SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF THE ISOMERS OF RH6(CO)14L2 CLUSTERS (L=NCME,PY,P(OPH)3), X-RAY CRYSTAL-STRUCTURE OF TRANS-RH6(CO)14(P(OPH3))2

The reaction of Rh6(CO)16 with two equivalents of Me3NO in the presenc e of a substituting ligand (L = NCMe, Py, P(OPh)3) affords a mixture o f isomers of the general formula Rh6(CO)14L2. The substituting ligands occupy two terminal sites in the structure of the parent cluster. For L = P(OPh)3 three isomers (I-III) were separated from the reaction mi xture. The structure of the first isomer (I) has been established by X -ray analysis. This compound crystallizes in two polymorphic modificat ions, monoclinic and triclinic; both of which consist of identical mol ecules and differ only in the packing of the molecules in the crystals . It should be noted that both polymorphic forms contain in the unit c ells the racemic mixture (RR and SS) of the Rh6(CO)14{P(OPh)3}2 molecu les. This octahedral cluster may be considered as a derivative of the parent Rh6(CO)16 cluster with two terminal CO groups in trans position s at Rh(l) and Rh(2) atoms substituted by phosphite ligands. The struc tures of two other disubstituted clusters, (II) and (III), were establ ished by P-31 NMR spectroscopy. Phosphite ligands in these compounds o ccupy the terminal sites at the adjacent rhodium atoms of the Rh6 octa hedron but differ in their mutual orientation. Other disubstituted der ivatives (L = NCMe, Py) are labile compounds and exist in solution as inseparable mixtures of the isomers characterized by H-1 and C-13 NMR spectroscopy. A mechanism of interconversion of the isomers is propose d.