Dynamic Disorder and Fluxionality in M3(CO)12 Clusters:  Variable-Temperature X-ray Diffraction Studies on Fe n Ru3 - n (CO)12 (n = 1, 2) and the Low-Temperature Phase of Fe3(CO)12

• Published in: Organometallics 1999-11, Vol.18 (24), p.5022-5033
• Main Authors: Farrugia, Louis J, Gillon, Amy L, Braga, Dario, Grepioni, Fabrizia
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om9902986

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 2 v symmetry with small C 2 distortions. Above 313 K, crystals of 1 undergo a phase transition to the centrosymmetric space group P21/n, becoming isomorphous and isostructural with the room-temperature phase of Fe3(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 M3(CO)12 cluster. In contradiction to our earlier studies, Fe3(CO)12 (3) has now been shown to undergo a phase transition at ∼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 M3(CO)12 clusters.