Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 2 / Section H / 33 / b / ii / Solution

In a skeletal description, each of the four naked Sn atoms retains an outward lone pair and contributes two electrons to the framework. Each aryl-substituted Ge atom contributes three electrons after assigning one to its Ge–aryl bond; it is not assigned an additional spectator lone pair in this counting. ThusThe PSEP prediction is a six-vertex closo octahedron, with the two protected Ge vertices and four naked Sn vertices; distortion does not change the topological classification. The alternative gross delocalized count $6(4)-2=22$ is not a listed jellium shell closure. Subtracting a lone pair from every atom would incorrectly remove electrons from the substituted Ge framework and give the wrong classification.