As observed by variable-temperature and -pressure C-13 NMR, intramolecular carbonyl scrambling in Ir2Rh2(CO)(12) and Ir-4(CO)(11)(mu-SO2) proceeds via a 'change of basal face' mechanism. In both cases the site exchange process has a positive activation volume suggesting that the transition states contain longer M-M distances compared to ground states of C-s symmetry. Transition state structures have been located by density functional calculations including relativistic effects. These structures contain a new symmetry plane which interchanges the indistinguishable starting and final geometries. Both transition state structures contain one significantly elongated M-M distance, bearing the bridging ligand unaffected by the site exchange. Differences in molecular volumes of ground and transition state geometries as calculated from Connolly surfaces and electron densities confirm the volume expansion in both cases. The sign of the activation volume is therefore a good criterion for distinguishing between the two main site exchange processes occurring in tetrahedral d(9) carbonyl clusters, i.e. the change of basal face process and the 'merry-go-round process, as the latter presents a negative activation volume.