Beryllium dichloride crystal

Nucleophilic carbene 4 (R = Me, R = H) has been shown to disrupt the polymeric structure of beryllium dichloride to form an ionic carbene complex, 10 (15). The crystal structure of 10 revealed that the cation possesses a distorted tetrahedral coordination geometry at the beryllium center. The average Be-Cl bond distance (2.083(7) A) is... 

Figure 8-10. Rods with 42 screw-axis, (a) Iron-chain in the vicinity of the Royal Palace in Madrid (photograph by the authors) (b) Beryllium dichloride chain in the crystal.

Real objects are not infinite. For symmetry considerations, it may be convenient to look only at some portions of the whole, where the ends are not yet in sight, and extend them in thought to infinity. A portion of an iron chain and a chain of beryllium dichloride in the crystal are shown in Figure 8-10. Translation from unit to unit is accompanied by a 90° rotation around the translation axis. A portion of a spiral stairway displaying screw-axis symmetry is shown in Figure 8-lla. The imaginary impossible stairway of Figure 8-llb indeed seems to go on forever. 

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