Tetrahedral and Square-Planar Five-Atom Molecules

TETRAHEDRAL AND SQUARE-PLANAR FIVE-ATOM MOLECULES 2.6.1. Tetrahedral XY4 Molecules (Tj,) 

Fundamental frequencies of XH4-type molecules are listed in Table 2.6a. The trends V3 Vi and V2 V4 hold for the majority of the compounds. The XH stretching frequencies are lowered whenever the XH4 ions form hydrogen bonds with counter- 

TABLE 2.5d. Vibrational Frequencies of WXYZ and Related Molecules (cm )  

In a series of ammonium halide crystals, the v(NH4 ) becomes lower and the 5(NH4 ) becomes higher as the N—H- X hydrogen bond becomes weaker in the order X = F Cl Br I. 

In the solid state, V3 and V4 may split into two or three bands because of the site effect. In some cases, the MX4 ions are distorted to a flattened tetrahedron (D2 ) or a structure of lower symmetry (C ) [892,912]. According to an X-ray analysis [913] the unit cell of [(CH3)2CHNH3]2 [CUCI4] contains two square-planar and four distorted tetrahedral [CuCU4] ions. 

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TETRAHEDRAL AND SQUARE-PLANAR FIVE-ATOM MOLECULES... 

The five-atom XY4 molecules and ligands commonly adopt tetrahedral and square-planar shapes. The normal modes of tetrahedral and square-planar XY4 are shown in Figure 5.5. Tetrahedral XY4 molecules show two normal modes that are infrared-active , while the square-planar XY4 molecules show three... 

The possible structures may be classified in terms of the coordination number of the central atom and the symmetry of the resulting molecule (Fig. 6.17). Two groups about a central atom will form angular (p2 orbitals, C, symmetry) or linear sp hybrid, D , symmetry) molecules three will form pyramidal (p, C3,) or trigonal planar (sp, Dy) molecules four will usually form tetrahedral (sp, Tj) or square planar (dsp, five usually form a trigonal bipyramidal (Dy,), more rarely a square pyramidal (C4,) molecule (both dsp hybrids, but using different orbitals, see Table 6.2) and six groups will usually form an octahedral molecule (d sp, O,). 

In addition to tetrahedral, another common shape for AB4 molecules is square planar. All five atoms lie in the same plane, with the B atoms at the corners of a square and the A atom at the center of the square. Which shape in Figure 9.3 could lead to a square-planar shape upon removal of one or more atoms ... 

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