Psi tetrahedron

IV) An anion neighbour of a central atom is replaced by a lone election pair (formation of a so-called psi-tetrahedron). 

The tetrahedron linkage described by (I) can be calculated from the composition of the compound, to be exact, from the anion to central atom ratio (n/m ). The linkage type (II), (III) and the formation of a psi-tetrahedron (IV) can be derived from a modified generalized 8 - N rule, I. e. one needs to calculate the VEC value of the compound. The anion sharing according to (I) is independent of the VEC value and can occur in addition to case (II), (III) and/or (IV). 

In these base tetrahedron drawings the central atoms are shown by filled small circles and the anions by larger open circles. Anions shared with an other tetrahedron are presented by half circles. A bond between anions or between central atoms is indicated by a short heavy line extending from the open or filled circle, respectively. In the case of a psi tetrahedron the lone electron pair on the central atom Is shown by a heavy bar alongside the filled circle. 

Figure 10 Examples for polycaVonIc valer ce compounds with anionic tetrahedron complexes where C C = 2, characterized by a psi tetrahedron, that means an anion neighbour of the central atom has been replaced by a lone electron pair.

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