Kinetic and computational study of dissociative substitution and phosphine exchange at tetrahedrally distorted cis-Pt(SiMePh2)2(PMe2Ph)2

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 degrees 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-d8, giving kex326 = 1.76 +/- 0.12 s-1, delta H++ = 117.8 +/- 2.1 kJ mol-1, and delta S++ = 120 +/- 7 J K-1 mol-1. An exchange rate constant independent of the concentrations of free phosphine, a strongly positive delta S++, 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 degrees to 80 degrees or 120 degrees. The origin of the tetrahedral twist is therefore not steric but electronic. The Si-Pt-Si angle is consistently less than 90 degrees, 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 sigma donors are cis to each other, causing a change in the metal's hybridization.