The stirring of [ortho-(HCC)-C₅H₄N] with [nido-B₁₀H₁₄] in benzene affords [6,9-{ortho-(HCC)-C₅H₄N}₂-arachno-B₁₀H₁₂] 1 in 93% yield. In the solid state, 1 has an extended complex three-dimensional structure involving intramolecular dihydrogen bonding, which accounts for its low solubility. Thermolysis of 1 gives the known [1-(ortho-C₅H₄N)-1,2-closo-C₂B₁₀H₁₁] 2 (13%), together with new [µ-5_N,6_C-(NC₅H₄-ortho-CH₂)-nido-6-CB₉H₁₀] 3 (0.4%), [µ-7_C,8_N-(NC₅H₄-ortho-CH₂)-nido-7-CB₁₀H₁₁] (0.4%) 4, binuclear [endo-6′-(closo-1,2-C₂B₁₀H₁₀)-µ-(1_C,exo-6′_N)-(ortho-C₅H₄N)-µ-(exo-8′_C,exo-9′_N)-(ortho-(CH₂CH₂)-C₅H₄N)-arachno-B₁₀H₁₀] (0.5%) 5, and [exo-6_C-endo-6_N-(ortho-(CHCH)-C₅H₄N)-exo-9_N-(ortho-(HCC)-C₅H₄N)-arachno-B₁₀H₁₁] 6. An improved solvent-free route to 2 is also presented. This set of compounds features an increasing cluster incorporation of the ethynyl moiety, initially by an effective internal hydroboration, affording an arachno to nido and then a nido to arachno:closo sequence of cluster geometry. An alternative low-temperature route to internal hydroboration is demonstrated in the room temperature reaction of [closo-B₁₁H₁₁][NⁿBu₄]₂ with CF₃COOH and [ortho-(HCC)-C₅H₄N], which gives [µ-1_C,2_B-[ortho-C₅H₄N-CH₂]-closo-1-CB₁₁H₁₀] 7 (40%) in which one carbon atom is incorporated into the cluster; a similar reaction with [ortho-(NC)-C₅H₄N] affords [NⁿBu₄][7-(ortho-NC-C₅H₄N)-nido-B₁₁H₁₂], 8 (68%) and stirring [ortho-(NC)-C₅H₄N] with [nido-B₁₀H₁₄] quantitatively affords the cyano analogue of 1, [6,9-{ortho-(NC)-C₅H₄N}₂-arachno-B₁₀H₁₂] 9. All compounds were characterised by single-crystal X-ray diffraction analysis and NMR spectroscopy.