Edge-sharing binuclear d⁸ complexes with XR bridges: Theoretical and structural database study of their molecular conformation

The binuclear complexes of d8 transition metal ions of the types [M2(μ-Y)(μ-XR)L4] and [M2(μ-XR)2L4] (where M = RhI, IrI, NiII, PdII, PtII, or AuIII; X = O, S, Se, or Te; Y = Cl or S) appear in a variety of molecular conformations in which the coordination planes around the two metal atoms are sometimes coplanar and sometimes bent, and the substituents at the bridging atoms adopt different orientations relative to the metal framework and to each other. Ab initio theoretical studies on a number of representative complexes, complemented with a structural database analysis, provides a rationale for the experimentally observed structures. The choice of molecular geometry is seen to result from the interplay of several factors including the conformational preference of the bridging atoms, the existence of weak metal ⋯ metal bonding in the bent structures, and different steric or hydrogen-bonding interactions implicating the terminal ligands and the substituents at the bridging atoms.