Electronic Interactions in Dinuclear Platinum and Palladium Ethynylferrocene and Ferrocenylvinylidene Complexes

The complex [Pd 2 (dppm) 2 (μ‐η 1 :η 1 ‐H−C=C−Fc)Cl 2 ] ( 2 ) and its vinylidene isomer [Pd 2 (dppm) 2 (μ‐C=CH−Fc)Cl 2 ] ( 3 ) [dppm = bis(diphenylphosphanyl)methane, Fc = ferrocenyl unit, CpFeC 5 H 4 ] have been synthesized and characterized by single‐crystal X‐ray diffraction. Both 2 and 3 are A‐frame structures with long Pd−Pd distances of 3.2055(9) Å ( 2 ) and 3.1002(2) Å ( 3 ), respectively, and square‐planar metal coordination. The cyclic voltammograms (CV) of the complexes are measured and compared with that of [Pt 2 (dppm) 2 (μ‐η 1 :η 1 ‐H−C=C−Fc)Cl 2 ] ( 1 ). The CV of 2 and 3 consist of a nearly reversible Fc‐oxidation, an irreversible oxidation and a small reversible couple. The E 1/2 values (vs. Cp 2 Fe + /Cp 2 Fe) of the Fc‐groups in complexes 1−3 follow the order 2 (−75±8 mV) > 1 (−108±10 mV) > 3 (−124±8 mV). The difference in the E 1/2 values is explained by the orbital interactions between the acetylene and vinylidene groups with Pt 2 and Pd 2 cores. Our analysis shows that the metal‐to‐ligand π back‐bonding and ligand‐to‐metal σ‐donation have determining effects on the electronic interactions between the dimetallic centers and the bridgehead ligands. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)