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

Ruthenocene and osmocene, under normal conditions isostructural to low-temperature ferrocene phase III, have been regarded as prototypical metallocenes exclusively present in the energetically favored eclipsed conformation. This strong preference contrasts with ferrocene, which exhibits 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 isostructural phases, where the cyclopentadienyl rings become disordered in two modes: seesaw tilts hinged on the metal cation and rotations about the molecular pseudo- axis. The transitions entropy change and the Fourier transformations of the diffraction data indicate the hindered rotations, with molecules dynamically disordered between the staggered and eclipsed conformations, whereas in the final structural models the electron density distribution around the rings, calculated from the atomic displacement parameters of refined atomic sites, is continuous. For the prototypical metallocenes, a common pattern of transformations leading to disordered conformations has been connected with intramolecular anagostic bonds CH··· ( = Fe, Ni, Ru, Os). Their strength correlates with the critical temperatures of phase transitions when the anagostic bonds are broken.