Experimental and Theoretical Studies of the Site Exchanges in Rh4(CO)12 and IrRh3(CO)12

As observed by variable temperature and pressure 13C NMR, the intramolecular scrambling of carbonyl ligands in Rh4(CO)12 and IrRh3(CO)12 is due to a merry-go-round process (3 μ2-CO ↔ 3 η1-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 C3v symmetry to an unbridged transition state, suggesting that bridged M−M distances are longer than unbridged M−M distances. Site exchange is faster in Rh4(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 Rh4(CO)12, as calculated from Connoly surfaces, agrees fairly ...