Skeletal electron count ($N_{\mathrm{skel}}$)

The number of electrons allocated to the connected cluster framework after separating external bonds and appropriate lone pairs. For ${\mathrm{B}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}^{\mathrm{q}-}$ with one terminal B–H bond per vertex it is $n+m+q$.

A pair of electrons allocated to cluster bonding. closo, nido and arachno counts distinguish the number of missing vertices relative to a closed parent polyhedron.