Pyramidal Stability of 16-Electron Half-Sandwich Intermediates [CpRu(P−P)]⁺ with P−P Ligands Forming Four- to Six-Membered Chelate Rings

This paper reports the synthesis, isomer separation, and X-ray characterization of the compounds (S_Ru,S_C)-/(R_Ru,S_C)-[CpRu(Chairphos)Cl], Chairphos = (S)-1,3-bis(diphenylphosphanyl)butane, and cis-/trans-[CpRu(Dppm-Me)Cl], Dppm-Me = 1,1-bis(diphenylphosphanyl)ethane. The Cl/I exchange reactions proceeded with predominant retention of the metal configuration, accompanied by some inversion, except for trans-[CpRu(Dppm-Me)Cl], which was stereospecifically converted to trans-[CpRu(Dppm-Me)I]. Temperature-dependent kinetic measurements afforded rates and activation parameters of the Cl/I exchange and epimerization reactions that follow basilica-type energy profiles. Dissociation of Cl⁻ from [CpRu(Chairphos)Cl] and [CpRu(Dppm-Me)Cl] gives pyramidal intermediates [CpRu(Chairphos)]⁺ and [CpRu(Dppm-Me)]⁺, which maintain the metal configuration. The 16-electron intermediates can react with excess I⁻ to form the iodo complexes with retention of the metal configuration, or they can change the metal configuration by pyramidal inversion, leading to formation of iodo complexes with inverted metal configuration. The kinetic measurements show that the pyramidal inversion via planar transition states depends on the P−Ru−P′ angles. It increases with decreasing chelate ring size, because small P−Ru−P′ angles resist planarization in the transition, which requires larger P−Ru−P′ angles.