Experimental and Theoretical Studies of the Site Exchanges in Rh(4)(CO)(12) and IrRh(3)(CO)(12).

As observed by variable temperature and pressure (13)C NMR, the intramolecular scrambling of carbonyl ligands in Rh(4)(CO)(12) and IrRh(3)(CO)(12) is due to a merry-go-round process (3 &mgr;(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(3)(v)() 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 IrRh(3)(CO)(12) where the apical position is occupied by an iridium atom.