Kinetic and Computational Study of Dissociative Substitution and Phosphine Exchange at Tetrahedrally Distorted cis-Pt(SiMePh2)2(PMe2Ph)2

• Published in: Inorganic chemistry 2000-11, Vol.39 (23), p.5271-5276
• Main Authors: Wendt, Ola F, Deeth, Robert J, Elding, Lars I
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic000492g

The substitution kinetics of Me2PhP in cis-Pt(SiMePh2)2(PMe2Ph)2 (1) by the chelating ligand bis(diphenylphosphino)ethane has been followed at 25.0 °C in dichloromethane by stopped-flow spectrophotometry. Addition of the leaving ligand causes mass-law retardation compatible with a dissociative process via a three-coordinate transition state or intermediate. Exchange of Me2PhP in 1 has been studied by variable-temperature magnetization transfer 1H NMR in toluene-d 8, giving k ex 326 = 1.76 ± 0.12 s-1, ΔH ⧧ = 117.8 ± 2.1 kJ mol-1, and ΔS ⧧ = 120 ± 7 J K-1 mol-1. An exchange rate constant independent of the concentrations of free phosphine, a strongly positive ΔS ⧧, and nearly equal exchange and ligand dissociation rate constants also support a dissociative process. Density functional theory (DFT) calculations for a dissociative process give an estimate for the Pt−P bond energy of 98 kJ mol-1 for R = R‘ = Me, which is in reasonable agreement with the experimental activation energy given the differences between the substituents used in the calculation and those employed experimentally. DFT calculations on cis-Pt(PR3)2(SiR‘3)2 (R = H, CH3; R‘ = H, CH3) are consistent with the experimental molecular structure and show that methyl substituents on the Si donors are sufficient to induce the observed tetrahedral twist. The optimized Si−Pt−Si angle in cis-Pt(SiH3)2(PH3)2 is not significantly altered by changing the P−Pt−P angle from its equilibrium value of 104° to 80° or 120°. The origin of the tetrahedral twist is therefore not steric but electronic. The Si−Pt−Si angle is consistently less than 90°, but the Si−Si distance is still too long to support an incipient reductive elimination reaction with its attendant Si−Si bonding interaction. Instead, it appears that four tertiary ligands introduce a steric strain which can be decreased by a twist of two of the ligands out of the plane; this twist is only possible when two strong σ donors are cis to each other, causing a change in the metal's hybridization.