Coordination octahedral

Just as four electron pairs experience the minimum repulsion when they are directed toward the corners of a tetrahedron, six electron pairs will try to point toward the corners of an octahedron. An octahedron is not as complex a shape as its name might imply it is simply two square-based pyramids joined base to base. You should be able to sketch this shape as well as that of the tetrahedron. 

At first, you might think that a coordination number of six is highly unusual it certainly violates the octet rule, and there are only a few molecules (SFg is one) where the central 

There are well known examples of 6-coordinate central atoms with 1,2, and 4 lone pairs. Since the corners of an octahedron are geometrically equivalent, the distinction between axial and equatorial atoms that we had to consider in the 5-coordinate case no longer exists thus the horizontal rectangles you see in the sketches below have no special significance and could just as well be placed in either of the two possible vertical planes. 

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Cobalt II) halides can be obtained by direct combination of the elements, or by dehydration of their hydrates. Anhydrous cobalt(II) chloride is blue, and the solid contains octahedrally-coordinated cobalt the hydrated salt C0CI2. bHjO is pink, with each cobalt surrounded by four water molecules and two chloride ions in a distorted octahedron. 

Addition of halide ions to aqueous copper(II) solutions can give a variety of halo-complexes for example [CuCl4] (yellow square-planar, but in crystals with large cations becomes a flattened tetrahedron) [CuClj] (red, units linked together in crystals to give tetrahedral or distorted octahedral coordination around each copper). 

The coefficients C are chosen to ensure that the function has a minimum at the appropriate reference bond angle. For linear, trigonal, square planar and octahedral coordination, Fourier series with just two terms are used with a Cq term and a term for n = 1, 2, 3 or 4, respectively ... 

Recently Desimoni et used the same bis(oxazoline) ligand in the magnesium(II) catalysed Diels-Alder reaction of the N-acyloxazolidinone depicted in Scheme 3.4. In dichloromethane a modest preference was observed for the formation of the S-enantiomer. Interestingly, upon addition of two equivalents of water, the R-enantiomer was obtained in excess. This remarkable observation was interpreted in terms of a change from tetrahedral to octahedral coordination upon the introduction of the strongly coordinating water molecules. 

MOMEC is a force field for describing transition metal coordination compounds. It was originally parameterized to use four valence terms, but not an electrostatic term. The metal-ligand interactions consist of a bond-stretch term only. The coordination sphere is maintained by nonbond interactions between ligands. MOMEC generally works reasonably well for octahedrally coordinated compounds. 

In the face-centred cubic structure tirere are four atoms per unit cell, 8x1/8 cube corners and 6x1/2 face centres. There are also four octahedral holes, one body centre and 12 x 1 /4 on each cube edge. When all of the holes are filled the overall composition is thus 1 1, metal to interstitial. In the same metal structure there are eight cube corners where tetrahedral sites occur at the 1/4, 1/4, 1/4 positions. When these are all filled there is a 1 2 metal to interstititial ratio. The transition metals can therefore form monocarbides, niU ides and oxides with the octahedrally coordinated interstitial atoms, and dihydrides with the tetrahedral coordination of the hydrogen atoms. 

Figure 13.5 A Mg + atom links GTP to the Ras protein. Mg Is coordinated to one oxygen atom each from the p and y phosphates of GTP as well as to the side chains of Set 17 and Thr 35 of Ras. Two water molecules complete the octahedral coordination of Mg.

Figure 7.7. The spinel strueture. The unit eell can be divided into octants - tetrahedrally coordinated cations A. octahedrally coordinated eations B, and oxygen atoms (large cireles) are shown in two oetants only (adapted from Smit and Wijn 1959).

Table 4.3 indicates that octahedral coordination is a common mode for Li. Less usual is planar 6-fold coordination (Fig. 4.8a), pentagonal pyramidal coordination (Fig. 4.8i) or irregular 6-fold coordination (Fig. 4.9a). Examples of 7-fold coordination are in Fig. 4.9b and c. Lithium has cubic 8-fold coordination in the metallic form and in several of its alloys with metals of large radius. It is also 8-coordinate in the dilithionaphthalene complex shown in Fig. 4.9d here the aromatic... 

While it remains true that tetrahedral and octahedral coordination modes are the predominant stereochemistries adopted by the group 13 metals, nevertheless increasing diversity is being achieved by carefully selecting appropriate electronic and geometric features to enhance the stabilization of unusual stereochemistries. Some representative examples follow. 

High-temperature neutron diffraction studies have shown that this latter phase has the cubic ordered Re03-type structure (p, 1047) with octahedral coordination of both types of Sn atoms by F (Sn -F 229 pm, Sn" -F 186 pm). The fi-phase also features octahedral coordination in a structure closely related to that of rhombohedral LiSbF6. 

Tin(IV) halides are more straightforward. Snp4 (prepared by the action of anhydrous HF on SnCU) is an extremely hygroscopic, white crystalline compound which sublimes above 700°. The structure (unlike that of CF4, SiF4 and GeF4) is polymeric with octahedral coordination... 

A recent addition to the many examples of octahedral coordination of O (Table 14.3) is the unusual volatile, hydrocarbon-soluble, crystalline oxo-alkoxide of barium [H4Ba6(/i(i-0)(0CH2-CH2,OMe)i4l, which forms rapidly when Ba granules are reacted with Me0CH2CH20H in toluene suspension. ... 

Figure 21.3 Two representations of the structure of perovskite, CaTi03, showing (a) the octahedral coordination of Ti, and (b) the twelve-fold coordination of Ca by oxygen. Note the relation of (b) to the cubic structure of Re03 (p. 1047).

These alkoxides are liquids or sublimable solids and, unless the steric effects of the alkyl chain prevent it, apparently attain octahedral coordination of the titanium by polymerization (Fig. 21.6). The lower alkoxides are especially sensitive to moisture, hydrolysing to the dioxide. Application of these organic titanates (as they are frequently described) can therefore give a... 

Most niobates and tantalates, however, are insoluble and may be regarded as mixed oxides in which the Nb or Ta is octahedrally coordinated and with no discrete anion present. Thus KMO3, known inaccurately (since they have no discrete MO3 anions) as metaniobates and metatantalates, have the perovskite (p. 963) stmcture. Several of these perovskites have been characterized and some have ferroelectric and piezoelectric properties (p. 57). Because of these properties, LiNb03 and LiTa03 have been found to be attractive alternatives to quartz as frequency filters in communications devices. 

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