Reactivity of [TpRuCl(PTA)(PPh₃)] with Alkynes and Propargylic Alcohols: Occurrence of Structurally Related Cationic vs Neutral Allenylidene Complexes with the Ruthenium Hydrotris(pyrazolyl)borate Moiety

[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 2 with HOTf (OTf = OSO₂CF₃) in CH₂Cl₂ yields the dicationic vinylidene compound [TpRu{CC(H)Ph}{PTAH}(PPh₃)](OTf)₂ (3), bearing a N-protonated PTA ligand. Reaction of 1 with 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 5 and 6 toward tertiary phosphines (PTA and PPh₂Me) has been investigated. Remarkably, both the cationic and the neutral allenylidene compounds undergo regioselective nucleophilic attack at the allenylidene C_α position 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 6 has been briefly considered, showing the formation of the carbonyl compound [TpRuCl(CO)(PTA)] (7).