Transition Metal Complexes with Bidentate Ligands Spanning trans‐Positions. 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( magnified image ) and trans‐[PtR(4‐PADA)( magnified image )][BF4] R=H, Me; X=Cl−, I−; L=PEt3, bis(3‐trifluoromethyl‐phenyl)benzylphosphine (4), magnified image =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]