As observed by variable temperature and pressure C-13 NMR, the intramolecular scrambling of carbonyl ligands in Rh-4(CO)(12) and IrRh3(CO)(12) is due to a merry-go-round process (3 mu(2)-CO <-> 3 eta(1)-CO) about any triangular face of the metal tetrahedron. Both cluster compounds have a negative activation volume on going from the. bridged ground-state structure of C-3v symmetry to an unbridged transition state, suggesting that bridged M-M distances are longer than unbridged M-M distances. Site exchange is faster in Rh-4(CO)(12) than in IrRh3(CO)(12) where the apical position is occupied by an iridium atom. Density functional calculations on the bridged and unbridged forms of both cluster compounds have been made at two levels of approximation (LDA and GGA) including relativistic effects for Ir and Rh. LDA reproduces best the experimental distances and shows that opening bridges shortens the M-M bonds. The difference in volume of the bridged and unbridged forms of Rh-4(CO)(12), as calculated from Connoly surfaces, agrees fairly well with the experimental activation volume. Calculations at the GGA level give the correct trends in energies. ELF maps and overlap population analysis indicate that iridium is more electropositive than rhodium, as suggested by the experimental results.