Unsolvated ruthenium(II) benzene dichloride: The beta polymorph

A novel polymorph of the unsolvated species [Ru2(benzene)2Cl4] (β form in the following) was serendipitously isolated as a polycrystalline powder. Its molecular and crystal structure was unraveled by means of state-of-the-art X-ray powder diffraction structure determination methods applied to laboratory data, and was compared to those of both the α polymorph and the CHCl3 solvate, throwing light on some discrepant results recently appeared in the literature. The thermal behavior of the α and β polymorphs was investigated by coupling thermogravimetric analyses to variable-temperature X-ray powder diffraction experiments. No temperature-stimulated phase transformation could be detected between the two polymorphs, each preserving its structural features up to decomposition, suggesting that kinetic, more than thermodynamic, factors regulate their isolation.