M_2E_2 four-member ring structure,M_2 (#mu-EH_2)_2(P2)_2(M=Pd or Pt;E=Si or Ge;P2=(PH_3)_2 or H_2PCH_2CH_2PH_2) versus #mu#-disilene and #mu#-digermene-bridged structures,M_2(#mu#-E_2H_4)(P2)_2.A theoretical study

摘要

M_2(EH_2)_2(P_2)_2 (M = Pd or Pt; E = Si or Ge; P2 = (PH_3)_2 or diphosphinoethane (H_2PCH_2CH,pH_2 dipe)) was theoretically vestigated with the DFT method. Natural bond orbital (nbo) analysis and the laplacian of electron density indicate that Pt_2(SiH_2)_2(P2)_2 and Pt_2(GeH_2)_2(P_2)_2 are characterized to be di(#mu#-silylene)- and di(#mu#-germylene)-bridged complexes, respectively,and M_2Ge_2 four-member ring structures, respectively. In other words, they should be represented as Pt_2(#mu#-SiH-2)_2(P2)_2 and Pt_2(##mu#-GeH_2)_2(P2)_2. On the other hand, the palladium(0) analogs are understood in terms of #mu#-disilene- and 1-digermene-bridged complexes in which the Si-Si and Ge-Ge bonding interactions are maintained. Thus, they should be presented as Pd2(p-Si_2H_4)(P2)_2 and Pd2_(p-Ge_2H_4)(P2)_2. The difference between platinum(0) and palladium(0) complexes is Lerpreted in terms of the difference in the strength of it-back donation interaction; in the platinum(0) complex, the it-back #pi#-nation interaction is so strong that the S-Si and Ge-Ge bonds are almost broken, while in the palladium(0) complex the it-back lation interaction is not so strong and thereby the Si-Si and Ge-Ge bonding interactions are still maintained. Also in thedisilene and digermene complexes, M(E_2H_4)(P2) (M = Pd or Pt; E = Si or Ge), a similar difference between .Platsnum(0) and palladium(O) complexes is observed; platinum(0) complexes are characterized to be a three-member metallacycle complex which involves an E—E single bond and two M—E covalent bonds, whereas the palladium(O) analogs are characterized to ik the usual disilene and digermene complexes in which the Si=Si and Ge=Ge double bonds are maintained. This is because the s-back-donating interaction is stronger in platinum(0) complexes than in palladium(0) complexes. In M(C_2H_4(P2) and M_2(#mu#-C_2H_4)(P2)_2, the C=C double bond is maintained, since the it-back donation is much weaker than those of Si and Ge analogs even platinum(0) complexes. Thus, these complexes are characterized to be the ethylene complex in which the C=C double bondthe Pt(0) and Pd(0) centers and not the three-member metallacycle complex.