Capped trigonal prisms

The structure of these complexes is not regular and has been described as 4 3 (C ) piano stool , which is obtained by slight distortion of a capped trigonal prism (C2d). 

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... 

Figure 9.23 The three lowest energy arrangements for seven points on the surface of a sphere assuming a I Ir6 repulsion law (a) the capped octahedron, (b) the capped trigonal prism, and (c) the pentagonal bi pyramid.

Let US consider the repulsive force model. The repulsive force is proportional to the inverse power n) of the distance (r) force l/r . Based on this consideration alone the pentagonal bipyramidal structure seems to be more stable for small values of n, the capped trigonal prism for intermediate values and the capped octahedron for large values of n, upto the limit of hard-sphere model. It is obvious, however, that such analysis cannot be applied for systems where all the hgands are not equal, i. e. MX Y 7- , and the ligands X and Y are very different from each other. 

The Yb(III) ion in [Yb(acac)3 H2O 0.5CeH6] is seven coordinated and the coordination polyhedron for this complex is identical with the polyhedron found for Yb(l) in the unsolvated complex, i.e. a capped trigonal prism. Like the unsolvated complex, the hemibenzene solvated ones are also linked in pairs by hydrogen bonds. The benzene molecule occupies a centre of S3unmetry in the complex. All intermolecular contacts involving benzene carbon atoms and acac carbons or oxygens are above 3.5 A (84). 

CTP (capped trigonal prism) complexes all the others are the PBP (pentagonal-bipyramidal) complexes. 

Pentagonal bipyramid Capped trigonal prism NbOfCp ]3-, [NbO(R2dtc)3J, MSfRjdtcy, [NbOF6]3, [M(02)F5]2 mf7]2-... 

Cap O = capped octahedroni CTP = capped trigonal prism, TP - trigonal prism. 

The anions [Mo(S2CNR2)X(CO)2j (R = Me or Et X = F or N3) are obtained by treatment of [Mo(S2CNR2)(CO)2] with X" the crystal structure of [NEt4][Mo(S2CNR2)F(CO)2] shows the geometry at molybdenum to be a fluoride-capped trigonal prism, very similar to that seen... 

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