Computations have been carried out on the title boranes and carboranes, model hypercarbon cluster systems chosen to explore how effectively an individual carbon atom, whilst bonding by a normal 2-electron 2-centre bond to an exo-hydrogen atom, can also bond to sets of three, four or five equivalent boron atoms within a series of carborane clusters which have carbon atoms in axial sites of C3v, C4v or C5v local symmetry. The calculated interatomic distances and bond orders and CH and BH group charges are reported, and the manner in which the introduction of CH units to replace BH− units in closo-borane cages perturbs the distribution of the skeletal electrons in these clusters is discussed.