New Osmocene and Ruthenocene Phases Reveal the Common Conformational Behavior Regulated by Anagostic Bonds in Prototypical Metallocenes

Ruthenocene and osmocene, at normal conditions isostructural to low-temperature ferrocene phase III, are regarded as prototypic metallocenes exclusively present in the energetically-favored eclipsed conformation. This strong preference contrasts with ferrocene, exhibiting the staggered, rotated, eclipsed, disordered and modulated conformations in its five polymorphic forms. Here we show that ruthenocene at 394.0 K and osmocene at 421.5 K transform to new higher-symmetry phases, disordered between the staggered and eclipsed conformers. For the prototypic metallocenes, a common pattern of transformations leading to disordered conformations has been connected with intramolecular anagostic bonds CH‧‧‧M (M = Fe, Ni, Ru, Os). Their strength correlates with the critical temperatures of phase transitions, when the anagostic bonds are broken.