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 mixture 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 THF gives pure 1 in 61% yield. Crystals of 1 are highly disordered, unlike those of its derivative [Ir2Rh2(CO)5(mu2-CO)3(norbomadiene)2] which 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 synchronous change of basal face, as shown by 2D- and VT-C-13-NMR. Substitution 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 the ligand L is chelating a Rh-atom of the basal IrRh, face. Carbonyl substitution 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 these substituted derivatives of 1 and is mainly due to the rotation of three terminal CO's about a local C3 axis on the apical Ir-atom.