Dynamic disorder and fluxionality in M3(CO)12 clusters:: Variable-temperature X-ray diffraction studies on FenRu3-n(CO)12 (n=1, 2) and the low-temperature phase of Fe3(CO)12

The metal atom disorder in the crystalline clusters Fe2Ru(CO)(12) (1) and FeRu2(CO)(12) (2) has been shown to be dynamic in origin. At and below 313 K, 1 crystallizes in the noncentrosymmetric space group Pn with two independent molecules in the asymmetric unit, both molecules having approximate C-2v symmetry with small C-2 distortions. Above 313 K, crystals of 1 undergo a phase transition to the centrosymmetric space group P2(1)/n, becoming isomorphous and isostructural with, the room-temperature phase of Fe-3(CO)(12) (3), These reversible changes are accompanied by an increase in the metal atom disorder, from a completely ordered structure at 223 K to a statistically disordered (1:1) "Star of David" structure at 323 K. Crystals of 2 show similar behavior, belonging to the noncentrosymmetric space group C2cb below 228 K and undergoing a phase transition to the centrosymmetric space group Ccmb above this temperature. An increase in the extent of disorder is observed in 2, from an ordered structure at 173 K to the extended Star of David disorder above the phase transition temperature. Compound 2 has an all-terminal carbonyl arrangement, with a strong D-3 distortion, and provides the first example of the D-3 structural type for a homoleptic M-3(CO)(12) cluster. In contradiction to our earlier studies, Fe-3(CO)(12) (3) has now been shown to undergo a phase transition at similar to 210 K to a second monoclinic phase with a partial ordering of the metal atom triangles. The asymmetric unit comprises four complete and one half-molecule of 3, and one of these molecules. (3a) is completely ordered. The thermal motion in 1 and the structures of 1-3 have been examined in the context of the various proposed fluxional mechanisms in M-3(CO)(12) clusters.