Lattice-Strain Coupled to Molecular Conformation and Disorder in Compressed Nickelocene

Abstract

Isothermal compression of nickelocene (NiCp2, where Cp denotes cyclopentadienyl ring C5H5–) to 1.3 GPa leads to the ordered phase I′, highly isostructural with that obtained at 0.1 MPa by isobaric cooling to 170 K and with phase I′ of ferrocene (FeCp2) above 3.24 GPa. However, the gradual ordering of Cp rings in NiCp2 and its anomalous crystal strain considerably differs from that in FeCp2. The disorder in NiCp2 molecules can be represented as various combinations of achiral staggered and eclipsed conformers, as well as R-/S-rotamers, which are differently stabilized by the crystal environment. The anomalous lattice strain has been correlated with the molecular conformation, represented as the convolution of disordered Cp rings. The coincidence of the phase transition at 1.3 GPa/296 K with the monoclinic angle assuming 90° value does not occur on lowering temperature at 170 K/0.1 MPa, which is due to a weaker coupling between the lattice strain and the molecular disorder. Compared to FeCp2, the energetic preference in NiCp2 for the eclipsed conformation is lower and the stronger crystal field, favoring the staggered conformation. Therefore, phase I′ of NiCp2 is stable below 170 K, while at 300 K NiCp2 transforms to phase I′ at much lower pressure than FeCp2.