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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.

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[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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