The mechanism of the platinum(II)-catalyzed intramolecular hydroamination of benzyl 4-pentenylamines has been evaluated under stoichiometric and catalytic conditions. Reaction of a benzyl 2,2-disubstituted 4-pentenylamine with [(PPh )Pt(μ-Cl)Cl] forms a thermally sensitive platinum amine complex that undergoes irreversible, intramolecular ligand exchange with the pendant C=C bond to form a reactive platinum π-alkene complex. The π-alkene complex undergoes rapid, outer-sphere C-N bond formation, evidenced by the anti addition of Pt and N across the complexed C=C bond, to form a thermally stable zwitterionic platinamethylpyrrolidinium complex. The zwitterionic complex is rapidly and exergonically deprotonated by free amine to form a neutral, bicyclic azaplatinacyclobutane complex that likely exists as a discrete 1:1 adduct with ammonium salt in the nonpolar reaction medium and that represents the resting state of the catalytic cycle. Turnover-limiting intramolecular protodemetalation of the azaplatinacyclobutane-ammonium adduct followed by ligand exchange releases the 2-methylpyrrolidine product. 3 2