 Solution

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.