The addition reactions of water, alcohols, and primary and secondary amines to the 10-acetonitrilium derivative of nido-carborane were studied. Hydrolysis of 10-MeC≡N-7,8-C₂B₉H₁₁ results in iminol 10-MeC(OH)=HN-7,8-C₂B₉H₁₁, which, on treatment with a base, gives amide [10-MeC(=O)HN-7,8-C₂B₉H₁₁]⁻. The reactions of 10-MeC≡N-7,8-C₂B₉H₁₁ with alcohols lead to imidates 10-ROC(Me)=HN-7,8-C₂B₉H₁₁ (R = Me, Et) as mixtures of E- and Z-isomers. In the solid state, 10-MeOC(Me)=HN-7,8-C₂B₉H₁₁ adopts E-configuration. The reactions of 10-MeC≡N-7,8-C₂B₉H₁₁ with primary amines result in amidines 10-RNHC(Me)=HN-7,8-C₂B₉H₁₁ (R = Me, Et) as mixtures of E- and Z-isomers. In the solid state 10-EtNHC(Me)=HN-7,8-C₂B₉H₁₁ was found to have the Z-configuration, which is stabilized by intramolecular N-H⋯H-B interactions between the NH group originating from the primary amine and the BH group of the carborane cage. These interactions are rather strong (3.7 kcal/mol) and are likely to persist in solution. The reactions of 10-MeC≡N-7,8-C₂B₉H₁₁ with secondary acyclic (Me₂NH, Et₂NH) and cyclic (piperidine, morpholine) amines result in amidines 10-R₂NC(Me)=HN-7,8-C₂B₉H₁₁ (R = Me, Et; R₂ = N(CH₂)₅, N(CH₂CH₂)₂O) as single isomers, which, according to single crystal X-ray diffraction data, have the E-configuration.