Tricapped coordination geometry

Chlorides. The oHve-green trichloride [10025-93-1], UCl, has been synthesized by chlorination of UH [13598-56-6] with HCl. This reaction is driven by the formation of gaseous H2 as a reaction by-product. The stmcture of the trichloride has been deterrnined and the central uranium atom possesses a riine-coordinate tricapped trigonal prismatic coordination geometry. The solubiUty properties of UCl are as follows soluble in H2O, methanol, glacial acetic acid insoluble in ethers. 

More illustrative is the structure of the fluorosilane which exhibits the same tricapped tetrahedron geometry. The three N-Si interactions take place in the frontal position towards the Si-F bond instead of the rear coordination observed in penta- and hexacoordinated structures. Furthermore, the benzylamino groups have the possibility not to be coordinated at silicon since these groups have the capability of free rotation around the carbon-nitrogen bond. 

As is common with the higher coordination geometries, the interconversion of Dzh Cav niay be easily achieved. The monocapped square antiprism (C4 ) can be generated from the s-tricapped trigonal prism by raising the points a and b (Fig. 27) and lowering the two caps c and d so that the four form the basal plane of the square antiprism. 

A by-product of the preparation of U(Et2NC02)4 from the tetrachloride and the amine is a product of composition U402(Et2NC02)i2 this is a tetramer in which there are two non-equivalent uranium(lV) sites. In one of them the coordination geometry is a distorted tricapped trigonal prism, and the geometry of the other does not fit any type of regular polyhedron.1 5... 

The malonate complex of Eu(III), Eu2(malonate)3 8H2O is interesting since there are two different coordination geometries, and only five of the water molecules are coordinated [167], One of the Eu ions is eight-coordinate in the form of a distorted square antiprism and the other Eu ion is nine-coordinate, distorted tricapped trigonal prism. In the analogous neodymium complex, six water molecules are coordinated because of the difference in ionic radius. The neodymium complex is nine-coordinate with monocapped antiprismatic geometry [168]. 

Tricapped trigonal prismatic is the most familiar example of nine-coordinate geometry, adopted for the [Ln(H20)9] + ions of all lanthanides in a number of crystalline salts. It is also found in a number of chlorides LnCls (Ln = La-Gd) and bromides LnBrs (Ln = La-Pr). This geometry is also sometimes adopted where polydentate ligands are involved, such as the [Ln(terpy)3] + ion (terpy = 2,2 6 ,2 -terpyridyl see Figure 4.7). 

Dipicolinates (pyridine-2,6-dicarboxylates) have been investigated in some detail with structures determined for several tris complexes such as Na3[Nd(dipic)3]-15H20 and Na3[Yb(dipic)3]13H20, typical of the early and later lanthanides, respectively. The lanthanides have essentially tricapped trigonal prismatic coordination geometries, isostruc-tural along the lanthanide series they have been the subject of important solution NMR studies. Mono and bis complexes can also be synthesized the mono complex [Nd(dipic)(H20)4] CIO4 has a polymeric structure in which each picolinate is bound to three different (nine-coordinate) neodymium ions. 

Figure 10.3 Coordination geometry of a tricapped trigonal-prism (TTP) and a capped square-antiprism (CSAP) form of Gd-DTPA and Gd-DOTA, respectively [14].

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