Tricoordinated Atoms

Tricoordinated atoms may also have two distinct geometric configurations, planar or pyramidal. Only one valence orbital remains, and it will be a pure p orbital in the planar case, hybridized in the pyramidal case. Both configurations for tricoordinated atoms are depicted in Figure 3.18. 

Figure 3.18. Group orbitals for a tricoordinated atom with planar (left) and pyramidal (right)...

Most of the tricoordinate atoms discussed above bear an unshared electron pair which formally occupies the position of a fourth substituent. These systems therefore show clear geometric similarities with the tetracoordinate centers to be considered... 

We shall discuss in this section large-amplitude motions which are either inversion as in ammonia, or are related to this type of motion. We shall concentrate on the so-called pyramidal inversion which occurs in molecules with tricoordinate atom whose stable position is not in the plane defined by the three atoms directly bounded to it. 

Starting from the classically expected structure with a short transannular bond and localized charges residing on the tricoordinate atoms the positive charges are delocalized over the central six sulfur atoms by a interaction which is further responsible for the three weak transannular bonds. Additionally the positive charge is delocalized onto all atoms—as found in the experiment—by a further 3p 3a interaction. The combination of both in-... 

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