Abstract
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)