A high-yielding synthesis of the 13-vertex cobaltacarborane 4-Cp-4,1,12-closo-CoC₂B₁₀H₁₂ is described and this compound used to prepare the known 14-vertex species 1,14-Cp₂-1,14,2,10-closo-Co₂C₂B₁₀H₁₂ (II) and 1-(p-cymene)-14-Cp-1,14,2,10-closo-RuCoC₂B₁₀H₁₂ (IV), the latter by a new route. The related species 1,14-Cp₂-2,10-Me₂-1,14,2,10-closo-Co₂C₂B₁₀H₁₀ (1) and 1,14-(η-C₉H₇)₂-1,14,2,10-closo-Co₂C₂B₁₀H₁₂ (2) are also reported. Polyhedral expansion of 4,1,8-CoC₂B₁₀ compounds affords a different isomer of the 14-vertex bimetallacarboranes, 1,14,2,9-Co₂C₂B₁₀, and three examples, 1,14-Cp₂-1,14,2,9-closo-Co₂C₂B₁₀H₁₂ (3), 1,14-Cp₂-2,9-Me₂-1,14,2,9-closo-Co₂C₂B₁₀H₁₀ (4) and 1,14-(η-C₉H₇)₂-1,14,2,9-closo-Co₂C₂B₁₀H₁₂ (5), are prepared and characterised. Patterns in ¹¹B NMR chemical shifts and in <δ¹¹B>, the weighted average ¹¹B chemical shift, within and between related isomers of the 14-vertex compounds II and 1–5 are discussed. Compounds II, IV, 2, 4 and 5 were studied crystallographically, with cage C atom positions in these and related bicapped hexagonal antiprismatic 1,14,2,x-M₂C₂B₁₀ species analysed by the Vertex-to-Centroid Distance method.