Crystal architecture of the cocrystalline salt [Ru(eta(5)-C5H5)(eta(6)-trans-PhCH=CHPh)][PF6]center dot 0.5trans-PhCH=CHPh and the reversible order-disorder phase transition in [Ru(eta 5-C5H5)(eta(6)-C6H6)][PF6]

The crystal architecture, stability and behaviour with temperature of the h exafluorophosphate salts [Ru(eta(5)-C5H5)-(eta(6)-arene)](+) (arene = benze ne 1 or trans-stilbene 2) have been investigated by variable temperature X- ray diffraction and differential scanning calorimetry. Compound 1 shows a r eversible high temperature phase transition to a semi-disordered phase at 3 32 K. It is pseudo-isomorphous with [Cr(eta(6)-C6H6)(2)][PF6] and with the room temperature form of [M(eta(5)-C5H5)(2)][PF6] (M = Co or Fe), the latte r species also undergoing reversible phase transitions. Crystalline 2 has b een characterised in its cocrystallised form [Ru(eta(5)-C5H5)(eta(6)-trans- PhCH=CHPh)][PF6] . 0.5trans-PhCH=CHPh. The trans-stilbene molecules fill in the voids formed in the packing when the small [PF6](-) anions assemble wi th the large and asymmetric [Ru(eta(5)-C5H5)(eta(6)-trans-PhCH=CHPh)](+) ca tions. The role of charge assisted C-H(delta+)... F(delta-) interactions is discussed.