Coordination dodecahedron

Rotating single-crystal measurements also permitted the extraction of the orientation of the magnetic tensor in the molecular reference frame and the experimental easy axis was found to coincide with the idealized tetragonal axis of the coordination dodecahedron of Dy. Crystal field calculations assuming idealized tetragonal symmetry permitted the reproduction of magnetic susceptibility data for gz = 19.9 and gxy 0 [121]. More elaborated calculations such as ab initio post Hartree-Fock CASSCF confirmed this simple analysis [119]. 

Fig. 5.18. Coordination dodecahedron in the tetrakis (hexafluoroacetylacetonato)yttrate(III) ion viewed down the two-fold axis through the yttrium ion (reprinted with permission from Bennett, M.J., F.A. Cotton, P. Legzdins and S.J. Lippard, 1968, Inorg. Chem. 7, 1770-1776).

TiCRjtftc) was prepared starting from Ti(NR2)4 4,5) or TiCl4 (6). The structure of the ethyl compound was elucidated by X-ray studies (7, S). The titanium atom is eight-coordinated by the sulfur atoms of the four chelating ligands and the coordination geometry closely approximates to that of a dodecahedron. 

Possible stereochemistries for eight-coordinate [Ln(H20)8]3+ are the dodecahedron, the square antiprism, and the cube, although the last seems less likely (295). The proposed nine-coordinate [Ln(H20)9]3+ tri-capped trigonal prism transition state is similar to the [Ln(H20)9]3+ structures observed in the solid state. Thus, the possible water exchange... 

Ten-coordinate (each NOs group is bidentate) structure dodecahedron with B-type vertices... 

Figure 9.34 The typical coordinating polyhedra for a dense regular monospheres packing (a) cuboctahedron, (b) anticuboctahedron, (c) dodecahedron, and (d) icosahedron.

Octacoordination is often encountered in lanthanide complexes. The preferred poly-hedra for eight coordination expected on the basis of interligand repulsivities are square antiprism (D ), dodecahedron with triangular faces (Z)2d), bicapped octahedron (D3(i), truncated octahedron (Z)2ft), 4,4-bicapped trigonal prism (C2v), distorted cube (C2v), and cube (0/,). The most commonly observed polyhedra for this coordination number are, however, the square antiprism and the dodecahedron. 

The compound Gd(H20)6 Cl3 is eight coordinate with six water molecules and two chloride ions coordinating to the metal ion 331). The third chloride remains ionic. The complex units are held together by hydrogen bonds through both the ionic and coordinated chloride ions. Each noncoordinated Cl" ion forms six Cl... H-0 bonds and each coordinated Cl- ion forms three Cl... H-0 bonds in addition to the Cl—Gd bond. The coordination geometry around Gd(III) has been described as distorted dodecahedron. In [Eu(H20)6 C12]C1, the coordination geometry has been described as distorted square antiprism 332). Similar compounds of Nd(III), Sm(III), Dy(III), Ho(III), and Er(III) are isostructural. 

A polyhedron based on bicapped dodecahedron possibly exists in decacoordi-nated [Ce(NOs)5]2- anion, which has been isolated (275) as triphenylethylphos-phonium salt (C2/c). The bicapped dodecahedron is a more appropriate description for the polyhedron than the bicapped square antiprism. However, Al-Karaghouli and Wood 215) from aesthetic point of view preferred to call the geometry of the coordination polyhedron as being trigonal bipyramid. 

---