Displacement of a cis-Olefin from a trans-Olefin Complex:  CpRu(CO)₂(trans-olefin)⁺

Ruthenium(II)−olefin complexes CpRu(CO)₂(η²-trans-olefin)⁺ (Cp = η⁵-C₅H₅; olefin = trans-3-hexene, trans-2-pentene, trans-3-octene, trans-4-octene, trans-5-decene) have been synthesized and characterized by IR, ¹H NMR, and ¹³C NMR spectroscopies. The reactions of these complexes with a wide range of ligands (L) result in the formation of CpRu(CO)₂(L)⁺ and the release of both cis- and trans-olefins:  [CpRu(CO)₂(trans-olefin)]BF₄ + L → [CpRu(CO)₂(L)]BF₄ + cis/trans-olefin. The relative amounts of cis- and trans-olefin released are controlled by several factors:  identity and amount of the incoming ligand L, identity of the olefin, temperature, and solvent. For 4-substituted pyridines, the cis/trans ratio increases as the electron-donating ability of the 4-substituent increases:  F₃C (18/82) < H (67/33) < CH₃ (74/26) < CH₃O (76/24). Increases in temperature, solvent polarity, and olefin side-chain length reduce the cis/trans ratio. A mechanism is proposed to account for the isomerization of trans-olefin ligands to their cis isomers during the substitution process.