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.