Coordination polyhedra octahedron

The real structures of these phases are more complex. The coordination of the Ti atoms is always six, but the coordination polyhedron of sulfur atoms around the metal atoms is in turn modulated by the modulations of the Sr chains. The result of this is that some of the TiS, polyhedra vary between octahedra and a form some way between an octahedron and a trigonal prism. The vast majority of compositions give incommensurately modulated structures with enormous unit cells. As in the case of the other modulated phases, and the many more not mentioned, composition variation is accommodated without recourse to defects. ... 

A formulation of structures by layers, such as that represented by the sequence (1) or (2), allows one to derive in a simple way the coordination of the atoms. In the case of perovskite, for example, the atom A of a layer (AX)C is surrounded by twelve atoms X, four located at the corners of the same mesh, and eight at the midpoints of the edges of the (BX2)0 layers above and below (AX)C. The coordination polyhedron is a cuboctahedron. In the case of the rock salt structure, the atom A of a layer (AX)C has coordination six, being surrounded by four atoms X at the corners of the same mesh, and by two atoms X at the center of the meshes (AX)0 above and below (AX)C. In this case the coordination polyhedron is an octahedron. 

The geometric shape of the fixed positions occupied by ligating atoms is the coordination polyhedron. The common coordination polyhedra are the tetrahedron, square plane, trigonal bipyramid, square pyramid, octahedron and trigonal prism, for coordination numbers four, five and six. Distortions from the idealized coordination polyhedra occur due to size requirements of ligands and electronic. effects such as the Jahn-Teller effect. Nevertheless, it is common practice to describe the overall geometry of the ligand environment as an idealized coordination polyhedron. 

In some cases, the CFSE attained by a transition metal ion in a regular octahedral site may be enhanced if the coordination polyhedron is distorted. This effect is potentially very important in most silicate minerals since their crystal structures typically contain six-coordinated sites that are distorted from octahedral symmetry. Such distortions are partly responsible for the ranges of metal-oxygen distances alluded to earlier, eq. (6.6). Note, however, that the displacement of a cation from the centre of a regular octahedron, such as the comparatively undistorted orthopyroxene Ml coordination polyhedron (fig. 5.16), also causes inequalities of metal-oxygen distances. 

The complex (f-BuCp)Sml2 3THF is a rare example of an iodide derivative and the Sm coordination polyhedron is a distorted octahedron [25]. The Sm-I bond distances are 3.107(1) and 3.186(2) A. In the complex Cp°Sml2 2THF, where Cp° = 2-methoxyethyl cyclopentadienyl, the Cp° ligand forces the two iodine atoms into cis positions while the iodines are in the transpositions in the complex (r-BuCp)Sml2 3THF. 

In these compounds the [Mn06] octahedron is clearly becoming unstable the known compounds are not generally robust, but there are several interesting exceptions. There are yet no indications of any coordination polyhedron other than the octahedron. 

An X-ray structural analysis of bis(benzoato)bis(quinoline)-nickel(II), Ni(quin)2-(QH5COO)2 shows that the centrosymmetrical islands of [Ni(quin)2(C6H5COO)2] (Fig. 39) are held together only by van der Waals forces (Fig. 40). The dihedral angle between the plane of O2C and that of the respective phenyl ring is 11.0°. The coordinated polyhedron of Ni(II) is a distorted octahedron. The bond lengths of Ni-O(l), Ni-0(2)... 

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