Mechanism of the Axial Ligand Substitution Reactions on the Head-to-Tail α-Pyridonato-Bridged cis-Diammineplatinum(III) Dinuclear Complex with Olefins

Reactions of the head-to-tail α-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex having equivalent two platinum atoms, Pt(N₃O), with p-styrenesulfonate and 4-penten-1-ol were studied kinetically. Under the pseudo first-order reaction conditions in which the concentration of the Pt^III dinuclear complex is much smaller than that of olefin, a consecutive basically four-step reaction was observed for the reaction with p-styrenesulfonate, but for the reaction with 4-penten-1-ol, the reaction was three step. The olefin π-coordinates to one of the two equivalent Pt atoms in the first step (step 1), followed by the second π-coordination of another olefin molecule to the other Pt atom (step 2). In the next step (step 3), the nucleophilic attack of water to the first π-coordinated olefin initiates its π-σ bond conversion on the Pt atom, and the second π-bonding olefin molecule on the other Pt atom is released. Finally, dissociation of the alkyl group on the Pt(N₃O) and reduction of the Pt^III dinuclear complex to the Pt^II dinuclear complex occur (step 4). The first water substitution with olefin (step 1) consists of two paths, the reaction of the diaqua dimer complex (path a) and the reaction of the aquahydroxo dimer complex (path b), whereas the second substitution (step 2) proceeds through three reaction paths: the normal path of the direct substitution of H₂O (path c), the path of the coordinated OH^- substitution (path d), and the path via the coordinatively unsaturated five-coordinate intermediate (path e). The reaction with p-styrenesulfonate proceeds through paths c, d, and e, whereas the reaction with 4-penten-1-ol proceeds through paths c and d. The third step (step 3) for the reaction with p-styrenesulfonate involves the coordinatively unsaturated intermediate, but that for the 4-pentene reaction does not. The reactivities of the HH dimer and HT dimer with olefins are compared and discussed.