Octahedral arrangement

Tris(2,4-pentanedionato)iron(III) [14024-18-1], Fe(C H202)3 or Fe(acac)3, forms mby red rhombic crystals that melt at 184°C. This high spin complex is obtained by reaction of iron(III) hydroxide and excess ligand. It is only slightly soluble in water, but is soluble in alcohol, acetone, chloroform, or benzene. The stmcture has a near-octahedral arrangement of the six oxygen atoms. Related complexes can be formed with other P-diketones by either direct synthesis or exchange of the diketone into Fe(acac)3. The complex is used as a catalyst in oxidation and polymerization reactions. 

The hexachloride, uranium hexachloride [13763-23-0], UCl, is best prepared by chlorination of UCl [10026-10-5] with SbCl. An alternative preparative approach is the disproportionation UCl [13470-21 -8] to UCl and UCl under reduced pressure. The obvious disadvantage of the second method is contamination by UCl, however, sublimation is a possible purification technique. Isostmctural with the hexafluoride, the hexachloride is monomeric with an octahedral arrangement of the chlorine atoms around the uranium center. 

There are two essential consequences of this relation. Because larger droplets sediment or rise much faster (a 5-p.m drop rises 625 times faster than a 0.2-p.m droplet), the process is equal to shearing, leading to enhanced flocculation. The ratio between flocculation due to shear and to diffusion of droplets is proportional to the cube of the radius. Secondly, flocculation to droplet aggregates means an enhanced sedimentation rate. Sis drops ia an octahedral arrangement gives approximately four times the sedimentation rate. 

Figure 5.4 The molecular structure of basic beryllium acetate showing (a) the regular tetrahedral arrangement of 4 Be about the central oxygen and the octahedral arrangement of the 6 bridging acetate groups, and (b) the detailed dimensions of one of the six non-planar 6-membeted heterocycles. (The Be atoms are 24 pm above and below the plane of the acetate group.) The 2 oxygen atoms in each acetate group are equivalent. The central Be-O distances (166.6 pm) are very close to that in BeO itself (165 pm).

We need six orbitals to accommodate six electron pairs around an atom in an octahedral arrangement, as in SF6 and XeF4, and so we need to use two d-orbitals in addition to the valence s- and p-orbitals to form six sp3d2 hybrid orbitals (Fig. 3.18). These identical orbitals point toward the six corners of a regular octahedron. 

Triphosphorus anion Pj" (16e) was calculated to be linear [11]. Honea et al. [12] prepared and isolated Si (16e) by low-energy deposition into a solid nitrogen matrix, and carried out a Raman spectra study to show that Si is a planar rhombus The Al " tetraanion (16e) stabilized by the three LP ions in the most stable structure of LijAl " is rectangular in a capped octahedral arrangement [13],... 

Focusing on reactions using the Fluid Matrix Technique, we have studied the interaction of chromium vapor with 2 at 200 K (13). The resulting film was found to contain metal complexes encapsulated within the polymer in which the isocyanide group adopts a well-defined octahedral arrangement around the chromium center, i.e. a species of type Cr(CN-[P])g. Since characterization of this metal complex within the polymer is not trivial we shall develop the analysis in a little detail. 

The size of the octahedral interstices follows from the construction of Fig. 7.2 (p. 53). There, it is assumed that the spheres are in contact with one another just as in a packing of spheres. A sphere with radius 0.414 can be accommodated in the hole between six octahedrally arranged spheres with radius 1. 

Figure 4.5 The octahedral arrangement of six equal circles on a sphere maximizes the area covered by the circles.

Anhydrous alumina can have a variety of structures, from an entirely disordered one (truly amorphous), through short-range ordered amorphous, to highly ordered in a tetrahedral or octahedral arrangement (y,y -or a-alumina) [cf. Section IV(3)]. 

Hexacoordinated silicon takes benefit from the favorable octahedral arrangement of ligands around a coordination center. As expected, negatively charged substituents are superior in stabilizing high silicon coordination numbers. Most recently however, compounds with chelate ligands, mono anionic derivatives and compounds with more electropositive donors have been added to the series of penta- and hexacoordinated silicon species. 

FIGURE 7.3 Anions surrounding a cation in an octahedral arrangement. Only the four in a plane are shown. 

For the NaCl crystal, the radius ratio is 0.54, which is well within the range for an octahedral arrangement of anions around each cation (0.414 - 0.732). However, because this is a 1 1 compound, there are equal numbers of cations and anions. This means that there must be an identical arrangement of cations around each anion. In fact, for 1 1 compounds, the environment around each type of ion must be identical. We can see that this is so from a very important concept known as the electrostatic bond character. If we predict (and find) that six Cl- ions surround each Na+, each "bond" between a sodium ion and a chloride ion must have a bond character of 1/6 because the sodium has a unit valence, and... 

Ligand (69) coordinates to nickel such that the four donors and the metal ion form a planar array whereas (68) coordinates around one face of an octahedral arrangement. Each complex type exhibits a characteristic kinetic inertness which no doubt arises from the operation of the macro-cyclic effect. Indeed, because of the inertness of the cation [Ni(tri)(H20)3]2+, its resolution into optical isomers has been possible... 

The anion [Osg(CO)18p has an octahedral arrangement of metal atoms of approximately Oh symmetry, and is crystallographically very similar to the [HRus(CO)w]- ion. This collection of structural data on electron-equivalent systems emphasizes some of the dangers in trying to predict the structure of complexes solely on the basis of electron counting procedures (220). 

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