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A closed deltahedral cluster described by $n+1$ skeletal electron pairs in the usual Wade model. borane examples include $[{\mathrm{B}}_{12}{\mathrm{H}}_{12}{]}^{2-}$.

For $[{\mathrm{B}}_{12}{\mathrm{H}}_{12}{]}^{2-}$, twelve BH vertices supply $12(2)$ skeletal electrons and the charge adds two:For the cobaltacarborane, nine BH vertices supply 18, two CH vertices supply six and CpCo supplies two by (a):Both are 12-vertex closo icosahedra with 13 skeletal electron pairs. In the cobalt compound, the Cp ring is external to the cage. cobalt caps the five-membered open face of the 11-vertex dicarbollide fragment and is a vertex of the resulting icosahedron, giving the usual adjacent-carbon cobaltacarborane framework. Counting all five Cp carbons as additional polyhedral vertices would be incorrect.

An electron-counting approach relating skeletal electron pairs to a cluster vertex count $n$. A closo deltahedron has $n+1$ pairs; nido and arachno frameworks have $n+2$ and $n+3$. The count excludes electrons in externally directed bonds or lone pairs. It is a structural guide with a defined domain, not a rule that every molecule with the same total valence count has the same shape.