M2E2 four-member ring structure, M2(μ-EH2)2(P2)2 (M=Pd or Pt; E=Si or Ge; P2=(PH3)2 or H2PCH2CH2PH2) versus μ-disilene and μ-digermene-bridged structures, M2(μ-E2H4)(P2)2. A theoretical study

M2(EH2)2(P2)2 (M=Pd or Pt; E=Si or Ge; P2=(PH3)2 or diphosphinoethane (H2PCH2CH2PH2; dipe)) was theoretically investigated with the DFT method. Natural bond orbital (nbo) analysis and the laplacian of electron density indicate that Pt2(SiH2)2(P2)2 and Pt2(GeH2)2(P2)2 are characterized to be di(μ-silylene)- and di(μ-germylene)-bridged complexes, respectively, which involve M2Si2 and M2Ge2 four-member ring structures, respectively. In other words, they should be represented as Pt2(μ-SiH2)2(P2)2 and Pt2(μ-GeH2)2(P2)2. On the other hand, the palladium(0) analogs are understood in terms of μ-disilene- and μ-digermene-bridged complexes in which the SiSi and GeGe bonding interactions are maintained. Thus, they should be represented as Pd2(μ-Si2H4)(P2)2 and Pd2(μ-Ge2H4)(P2)2. The difference between platinum(0) and palladium(0) complexes is interpreted in terms of the difference in the strength of π-back donation interaction; in the platinum(0) complex, the π-back donation interaction is so strong that the SiSi and GeGe bonds are almost broken, while in the palladium(0) complex the π-back donation interaction is not so strong and thereby the SiSi and GeGe bonding interactions are still maintained. Also in the mononuclear disilene and digermene complexes, M(E2H4)(P2) (M=Pd or Pt; E=Si or Ge), a similar difference between platinum(0) and palladium(0) 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(0) analogs are characterized to be the usual disilene and digermene complexes in which the SiSi and GeGe double bonds are maintained. This is because the π-back-donating interaction is stronger in platinum(0) complexes than in palladium(0) complexes. In M(C2H4)(P2) and M2(μ-C2H4)(P2)2, the CC double bond is maintained, since the π-back donation is much weaker than those of Si and Ge analogs even in platinum(0) complexes. Thus, these complexes are characterized to be the ethylene complex in which the CC double bond coordinates with the Pt(0) and Pd(0) centers and not the three-member metallacycle complex.