Heptagonal bipyramid and flattened oblatocloso niobium and tantalum dicarbaborane structures

Low-energy Cp ₂ M ₂ C ₂ B ₅ H ₇ (M = Nb, Ta) structures include heptagonal bipyramids and oblatocloso (6,6) deltahedra, whose intradeltahedral M–M distances differ by ∼0.4 Å, indicating markedly different degrees of flattening.