The relative energies of triangular face sharing condensed macro polyhedral carboranes: CB20H18 and C2B19H18+ derived from mono- and di-substitution of carbons in (4) B21H18− is calculated at B3LYP/6-31G* level. The relative energies, H⋯H non-bonding distances, NICS values, topological charge analysis and orbital overlap compatibility connotes the face sharing condensed macro polyhedral mono-carboranes, 8 (4-CB20H18) to be the lowest energy isomer. The di-carba- derivative, (36) 4,4′a-C2B19H18+ with carbons substituted in a different B12 cage in (4) B21H18− in anti-fashion is the most stable isomer among 28 possibilities. This structure has less non-bonding H⋯H interaction and is in agreement with orbital-overlap compatibility, and these two have the pivotal role in deciding the stability of these clusters. An estimate of the inherent stability of these carboranes is made using near-isodesmic equations which show that CB20H18 (8) is in the realm of the possible.