Oxygen pentagon

Figure 4.11 Molecular structures of typical crown-ether complexes with alkali metal cations (a) sodium-water-benzo-I5-crown-5 showing pentagonal-pyramidal coordination of Na by 6 oxygen atoms (b) 18-crown-6-potassium-ethyl acetoacetate enolate showing unsymmelrical coordination of K by 8 oxygen atoms and (c) the RbNCS ion pair coordinated by dibenzo-I8-crown-6 to give seven-fold coordination about Rb.

The smaller cavities are distorted pentagon dodecahedra (distance of oxygen atoms to center varies between 3.77 and 3.95 A) their average free diameter is only about 5.0 A. The larger cavities are almost spherical the oxygen atoms lie at the vertices of hexadeca-hedra and their free diameter is 6.7 A. The distances have been calculated on the basis of a unit cell edge of 17.40 A as found for propane hydrate. 

The oxygen atoms placed in the above positions form, in each unit cell, a framework consisting of two pentagonal dodecahedra, centered at... 

Figure 4.37 Structure of Ta76W6O203 (a) projection down 001 (b) metal only positions and (c) undulating chains of pentagonal bipyramids, shaded. The metal atoms are small circles and the oxygen atoms are large circles. [Data from S. Schmid, J. G. Thompson, A. D. Rae, B. D. Butler, and R. L. Withers, Acta Crystallogr, B51, 698-708 (1995).]...

Figure 8.7 Structural changes of ReOx species in HZcvd catalyst preparation and the catalytic reaction conditions, and a proposed structure of active [Re6017] cluster in the ZSM-5 pore channel, where the [Re6013] cluster is bound to the pentagonal rings of the zeolite inner wall via three lattice oxygen atoms, and the oxygen atoms are tentatively arranged on the Re6 octahedron.

In certain complexes, even though the coordination number is the same, two molecules in the same unit cell exhibit different conformations. Thus, in Eu(thd)3 DMSO, the gross geometry around Eu(III) is a distorted pentagonal bipyramid with the oxygen of the DMSO occupying one of the apices 283, 284), but the structure of the two molecules in the unit cell differs in detail. A similar situation also exists in Eu(thd)3(DMF)2 283, 284). 

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