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
S. Sakaki et al., 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, J ORGMET CH, 635(1-2), 2001, pp. 173-186
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.