SYNTHESIS, REACTIVITY, AND FLUXIONAL BEHAVIOR OF [IR2RH2(CO)12], AND CRYSTAL-STRUCTURE OF [IR2RH2(CO)8(NORBORNADIENE)2]

The synthesis of [Ir2Rh2(CO)12] (1) by the literature method gives a m ixture 1/[IrRh3(CO)12] which cannot be separated using chromatography. The reaction of [Ir(CO)4]- with 1 mol-equiv. of [Rh(CO)2(THF)2]+ in T HF gives pure 1 in 61% yield. Crystals of 1 are highly disordered, unl ike those of its derivative [Ir2Rh2(CO)5(mu2-CO)3(norbomadiene)2] whic h were analysed using X-ray diffraction. The ground-state geometry of 1 in solution has three edge-bridging CO's on the basal IrRh2 face of the metal tetrahedron. Time averaging of CO's takes place above 230 K. The CO site exchange of lowest activation energy is due to one synchr onous change of basal face, as shown by 2D- and VT-C-13-NMR. Substitut ion of CO by X- in 1 takes place at a Rh-atom giving [Ir2Rh2(CO)8(mu2- CO)3X]-(X = Br, I). Substitution by bidentate ligands gives [Ir2Rh2(CO )7(mu2-CO)3(eta4-L)] (L = norbornadiene, cycloocta-1,5-diene) where th e ligand L is chelating a Rh-atom of the basal IrRh, face. Carbonyl su bstitution by tridentate ligands gives [Ir2Rh2(CO)6(mu2-CO)3(mu3-L)] ( L = 1,3,5-trithiane, tripod) with L capping the triangular basal face of the metal tetrahedron. Carbonyl scrambling is also observed in thes e substituted derivatives of 1 and is mainly due to the rotation of th ree terminal CO's about a local C3 axis on the apical Ir-atom.