Structure of vanadocene in the temperature interval 108–357 K and the nature of the ring disorder

Abstract

The crystal structure of vanadocene, bis(π5–2,4-cyclopentadien-1-yl)vanadium, Cp2V, has been studied at the four temperatures 108, 170, 297 and 357 K by X-ray diffraction in order to elucidate the nature of the Cp-ring disorder and compare the results obtained with similar data for formally isostructural ferrocene, cobaltocene and nickelocene. A careful analysis of the anisotropic displacement parameters (ADP's) for Cp2V showed that at 108 K the crystal structure is ordered (staggered Cp2V molecules occupy centres of symmetry in the space group P21/n, Z = 2), which distinguishes vanadocene from its formal analogues and some Cp-ring disorder starts at ca 170 K. A model of this disorder has been proposed with two distinct orientations of the ring, differing in temperature-dependent occupancy factors and by a rotation in the ring plane of almost 36°. The fractional contributions of the second (minor) ring orientation are 0.05, 0.17 and 0.35, respectively, at 170, 297 and 357 K. The energetic characteristics of the ring rotation motion in the Cp2V crystal were estimated from ADP analysis and compared with similar data for other 3d-metallocenes.