Pentagonal bipyramids stereochemistry

UV irradiation of [Ta I(CO)6] in the presence of dppe provided, after chromatography on silica gel, the complex TaIH(CO)4(dppe) whose 111 and 31P NMR were interpreted in terms of an H-capped octahedral structure. 4 Photolysis converted this complex to TaH(CO)2(dppe)2 which, in the solid state, exhibited pentagonal bipyramidal stereochemistry with axial CO ligands.644... 

Complexes with mixed 0-and A -donor ligands such as edta and Schiff bases are well known and [Fe(edta)(H20)] and [Fe(salen)Cl] are examples of 7-coordinate (pentagonal bipyramidal) and 5-coordinate (square-pyramidal) stereochemistries respectively. 

Figure 55 Stereochemistries corresponding to the locations marked on the upper left-hand side of Figure 54. (P) Pentagonal bipyramids (O) capped octahedra (T) capped trigonal prisms...

The stereochemistry at each of these locations is shown in Figure 55. It can be seen that the capped octahedron can be considered as an intermediate between the pentagonal bipyramid and the capped trigonal prism, but it is much closer in energy and stereochemistry to the latter. 

Structurally characterised molecules with seven identical ligands are restricted mainly to some fluoro complexes [MF7]-, aquo complexes [M(H20)7]2+, isonitrile complexes [M(CNR)7]2+ and cyanide complexes [M(CN)7]. The stereochemistries are distributed along the long valley connecting the capped trigonal prism and the pentagonal bipyramid. 

In the calcium polyiodide compound [Ca(H2O)7](I10) the stereochemistry appears to be intermediate between a capped octahedron and a pentagonal bipyramid, whereas [Sr(H20)7](I12) and [Ca(H20)7]2[CdgCl16(H20)2]-H20 contain capped trigonal prismatic cations.246 247... 

At small normalized bites, stereochemistry B is a pentagonal bipyramid with the bidentate ligand spanning one of the pentagonal edges, and exists as a minimum on the potential energy surface at b 90°, 4>c 180°. Above b = 1.2 this minimum divides into two minima (Figure 63)... 

Stereochemistry C is intermediate between a pentagonal bipyramid and a capped trigonal prism, and the repulsion energy calculations show that as the normalized bite increases, the bidentate ligand DE progressively swings further away from the pentagonal bipyramid position towards the... 

As for five coordination, a mixture of unidentate ligands may lead to more well-defined stereochemistries due to the different types of polyhedral vertices available and the different spatial requirements of the different hgands. For example, the sterically least hindered sites in these structures are the axial sites of a pentagonal bipyramid as these are the only ones with no neighboring ligands... 

A DMPE adduct of U(BH3CH3)3 has been described. It contains two molecules of phosphine per uranium atom. The complex is tetragonal, space group P43 2i2, Z = 4, with a = 1129.7 and b = 2303 pm. The H3BCH3 units are tridentate and the phosphines bidentate. The stereochemistry around the uranium is effectively a pentagonal bipyramid. 

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