Multicentre bonding

A hydrogen atom associated with more than one skeletal centre. Removal as a proton must be distinguished from hydride transfer; the cluster charge and electron count change differently in the two operations.

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.

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.