Trigonal pyramidal VSEPR structure

S4.8 If the N atom lone pair of (H3Si)3N is delocalized onto the three Si atoms, it cannot exert its normal repulsion influence as predicted by VSEPR rules. The N atom of (HsSifsN is trigonal planar, whereas the N atom of (HaOsN is trigonal pyramidal. The structures of (H3Si)3N and (H3C)3N, excluding the hydrogen atoms are ... 

Thus, according to this alternate approach, NH3 belongs to the VSEPR class AX3E. Molecules of this type adopt a trigonal pyramidal structure. 

SOLUTION The Lewis structure of nitrogen trifluoride is shown in (22) we see that the central N atom has four electron pairs. According to the VSEPR model, these four electron-rich regions adopt a tetrahedral arrangement. Because one of the pairs is a lone pair, the molecule is expected to be trigonal pyramidal (23). Spectroscopic measurements confirm this prediction. 

The structures of the element trihalides EX3 are covered in a number of textbooks on structural inorganic chemistry (4, 5), and these will not be discussed in great detail here. It is, however, worth mentioning some of the salient structural features. In most cases, a molecular trigonal pyramidal EX3 unit consistent with VSEPR theory predictions is readily apparent in the solid-state structure, although there are usually a number of fairly short intermolecular contacts or secondary bonds present. A general description of the structures as molecularly covalent but as having a tendency toward macromolecular or polymeric networks is therefore reasonable. Only in the case of the fluorides is an ionic model appropriate. 

Section 8.13 molecular structure valence shell electron-pair repulsion (VSEPR) model linear structure trigonal planar structure tetrahedral structure trigonal pyramid trigonal bipyramid octahedral structure square planar structure... 

The heavier atoms of Group 13 appear not to form p -p bonds. Five- and six-coordinate structures (schemes (4) and (5)) are almost entirely restricted to the heavier atoms, although at least one complex containing five-coordinate boron is known. According to VSEPR theory, scheme (4) structures should be trigonal bipyramidal. However, InClf- and TlClf are found to be square pyramidal in crystalline solids. As noted in Section 8.2, this shape (also adopted by MnClf-) is possibly favoured by crystal packing requirements. 

The amended VSEPR model predicts two forms of five-coordination, and experimental chemistry has clearly identified many examples of both forms. These limiting structures are square-based pyramidal (or, simply, square pyramidal) and trigonal bipyramidal (Figure 4.13). The classical square-based pyramidal shape is formed simply by cleaving off one bond from an octahedral shape, which leaves the metal in the same plane as the four square-based ligands. In reality, almost no complexes exhibit this shape, but rather adopt a distorted... 

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