Bipyramidal structure, trigonal

Similarly, the classical tetrahedrane D with six 2c2e bonds may be classified as a nido-cluster having 2n + 4 SE. It is derived from the doso-structure (trigonal-bipyramid), with one apical vertex missing. 

Aluminum has 3d orbitals relatively accessible, and not only may the valency of aluminum rise above four, but some d character may be present in the bonds of the tetravalent and also in the bonds of the trivalent aluminum compounds. At present only few organic aluminum compounds with five- and six-coordinated aluminum are known (sp3d and sp3d2 hybrids see Sections III,D and IV,C). The differences between the behavior of aluminum and boron compounds can partially be explained by the possibility of formation of these structures (trigonal bipyramid, octahedron). 

The structure of sulphur tetrafluoride, and probably also SeF and TeF4, is trigonal bipyramidal with one position occupied by a lone pair of electrons ... 

Complexes of titanium(III) can be made from the trichloride— these are either approximately octahedral, 6-coordinate (for example TiClj.SL (L = ligand) and [TiCljfHjOj, formed when TiCls dissolves in aqueous hydrochloric acid), or 5-coordinate with a trigonal bipyramid structure. 

The stereochemistry of Mg and the heavier alkaline earth metals is more flexible than that of Be and, in addition to occasional compounds which feature low coordination numbers (2, 3 and 4), there are many examples of 6, 8 and 12 coordination, some with 7, 9 or 10 coordination, and even some with coordination numbers as high as 22 or 24, as in SrCdn, BaCdn and (Ca, Sr or Ba)Zni3. " Strontium is 5-coordinate on the hemisolvate [Sr(OC6H2Bu3)2(thf)3]. jthf which features a distorted trigonal bipyramidal structure with the two aryloxides in equatorial positions. ... 

Figure 10.2 Crystal structure of GeF2 (a) projection along the chains, and (b) environment of Ge (pseudo trigonal bipyramidal). The bond to the unshared F is appreciably shorter (179 pm) than those in the chain and there is a weaker interaction (257 pm) linking the chains into a 3D structure.

Turning finally to compounds with bonds from the heavier Group 14 elements to heavier Group 15 elements we may note compounds such as [Sn C(PMe2)3)2] which has the pseudo trigonal bipyramidal structure (6). This complex, which has Sn bonded exclusively to four P atoms, is formed as yellow crystals by the... 

Figure 12.22 Structures of (aj (Cl PNMe) , and (b) jCl(S)PNMc 2- Note the difference in length of the axial P-N and equatorial P-N bonds (and of the axial and equatorial P-Cl bonds) about the trigonal bipyramidal P atoms in (a).

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