The nucleophilic addition of 2-pyridyl carbamidoxime to the C≡N group of the aliphatic nitrilium closo-decaborate clusters (PhCH₂PPh₃)[B₁₀H₉NCR] (R = Me, Et, and ⁱPr) proceeds to give 2-iminium closo-decaborates (68–88% isolated yields). A systematic kinetic and theoretical study of the nucleophilic addition indicates that the reactivity of the nitrile ligand at the boron cluster unexpectedly increases in the order of R: Me < Et < ⁱPr. It was shown theoretically, that the electrophilicity of the C≡N group is primarily determined by hyperconjugation with the neighboring aliphatic moiety, which induced by substantial (ca. +0.6 e) positive charge at the nitrile C atom.