Ab initio calculations have been carried out for
Pt(II) dimers with S2- and/or
RS- bridging ligands in order to
elucidate the factors governing the structural preference, planar or
hinged, of these complexes. Calculations
have been performed for systems which model all structurally
characterized Pt(II) binuclear complexes. All
possible determinants of the hinge angle have been taken into account
in the ab initio calculations. These show
that electronic rather than steric effects govern the geometry of the
central Pt2S2 ring. Only one minimum
of
energy has been found for each complex, but the energetic cost of the
hinge motion is low. Complexes with two
S2-, or with one
S2- and one RS-, bridging
ligands are hinged; the decrease of the through-ring
antibonding
interaction between the in-plane sulfur p orbitals with folding appears
to be the determinant for hinging. Complexes
with two RS- bridging ligands can be planar or hinged
depending on the relative orientation, syn or
anti, of the
R groups, on the nature, L4 or
L2L2‘, of the terminal ligands, and in the
latter case on their disposition, cis or
trans. However, the aromatic or aliphatic nature of
bridging thiolates does not influence the geometry of the
Pt2S2 ring significantly. Conclusions
drawn from platinum dimers can be extended to the palladium
analogs.
[H3AlXH3]- (X = C, Si, and Ge) and H3AlYH3 (Y = N, P, and As) have been investigated as donor−acceptor complex types at the G2(MP2) level of theory. Both staggered and eclipsed conformations have
been examined. For all complexes, the first one is found to be favored. The G2(MP2) results show that the
anionic complexes are more stable than the neutral ones. They show also that this stability decreases when
going from carbon to germanium for [H3AlXH3]- complexes and from nitrogen to arsenic for H3AlYH3
complexes. The interaction diagrams prove that the evolution of complexation energy depends on the
coordination mode. In fact, this is a simple “HOMO−LUMO” interaction for [H3AlXH3]- anionic complexes,
while for the H3AlYH3 neutral ones it is a result of two interaction types: interaction between “a1” symmetry
fragments orbital (stabilizing) and interactions between “e” symmetry fragments orbital (destabilizing). A
linear relationship has been established and discussed between the G2(MP2) complexation energy and the
ligand G2(MP2) proton affinity, whereas no correlation has been found with the charge transfer.
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