Pentagonal pyramidal structure, distorted

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.

The distorted pentagonal-pyramidal coordination has been identified also in Mo and W oxo diperoxo siloxane complexes64 (14-17). Crystal structures for species 14, 15 and 16 have been solved, while for the tungsten dinuclear compound, structural identification was suggested on the basis of similar IR and Raman spectra obtained for complexes 16 and 1764. 

A third type of crown ether magnesium complex includes compounds containing an Mg-C bond to a carbon atom of the macrocycle, formed as internally com-plexed Grignard reagents 101 [375, 387, 388]. The crystal structure determination of 101 (X=Br) shows that the metal coordination is distorted pentagonal pyramidal, with bromine in apical position (Mg-Br 2.517 A) and two normal Mg-O distances (2.12 and 2.13 A) and two large interatomic distances (2.33 and 2.49 A) [388]. 

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