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. 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.

The monosubstituted [Ir4(CO)IIL] clusters (L = P(OPh),, 1; L = (P(OMe),, 2; L = P(OCH2),CEt, 3) were obtained in good yields by the reaction of [Ir4(C0), ,I]-with the corresponding phosphite. In the solid state, cluster 3 has a C, geometry with all terminal ligands as shown by an X-ray analysis. Three isomers are present in solution: one with terminal ligands (A) and two with three edge-bridging CO's and with L in axial (B) or radial (C) position (see Scheme). The thermodynamic and kinetic parameters of isomerisations B p A and A$C were determined by simulation of the variable-temperature "P-NMR spectra. The three isomers correspond to three minima on the kinetic pathway of CO scrambling, whose relative energies vary independently within a small range (1-9 kJ mol-' at 298 K). At low temperature, isomer C is always the least stable and is not observed for 1 which bears the most bulky phosphite ligand. The isomerisations are due to two intramolecular merry-go-rounds of CO groups about two unequivalent faces of the unbridged species A.Introduction. - . We report here on a thermodynamic and kinetic study of the solution equilibria involving the monosubstituted derivatives of [1r4(CO),J with three different trialkyl phosphites, and on the crystal structure of [Ir,(CO),,{P(OCH,),CEt)].

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