Coaxial versus perpendicular structures for a range of binuclear cyclopentadienylpalladium derivatives

Binuclear palladium complexes of planar hydrocarbon ligands are of particular interest since both the coaxial structure (η⁵-Me ₅C₅)₂Pd₂(μ-CO)₂ and the perpendicular structure (μ-C₆H₆)₂Pd ₂(Al₂Cl₇)₂ have been synthesized as stable compounds. Here we report theoretical studies on a family of related compounds. For the formal Pd(ii) derivatives Cp₂Pd₂X ₂ (Cp = η⁵-C₅H₅; X = F, Cl, CN) the perpendicular structures with direct Pd-Pd bonds are predicted to lie in energy below the isomeric coaxial structures. These coaxial structures have long Pd⋯Pd distances indicating the absence of palladium-palladium bonds. The lowest energy perpendicular Cp₂Pd₂X₂ structures (X = F, CN) and a higher energy similar Cp₂Pd ₂Cl₂ structure contain a substituted η⁴- C₅H₅X cyclopentadiene ligand obtained by the addition of X to one of the Cp ligands. For the formal Pd(i) complexes Cp₂Pd ₂L₂ (L = CO and CS) the coaxial structures lie in energy below the isomeric perpendicular structures with a particularly large energy separation of ∼19 kcal mol^-1 for the thiocarbonyl derivatives. However, for Cp₂Pd₂(CNCH₃)₂ the coaxial and perpendicular isomers have essentially the same energies.