INTRAMOLECULAR DYNAMICS OF TETRANUCLEAR IRIDIUM CARBONYL CLUSTER COMPOUNDS - PART VI - DERIVATIVES WITH BIDENTATE LIGANDS - CRYSTAL-STRUCTURES OF (MU-[1,1-BIS(METHYLTHIO-CHI-S)ETHANE])TETRAIRIDIUM ([IR-4(CO)(10)(MU(2)-(MES)(2))CHME)]), BIS-[DIPHENYLPHOSPHINE]-CHI-P-CHI-P'))TETRAIRIDIUM ([IR-4(CO)(10)(MU(2)-(PH2P)(2)CHME)]), AND NE-1,3-DIYLBIS[DIPHENYLPHOSPHINE]-CHI-P-CHI-P'))TE

The disubstituted clusters [Ir-4(CO)(10)(mu(2)-L-L)] with one edge-bri dging ligand have a ground-state geometry with all COs terminal (L-L = (MeS)(2)CHMe, cluster 1) or with three edge-bridging COs (L-L = (Ph2P )(2)CHMe, cluster 2; L-L = Ph2P(CH2)(3)PPh2, cluster 3) in the solid s tate and in solution. A comparative C-13-NMR study of 1-3 shows that t heir respective ground-state geometries are merely relative minima of energy in the same kinetic profile of successive fluxional processes c onsisting of a merry-go-round of six COs about a unique triangular fac e and the rotation of terminal COs about one Ir-atom. The factors affe cting the activation energy of the merry-go-round result from the rela tive bites of the bidentate ligands in the ground-state geometry, as s hown by a comparison of the molecular structures of 2 and 3.