SYNTHESIS AND X-RAY-POWDER DIFFRACTION CHARACTERIZATION OF (OC)(2)RHCL2RH(COD) (COD = CYCLOOCTA-1,4-DIENE)

In order to elucidate the nature and the structure of the elusive (OC) (2)Rh(Ph3SiO)(2)Rh(cod) (cod)=cycloocta-1,5-diene) complex, an importa nt model compound for surface catalysis, (OC)(2)RhCl2Rh(cod) has been synthesized, and structurally characterized by ab initio X-ray powder diffraction. Crystals of (OC)(2)RhCl2Rh(cod) are monoclinic, space gro up P2(1)/c, a = 6.659(1), b = 12.274(1) and c = 16.096(1) Angstrom, be ta = 92.176(5)degrees, Z = 4, rho(calc),,,, = 2.209 g cm(-3). The stru cture has been solved, from powder diffraction data only, by Patterson and Fourier-difference methods and has been ultimately refined, by th e Rietveld method, down to R-p, = 0.116 and R-wp, = 0.154 for 4050 dat a points collected in the 12-93 degrees (2 theta) range. The molecule contains two square-planar rhodium atoms, one bearing two terminal car bonyls and the other bound to the chelating cod fragment, and two chlo rine atoms bridging the Rh ... Rh vector. The Rh2Cl2, core is markedly non-planar, the dihedral angle about the Cl ... Cl hinge being 135.4( 6)degrees.