Coordination polyhedra centers

The coordination polyhedron results when the centers of mutually adjacent coordinated atoms are connected with one another. For every coordination number typical coordination polyhedra exist (Fig. 2.2). In some cases, several coordination polyhedra for a given coordination number differ only slightly, even though this may not be obvious at first glance by minor displacements of atoms one polyhedron may be converted into another. For example, a trigonal bipyramid can be converted into a tetragonal pyramid by displacements of four of the coordinated atoms (Fig. 8.2, p. 71). 

For mixed lanthanide-transition metal clusters, Yukawa et al. have synthesized an octahedral [SmNi6] cluster by the reaction of Sm3+ and [Ni(pro)2] in nonaque-ous medium [66-68]. The six [Ni(pro)2] ligands use 12 carboxylate oxygen atoms to coordinate to the Sm3+ ion, which is located at the center of an octahedral cage formed by six nickel atoms. The coordination polyhedron of the central Sm3+ ion may be best described as an icosahedron. The [SmNir, core is stable in solution but the crystal is unstable in air. The cyclic voltammogram shows one reduction step from Sm3+ to Sm2+ and six oxidation steps due to the Ni2+ ions. Later, similar [LaNis] and CjdNif> clusters were also prepared. 

Furthermore, protonation results in a significant distortion of the coordination polyhedron, i.e., the metal ion is displaced from the plane formed by the four cyano ligand carbon atoms toward the oxo along the M = 0 axis by as much as 0.34 A, which represents about 20% of the total metal-oxo bond length. In spite of this distortion stronger metal-cyano bonds are observed crystallographically, suggesting a better n back-donation by the metal center to the cyano carbons since d-ff overlap is increased. This observation is in line with both the 13C and 15N chemical shift and kinetic data (Section V) for the protonated complexes (8). 

This reasoning also holds for the reactivity on the Mo(IV) center as illustrated in Fig. 19b and confirmed experimentally (7). A deviation of the experimental points for the inversion of the coordination polyhedron was obtained from the oxygen exchange on the oxo site in the [MoO(OH2)(CN)4]2 complex at pH <6 (Fig. 19b was observed and was interpreted in Section VI,B). 

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. 

According to an X-ray study (86MI2) the halogen atoms (X = Br, Cl) in 10-Te-4 telluranes 120 take the axial positions in the trigonal-bipyramidal coordination polyhedron of the tellurium center. 

These points are to be supplemented by the coordinates of center points of all the polyhedron faces, Table 3.32. The coordinates of the polyhedron center point are found by averaging appropriate coordinates of all the eight design vertices and the centroid coordinates of faces by averaging the coordinates of the points belonging to the face, Table 3.32. 

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