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[TpRuCl(PTA)(PPh3)] (1; PTA = 1,3,5-triaza-7-phosphaadamantane) reacts with phenylacetylene, yielding the alkynyl complex [TpRu(CCPh)(PTA)(PPh3)] (2) or the neutral vinylidene complex [TpRuCl{CC(H)Ph}(PTA)] (4), depending on the solvent and the reaction conditions (Tp = hydrotris(pyrazolyl)borate). Protonation of 2with HOTf (OTf = OSO2CF3) in CH2Cl2yields the dicationic vinylidene compound [TpRu{CC(H)Ph}{PTAH}(PPh3)](OTf)2(3), bearing a N-protonated PTA ligand. Reaction of 1with 1,1-diphenyl-2-propyn-1-ol affords the new cationic compound [TpRu(CCCPh2)(PTA)(PPh3)]PF6(5) or neutral allenylidene compound [TpRuCl(CC=CPh2)(PTA)] (6), depending on the reaction solvent (MeOH or toluene, respectively). The reactivity of complexes 5and 6toward tertiary phosphines (PTA and PPh2Me) has been investigated. Remarkably, both the cationic and the neutral allenylidene compounds undergo regioselective nucleophilic attack at the allenylidene Cαposition to give the σ-allenyl-phosphonium complexes [TpRu{C(L)CCPh2}(PTA)(PPh3)]PF6(L = PPh2Me (8), PTA (9)) and [TpRuCl{C(L)CCPh2}(PTA)] (L = PTA (10), PPh2Me (11)), respectively. Noticeably, the reaction goes to completion in the case of the neutral allenylidene complex 6, whereas an equilibrium between the phosphonioallenyl adduct and the educt species is observed in the case of the cationic allenylidene compound 5. The thermodynamic parameters for such an equilibrium have been determined by NMR methods at different temperatures. Finally, the hydrolysis of the neutral allenylidene 6has been briefly considered, showing the formation of the carbonyl compound [TpRuCl(CO)(PTA)] (7). [ABSTRACT FROM AUTHOR]