Metal coordination polyhedra

Much less common are dimers, where the metal coordination polyhedra share a common face. As examples can be taken [(Bu SiO)Ba([i-OSiBu 3)3Ba(THF)] built up of 2 tetrahedra [497] or bimetallic t-butoxides... 

Inversion of the Metal Coordination Polyhedron along the O-M-O Axis... 

Fig. 5.49. 300 MHz H NMR spectrum of nickel(II)-substituted azurin at pH 7.0 and 303 K, and a schematic drawing of the metal coordination polyhedron (adapted from [97]).

Fig. 17. The metal coordination polyhedron in Nd2Eu203F6 (a) derived as NdiEuiOiF, + F and (b) derived as Nd2Eu203F6 + O.

Fig. 18. (a) The space lattice and linkage of metal coordination polyhedrons (b) The projection of the linkage of the metal coordination polyhedrons in the directions of [100],... 

Oxygen chelates such as those of edta and polyphosphates are of importance in analytical chemistry and in removing Ca ions from hard water. There is no unique. sequence of stabilities since these depend sensitively on a variety of factors where geometrical considerations are not important the smaller ions tend to form the stronger complexes but in polydentate macrocycles steric factors can be crucial. Thus dicyclohexyl-18-crown-6 (p. 96) forms much stronger complexes with Sr and Ba than with Ca (or the alkali metals) as shown in Fig. 5.6. Structural data are also available and an example of a solvated 8-coordinate Ca complex [(benzo-l5-crown-5)-Ca(NCS)2-MeOH] is shown in Fig. 5.7. The coordination polyhedron is not regular Ca lies above the mean plane of the 5 ether oxygens... 

Figure 6.1 The icosahedron and some of its symmetry elements, (a) An icosahedron has 12 vertices and 20 triangular faces defined by 30 edges, (b) The preferred pentagonal pyramidal coordination polyhedron for 6-coordinate boron in icosahedral structures as it is not possible to generate an infinite three-dimensional lattice on the basis of fivefold symmetry, various distortions, translations and voids occur in the actual crystal structures, (c) The distortion angle 0, which varies from 0° to 25°, for various boron atoms in crystalline boron and metal borides.

Figure 6.10 Cubic MBs showing (a) boron octahedra (B-B in range 170-174 pm), and (b) 24-atom coordination polyhedron around each metal atom.

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. 

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. ... 

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). 

Shinn and Eick 212) found that the orthorhombic crystals of La2(C03)3-8H2O contains two distinctive decacoordinated La(III) ions (space group Peon, Z—A, a =8.934, 6 = 9.580, c = 17.00 A). The coordination polyhedron of La is made up of ten oxygen atoms from water molecules, bidentate carbonates and xmidentate carbonates. One fourth of the water molecules in this compound are not bonded to the metal ion but these are situated between layers. The coordination polyhedron is assumed to be distorted Civ The oxygens of the bidentate carbonate ion form longest La—O bond [La(l)—0(bident. CO3) =2.73 A, La(2)— O (bident. C03)=2.74A], whereas the water molecules are nearer to the lan-thanium ion (La—OH2 =2.63 A). 

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