Transition Metal Complexes with Bidentate Ligands Spanning trans-Positions. Part XII. Steric Effects in the Kinetics and Mechanism of Substitutions at Hydride and Methyl Bisphosphine Platinum(II)) Complexes.

Ligand substitution reactions on square-planar platinum (II) complexes of the types trans-[PtRXL2], trans-[PtR(4-PADA)L2][BF4], trans-[PtRX(LL)] and trans-[PtR(4-PADA)(LL)][BF4] R=H, Me; X=Cl−, I−; L=PEt3, bis(3-trifluoromethyl-phenyl)benzylphosphine (4), LL=the trans-spanning 2,11-bis[bis(3-trifluoro-methylphenyl)phosphinomethyl]benzo [c]phenanthrene (3); 4-PADA (=pyridine-4-azo-4′-(N, N-dimethyl)aniline) have been studied at 30° using stopped-flow and conventional spectrophotometry, methanol as solvent, and 2.5 × 10−2 M ionic strength (LiClO4 as supporting electrolyte). 4-PADA was used as indicator ligand, as its absorption spectrum differs from those spectra where it is complexed. The expected steric effects of the bulky ligands, especially of 3, on the rates and mechanisms of all the reactions studied are small. All reactions take place by the usual two-term rate law. Noteworthy, for the complexes with the bulky ligands 3 and 4, the direct reaction path with the entering nucleophile is predominant. There is no preference for a solvent or dissociative path. The reactivity order for the hydride complexes is trans-[PtHX (PEt3)2]<trans-[PtHX(4)2]<trans-[PtHX (3)]. However, for the corresponding methyl complexes, there is some retardation by ligand 3, probably due to an interaction between the methyl group and the hydrocarbon moiety of 3, which inhibits the fluxional behavior of this ligand. The results have some relevance for the understanding of olefin-insertion reactions of hydride complexes containing these phosphine ligands.