Trigonal-bipyramidal geometry preferences

Note The graphical description of the n-sulfurane shows why the endocyclic sulfur is coordinated by nitrogen rather than by sulfur. In a trigonal bipyramidal geometry nitrogen is preferred over sulfur as the axial ligand. 

There are several reasons for the extensive use of copper Lewis acids in stereoselective transformations (i) predictable coordination geometry about the metal, (ii) ready availability, (iii) two oxidation states, and (iv) moderate Lewis acidity. Copper(II) complexes usually adopt a square planar, square pyramidal, or trigonal bipyramidal geometry, whereas Cu(I) complexes have a preference for tetrahedral geometry (Figure 1). 

The crystal structure has been determined by X-ray diffraction methods. This compound is obtained as a mixture of two isomers with the alkyl group located in equatorial or axial positions in a pseudo-trigonal-bipyramidal geometry. In the crude reaction product the equatorial complex is present with a slight preference over the axial in a ratio of 1.8 1. Slow conversion between both isomers is observed at room temperature.81... 

Tetraorganoantimony(V) alkoxides and triorganoantimony(V) dialkoxides are monomeric, five-coordinate compounds, e.g. PluSbOMe and Ph3Sb(OMe)2 [420]. This is not surprising, because the trigonal bipyramidal geometry found in these compounds is a preferred coordination mode of antimony(V). Despite this, tri-phenyl(4-nitrocatecholato)antimony(V) was found to dimerize via Sb- - -O second-... 

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