Tuning the palladium-silicon bond: Bond analysis of bisphosphine silyl palladium hydrides

Density functional calculations (PW91) have been carried out for 33 complexes of the type (dhpe)M(H)Si(R-1)(R-2)(R-3) (dhpe = 1,2-bis(dihydrophosphino)ethane; M = Pd, Pt; R = H, Me, Ph, F, Cl). Bond snapping energies for the M-Si bond have been obtained, and the variation of silyl substitutents R allows one to tune the M-Si bond strength over a range of about 60 kJ/mol. On the basis of the results obtained from a Hirshfeld charge analysis, the Pd-Si bond is characterized by a sizable amount of charge transfer from the transition-metal fragment to the main-group ligand. This is reflected in the optimized Pd-Si bond length, which correlates well with the amount of charge transferred. Experimentally observed trends for the relative stability of bisphosphine silyl palladium hydrides can be successfully reproduced using a simplified approximation based on reaction energies derived from bond snapping energies. On average, the Pt-Si bond is stronger than the Pd-Si bond by 26 kJ/mol. The enhanced platinum bond strengths relative to those of palladium are attributed to relativistic effects.