Equatorial atoms

The common structural element in the crystal lattice of fluoroaluminates is the hexafluoroaluminate octahedron, AIF. The differing stmctural features of the fluoroaluminates confer distinct physical properties to the species as compared to aluminum trifluoride. For example, in A1F. all corners are shared and the crystal becomes a giant molecule of very high melting point (13). In KAIF, all four equatorial atoms of each octahedron are shared and a layer lattice results. When the ratio of fluorine to aluminum is 6, as in cryoHte, Na AlF, the AIFp ions are separate and bound in position by the balancing metal ions. Fluorine atoms may be shared between octahedrons. When opposite corners of each octahedron are shared with a corner of each neighboring octahedron, an infinite chain is formed as, for example, in TI AIF [33897-68-6]. More complex relations exist in chioUte, wherein one-third of the hexafluoroaluminate octahedra share four corners each and two-thirds share only two corners (14). 

FIGURE 3.4 (.0 A lone pair in an axial position is close to three equatorial atoms, (bi In an equatorial position, a lone pair is close to only two atoms, a more favorable arrangement. 

Sn this is not so clear. The 22 valence electrons of the Sn ion could be accommodated in exact agreement with the octet rule according to the formula given in the margin. However, calculations with the electron localization function show that lone electron pairs are also present at the equatorial atoms therefore, only six electron pairs remain for the bonds. This corresponds to the number expected according to the Wade rules, as for bo-ranes ( + 1 multicenter bonds in a closo cluster with n = 5 vertices, cf. p. 144). We will deal with the bonding in such cluster compounds in Section 13.4. 

There are well known examples of 6-coordinate central atoms with 1,2, and 4 lone pairs. Since the corners of an octahedron are geometrically equivalent, the distinction between axial and equatorial atoms that we had to consider in the 5-coordinate case no longer exists thus the horizontal rectangles you see in the sketches below have no special significance and could just as well be placed in either of the two possible vertical planes. 

A class of fluxional molecules of great importance are those with a tbp configuration. When all five appended groups are identical single atoms, as in AB5, the symmetry of the molecule is Dih. The two apical atoms Bj and B2 (Fig. 1-7) are equivalent but distinct from the three equatorial atoms B3, B4, B5, which are equivalent among themselves. In general, experiments such as measuring nmr spectra of B nuclei, which can sense directly the kind of environmental difference represented by B], B2 versus B3, B4, B5, should indicate the presence of two sorts of B nuclei in tbp... 

Careful purification of solutions of fluorosilane and fluorosilicate anions and a decrease in concentration and temperature result in a fine structure of F NMR spectra due to a non-equivalent arrangement of ligands (Table 16). For example, in the F NMR spectra of octahedral complexes of the type RSiFj" a doublet (equatorial atoms) and a quintuplet are observed whose mutual arrangement depends on the nature of the ligand In fluorine derivatives of penta- or hexa-coordinate silicon with slow exchange, the Hgip values (if they can be determined) are greater for the fluorine atoms with chemical shifts in low field. 

This structure is called a trigonal bipyramid. There are two different bond angles in this structure. Between the atoms in the trigonal plane, the angle is 120°, while between the axial and equatorial atoms the angle is only 90°, i.e. a right angle. This is an important shape in the transition of one saturated carbon centre to another. 

The compound (12) has six tin atoms arranged in octahedral geometry with an S4 axis of symmetry, (Figure 2.12.13). Considering the lone pair in the geometrical arrangement, both the axial and equatorial atoms are in five-coordinate square-based pyramidal geometries, and in contrast to other reported structures,no p,3-ONeoPent bonds are formed due to the steric bulk of ONeoPent. 

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