Molecular crystals under high pressure: theoretical and experimental investigations of the solid-solid phase transitions in [Co2(CO)6(XPh3)2] (X = P, As).

An accurate analysis of the solid-state structural transformations occurring in species of the general formula [Co(2)(CO)(6)(XPh(3))(2)] (X = P, As) by experimental X-ray diffraction under non-ambient conditions (on variation of pressure or temperature) and by ab initio theoretical modelling is presented. After a crystal-to-crystal phase transition, the conformation of carbonyl ligands about the Co-Co bond changes from staggered to (almost) eclipsed. The analysis of high-pressure and low-temperature structures sheds light on the peculiar behaviour of the metal-metal bond, the most flexible in the molecule, which is initially compressed, then elongated due to increased intra-carbonyl repulsion and eventually compressed again. Theoretical calculations in the solid state allow the thermodynamic quantities of the transformation to be computed and prediction of the distortions produced by the external stress. The change in molecular symmetry is induced by the increase in the high internal energy associated with the staggered carbonyl conformation, which does not allow efficient packing if the crystalline volume is reduced. The eclipsed conformer, though much less stable in isolation, guarantees better entanglement between molecules.