Theoretical study of silyl-bridged dinuclear palladium(I) and platinum(I) complexes, M2 (μ-η2-H•••SiH2)2(PH3)2 (M = Pd or Pt).: New insight into the bonding nature

Silyl-bridged dinuclear palladium(I) and platinum(I) complexes with the composition M-2-(SiH3)(2)(PH3)(2) (M = Pd or Pt) were theoretically investigated with DFT, MP2 to MP4(SDQ), and CCSD(T) methods. These complexes are more stable than two M(PH3)(SiH3) complexes by 80.6 kcal/mol for M = Pd and 105.6 kcal/mol for M = Pt, where the values calculated with the CCSD(T) method are given hereafter. Although this complex is understood to take the silyl-bridged form in a formal sense, the NMR chemical shifts of Si and H atoms and the Laplacian of electron density indicate that the electronic structure of the SiH3 group somewhat shifts toward that of the silylene + hydride groups and the agostic interaction is responsible for this interesting electronic structure. These complexes are represented as M-2(mu-eta(2)-H center dot center dot center dot SiH2)(2)(PH3)(2), in which the formula of H center dot center dot center dot SiH2 indicates that this group is not a pure silyl group but possesses the characteristics of the hydride and mu-silylene groups to a considerable extent. The planar geometry of this compound comes from the presence of the three-center two-electron (3c-2e) interaction between the silyl spa orbital and the M-M moiety, while these complexes become nonplanar in the absence of the 3c-2e interaction. The agostic interaction between the Si-H bond and the M center contributes to the stabilization energies of 8.0 and 17.3 kcal/mol for M = Pd and Pt, respectively. The stronger agostic interaction and the larger stabilization energy of Pt-2(mu-eta(2)-H center dot center dot center dot SiH2)(2)(PH3)(2) than those of the Pd analogue result from the fact that the d orbital of Pt expands more than that of Pd.