis is also shown (—). The ordinate axis refers to the change in the J-band vacancy/atom determined from XANES spectra normalized with respect to total number of surface atoms present (based on cluster calculations on a cubo-octahedron model). 

Acknowledgements. I thank heartily N.P. Lazarev and I.M. Mikhailovskij for their active cooperation in investigating some problems of metallic-glass physics that found reflection in the present chapter and T. Ninomia for discussing the details of the tetrahedron-octahedron model. Helpful discussions with V. Baryakhtar and F. Hensel are most appreciated. 

Fig. 9.9 Octahedron models of aqua copper and ammine copper complexes...

Problem Students know the ionic symbols like Na+(aq) and Cl (aq) with the (aq)-symbol one will show that about four to six water molecules are surrounding one ion attached by electrostatic forces. With a simple experiment about the dissolving process of three different copper salts, the student will conclude that the Cu2 + (aq) is responsible for the same blue color of all three solutions. Adding ammonia solution to all three liquids, an identical deep-violet solution results. What particles are now responsible for this new color The teacher has to introduce copper complexes, symbolized by special symbols [Cu(H20)6]2+ and [Cu(NH3)4(H20)2]2+. These six ligands in both complexes combine with the central ion by a special structure which can be shown by octahedron models (see Figs. 9.2 and 9.9). 

Figure A4.1 Two views of the completed octahedron model (a) with a Q axis vertical (b) with a C4 axis vertical.

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). 

Fig. 1. Methods for representing SiO and AlO tetrahedra by means of (a) baH-and-stick model, (b) soHd tetrahedron, (c) skeletal tetrahedron, and (d) spare-filling of packed spheres (1). (e) Linking of four tetrahedra in a four-membered ring, (f) Secondary building unit called tmncated octahedron as...

Fig. 4. A model representing a possible structure for TiO-i (Structured), composed of staggered strings of oclahedra (Fig. 3) combined by sharing octahedron corners only.

Fig. 6. A photograph of a model representing one half of the unit cube. The arrangement of the six 24 e octahedra sharing edges with an 8e octahedron is clearly shown.

The structure of CaB contains bonding bands typical of the boron sublattice and capable of accommodating 20 electrons per CaB formula, and separated from antibonding bands by a relatively narrow gap (from 1.5 to 4.4 eV) . The B atoms of the B(, octahedron yield only 18 electrons thus a transfer of two electrons from the metal to the boron sublattice is necessary to stabilize the crystalline framework. The semiconducting properties of M B phases (M = Ca, Sr ", Ba, Eu, Yb ) and the metallic ones of M B or M B5 phases (Y, La, Ce, Pr, Nd ", Gd , Tb , Dy and Th ) are directly explained by this model . The validity of these models may be questionable because of the existence and stability of Na,Ba, Bft solid solutions and of KB, since they prove that the CaB -type structure is still stable when the electron contribution of the inserted atom is less than two . A detailed description of physical properties of hexaborides involves not only the bonding and antibonding B bands, but also bonds originating in the atomic orbitals of the inserted metal . ... 

Views of sulfur hexafluoride (a) Lewis stmeture (b) ball-and-stick model (c) ball-and-stick model showing the triangular faces of the octahedron. 

The four-coordinate model complex RuHCl(PH3)2 is not planar but has a saw-horse geometry withtrans phosphines and H-Ru-Cl = 101.3°. This angle illustrates that a d6 tetra coordinated complex prefers to be a piece of an octahedron with two empty coordination sites in order to keep the six electrons of the metal in nonbonding orbitals (essentially similar to the t2g set of an octahedron). 

Let US consider the repulsive force model. The repulsive force is proportional to the inverse power n) of the distance (r) force l/r . Based on this consideration alone the pentagonal bipyramidal structure seems to be more stable for small values of n, the capped trigonal prism for intermediate values and the capped octahedron for large values of n, upto the limit of hard-sphere model. It is obvious, however, that such analysis cannot be applied for systems where all the hgands are not equal, i. e. MX Y 7- , and the ligands X and Y are very different from each other. 

Hoefdraad was able to explain his results with a relatively simple model. Following Schmidtke 6) he assumed the sequence of ligand m. o. s in an octahedron to be... 

He, F.-C., Liu, L.-B., and Li, X.-V. (1990). Molecular models constructed in an easy way. Part 1. Models of tetrahedron, trigonal bipyramid, octahedron, pentagonal bipyramid and capped octahedron, y. Chem. Educ. 67,556—558. 

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