The structures and energetics of the dimethylamino substituted cobaltdicarbaboranes (Me2N)2C2Bn–3Hn–3CoCp (n = 8–12) have been investigated by density functional theory. The lowest energy structures are based on the most spherical closo deltahedra in accord with the Wade-Mingos rules. Among the nine possible icosahedral 12-vertex (Me2N)2C2B9H9CoCp structures the four structures with adjacent carbon atoms are the highest energy structures similar to the unsubstituted CpCoC2B9H11 system. However, the C–C edges in these 12-vertex structures stretch to non-bonding distances of at least ∼2.0 Å thereby converting the closo icosahedron into an isonido polyhedron with a tetragonal face with the carbon atoms on the diagonal. The lowest energy structures of the subicosahedral (Me2N)2C2Bn–3Hn–3CoCp (n = 8–11) systems are based on the corresponding closo deltahedra with a strong energetic preference for degree 4 rather than degree 5 carbon vertices.