Examples of anomalous symmetry

Special interest attaches to molecules or crystals that possess symmetry which is lower or higher than might be expected. This statement calls for a brief explanation. The packing of equal spheres is discussed in Chapter 4 where we shall see that the majority of metals adopt certain highly symmetrical structures. Some metals, however, crystallize with less symmetrical variants of these structures, and these are obviously of interest since the lower symmetry presumably indicates some peculiarity in the bonding in such crystals. Some crystals containing only highly 

The subject of isomerism is closely concerned with symmetry. Consider, for example, the three molecules (i) O2X2, (ii) N2X2, and (iii) C2X2. In a molecule of type (i), formed by oxygen and sulphur, the two X atoms are not coplanar with the two 0 (S) atoms the molecule is enantiomorphic, that is, it cannot be brought into coincidence with its mirror image it exists in left- and right-handed forms. 

Description of the molecular geometry requires a knowledge of the dihedral angle XOO/OOX. On the other hand all atoms of (ii) are coplanar, and such molecules exist in cis and trans forms. The four atoms of (iii) are collinear, it has cylindrical symmetry, and there are no isomers. A number of molecules of type (iv) are formed 

The dithionite ion (S2 O4 ) provides an example of a group of this kind with the eclipsed configuration. 

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