Unsolvated ruthenium(II) benzene dichloride: The beta polymorph

A novel polymorph of the unsolvated species [Ru2(benzene)2Cl4] (beta 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 alpha polymorph and the CHCl3 solvate, throwing light on some discrepant results recently appeared in the literature. The thermal behavior of the alpha and beta 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.