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

M-2(EH2)(2)(P2)(2) (M = Pd or Pt; E = Si or Ge; P2 = (PH3)(2) or diphosphin oethane (H2PCH2CH2PH2; dipe)) was theoretically investigated with the DFT m ethod. Natural bond orbital (nbo) analysis and the laplacian of electron de nsity indicate that Pt-2(SiH2)(2)(P2)(2) and Pt-2(GeH2)(2)(P2)(2) are chara cterized to be di(p-sitylene)- and di(mu -germylene)-bridged complexes, res pectively, which involve M2Si2 and M2Ge2 four-member ring structures, respe ctively. In other words, they should be represented as Pt-2(mu -SiH2)(2)(P2 )(2) and Pt-2(p-GeH2)(2)(P2)(2). On the other hand, the palladium(O) analog s are understood in terms of mu -disilene- and mu -digermene-bridged comple xes in which the Si-Si and Ge-Ge bonding interactions are maintained. Thus, they should be represented as Pd-2(mu -Si2H4)(P2)(2) and Pd-2(mu -Ge2H4)(P 2)(2). The difference between platinum(O) and palladium(O) complexes is int erpreted in terms of the difference in the strength of ir-back donation int eraction; in the platinum(O) complex, the pi -back donation interaction is so strong that the Si-Si and Ge-Ge bonds are almost broken, while in the pa lladium(O) complex the pi -back donation interaction is not so strong and t hereby 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(O) and palladium( O) complexes is observed; platinum(O) complexes are characterized to be a t hree-member metallacycle complex which involves an E-E single bond and two M-E covalent bonds, whereas the palladium(O) analogs are characterized to b e the usual disilene and digermene complexes in which the Si Si and Ge--Ge double bonds are maintained. This is because the Tc-back-donating interacti on is stronger in platinum(O) complexes than in palladium(0) complexes. In M(C2H4)(P2) and M-2(mu- C2H4)(P2)(2), the C=C double bond is maintained, si nce the pi -back donation is much weaker than those of Si and Ge analogs ev en in platinum(O) complexes. Thus, these complexes are characterized to be the ethylene complex in which the C=C double bond coordinates with the Pt(O ) and Pd(O) centers and not the three-member metallacycle complex. 0 2001 P ublished by Elsevier Science B.V.