An attempt has been made to elucidate the low-energy modes of vibration of the icosahedral envelope of carbonyl ligands in the series of dodecacarbonyl clusters, Co4(CO)12, RhCo2(CO)12, Ir2Co2(CO)12, and Fe3(CO)12, from an examination of their crystalline structures. It was hoped that this approach would give additional evidence necessary for a clearer understanding of the fluxional behaviour of such species both in the solid and solution. No evidence for a “frozen-in” distortion towards a cuboctahedral geometry was detected. There are, however, large distortions of the carbonyl icosahedron which arise from the encapsulated metal core.