Three types of N(H)-nucleophiles, viz. hydrazine, acetyl hydrazide, and a set of hydrazones, were used to study the nucleophilic addition to the CN group of the 2-propanenitrilium closo-decaborate cluster (Ph₃PCH₂Ph)[B₁₀H₉NCEt], giving N-closo-decaborato amidrazones. A systematic mechanistic study of the nucleophilic addition is provided and included detailed synthetic, crystallographic, computational and kinetic work. As a result, two possible mechanisms have been proposed, which consist of firstly a consecutive incorporation of two Nu(H) nucleophiles, with the second responsible for a subsequent rapid proton exchange. The second possible mechanism assumes a pre-formation of a dinuclear [Nu(H)]₂ species which subsequently proceeds with the nucleophilic attack on the boron cluster. The activation parameters for hydrazones indicate a small dependence on bond formation [ΔH^‡ = 6.8-15 kJ mol^-1], but significantly negative entropies of activation [ΔS^‡ ranges from -139 to -164 J K^-1 mol^-1] with the latter contributing some 70-80% of the total Gibbs free energy of activation, ΔG^‡. In the X-ray structure of (Z)-(Ph₃PCH₂Ph)[B₁₀H₉N(H)C(Et)NHNCPh₂], very rare intramolecular non-covalent B-H⋯π(Ph) interactions were detected and studied by DFT calculations (M06-2x/6-311++G∗∗ level of theory) and topological analysis of the electron density distribution within the framework of Bader's theory (QTAIM method). The estimated strength of these non-covalent interactions is 0.8-1.4 kcal mol^-1.