The synthesis of propionitrilium derivatives of cobalt and iron bis(dicarbollides) [8-EtCN-3,3′-M(1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)] (M = Co, Fe) by the reaction of the cesium salts of the parent bis(dicarbollide) anions [3,3′-M(1,2-C₂B₉H₁₁)₂]⁻ with propionitrile in the presence of tert-butyl bromide is described. The nucleophilic addition reactions of water, alcohols, and thiols with the –N⁺C triple bond of the prepared nitrilium derivatives were studied and the corresponding iminols [8-EtC(OH)HN-3,3′-M(1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)] (M = Co, Fe), imidates [8-EtC(OR)HN-3,3′-M(1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)] (M = Co, R = Me, Et, i-Pr, Bu; M = Fe, R = Et), and thioimidates [8-EtC(SR)HN-3,3′-Co(1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)] (R = Et, Bu, Hx) were synthesized as mixtures of E and Z-isomers, some of which could be separated by the column chromatography on silica. The molecular structure of [(Z)-8-EtC(OiPr)HN-3,3′-Co(1,2-C₂B₉H₁₀)(1′,2′-C₂B₉H₁₁)] was determined by single-crystal X-ray diffraction study.