Coaxial versus perpendicular structures for a range of binuclear cyclopentadienylpalladium derivatives

Abstract

Binuclear palladium complexes of planar hydrocarbon ligands are of particular interest since both the coaxial structure (η5-Me5C5)2Pd2(μ-CO)2 and the perpendicular structure (μ-C6H6)2Pd2(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 Cp2Pd2X2 (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 Cp2Pd2Cl2 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 Cp2Pd2L2 (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.