HOLVETICA CHIMICA AC'TA - Vol. 77 (1994) 547 51. Variable-Temperature and -Pressure 3'P-NMR Study of the Intramolecular PPh, Migration in the Cluster Compound (Ir,Rh,(CO),,PPh,] by Gdbor Laurenczy, Giacomo Bondietti, AndrC E. Merbach, Bertrand Moullet, dnd Raymond Roulet" In\titut de Chimie Minerale et Andlytique, Universite de Ldusdnne, Pldce du Chiteau 3, CH-1005 Ldusnnnc (l4.X.93) The reaction of [Ir2Rhz(CO)lz] with 1 mol-equiv. of PPh; yields [Ir,Rh,(CO),,PPh,] (1) as a mixture of two isomers with the phosphine ligand axially bound either to one basal Rh-atom in the kinetically preferred isomer 1R or to one basal Ir-atom in the thermodynamically preferred isomer 11. Both isomers are fluxional on the "C-NMR time scale at low temperature due to CO scrambling. Around room temperature, a new type of fluxional process starts to operate which is responsible for the isomerisation 1R F=? 11, i.r. the intramolecular migration of the reputedly inert PPh, ligand from one metal centre to another. The activation volumes ofconversions 1R + 11 and 11 + 1R are both positive, indicating that the migration of PPh, is dissociative in character. This article reports the first application of variable pressure 3'P-NMR to mechanistic studies. Introduction. - The fluxional behaviour of the cluster compounds derived from [Ir,(CO),,I [1-31, [Ir,Rh(CO),,I (41, [Ir2Rh2(CO),,1 [51, and [Rh,(C0),21[61 are due to CO scrambling processes such as the merry-go-round of six CO's about a triangular face of the metal tetrahedron, changes of basal face, or the rotation of three terminal CO's located on one metal centre. We report here on a new type of intramolecular rearrangement, distinct from CO scrambling, which was identified by a variable-temperature and variable-pressure "P-NMR study of the cluster compound [Ir2Rh,(CO),,PPh,] (1). Results and Discussion. - The reaction of [1r,Rh2(C0)12] with 1 mol-equiv. of PPh, in CH,Cl, at 0" followed by column chromatography at 0" gave [Ir2Rh2(CO)lIPPh3] as a single isomer 1R (Scheme). The same isomer could also be obtained in slightly higher yield (75%) by reacting [PPh,][Ir,Rh,Br(CO),,] [5] with PPh, and AgBF, (1 mol-equiv.) in CHzClz at 0". The same reactions effected at room temperature gave an equilibrium mixture of the two isomers 11 and 1R ([lI]/[lR] z 3) which could not be separated by chromatography or fractional crystallisation. The "P-NMR spectrum of a freshly pre- pared sample of 1R in CD2C12at 273 K consists of a doublet at 6 +10.6 ppm ('J(P,Rh) = 119.3 Hz) relative to external 85% H,PO,. The chemical-shift difference of the coordinated and free ligand (AS = +5.3 ppm) suggests that the Rh-bonded PPh, ligand is located in axial position from a comparison with the corresponding A6 values of [lr,(CO),,PPh,] [6] and of the axial isomer of [Ir,(CO),,PEt,] [la]. The "C-NMR spectrum of a sample of 1R enriched in "CO (ca. 30 %) in CD2C1, at 245 K presents 1 1 resonances of equal intensities (see Exper. Part). Three resonances with C,Rh-couplings have 6's in the region of bridging CO's indicating that two Rh-atoms and one Ir-atom form the basal face of the metal tetrahedron. Two of the three resonances with 6's in the region of radial CO's couple with Rh, and one of these two resonances presents an additional geminal