Mixed‐Valence Ruthenium Complexes Rotating through a Conformational Robin–Day Continuum

The conformational energy landscape and the associated electronic structure and spectroscopic properties (UV/Vis/near‐infrared (NIR) and IR) of three formally d⁵/d⁶ mixed‐valence diruthenium complex cations, [{Ru(dppe)Cp*}₂(μ‐C≡CC₆H₄C≡C)]⁺, [ 1 ]⁺, [trans‐{RuCl(dppe)₂}₂(μ‐C≡CC₆H₄C≡C)]⁺, [ 2 ]⁺, and the Creutz–Taube ion, [{Ru(NH₃)₅}₂(μ‐pz)]⁵⁺, [ 3 ]⁵⁺ (Cp=cyclopentadienyl; dppe=1,2‐bis(diphenylphosphino)ethane; pz=pyrazine), have been studied using a nonstandard hybrid density functional BLYP35 with 35 % exact exchange and continuum solvent models. For the closely related monocations [ 1 ]⁺ and [ 2 ]⁺, the calculations indicated that the lowest‐energy conformers exhibited delocalized electronic structures (or class III mixed‐valence character). However, these minima alone explained neither the presence of shoulder(s) in the NIR absorption envelope nor the presence of features in the observed vibrational spectra characteristic of both delocalized and valence‐trapped electronic structures. A series of computational models have been used to demonstrate that the mutual conformation of the metal fragments—and even more importantly the orientation of the bridging ligand relative to those metal centers—influences the electronic coupling sufficiently to afford valence‐trapped conformations, which are of sufficiently low energy to be thermally populated. Areas in the conformational phase space with variable degrees of symmetry breaking of structures and spin‐density distributions are shown to be responsible for the characteristic spectroscopic features of these two complexes. The Creutz–Taube ion [ 3 ]⁵⁺ also exhibits low‐lying valence‐trapped conformational areas, but the electronic transitions that characterize these conformations with valence‐localized electronic structures have low intensities and do not influence the observed spectroscopic characteristics to any notable extent.