The compound chloro(dioxygen)tris(triphenylphosphine)rhodium(I), RhCl(02)(P(C6H5)3)3-2CH2Cl2, has been identified as one of the species formed by the oxygenation of RhCl(P(C6H5)3)3. The crystal structure of this compound has been determined from three-dimensional x-ray data collected on a manual four-circle diffractometer at room temperature. The compound crystallizes in the orthorhombic space group Pbca with 8 molecules per unit cell (pobsd = 1.411, pcalcd = 1.416 g cm"3). The axial parameters are a = 24.817 (4), b = 18.359 (2), c = 23.200 (4) A. Least-squares refinement of absorptionand decomposition-corrected intensity data converged at a conventional R factor of 0.049 based on 2073 significant reflections. The geometry about the rhodium atom may be described as a trigonal bipyramid with three phosphine ligands occupying one equatorial (Rh-P(2) = 2.357 (3) Á) and the two axial positions (Rh-P(l) = 2.365 (4), Rh-P(3) = 2.391 (4) Á). The remaining equatorial sites are occupied by the chlorine (Rh-Cl(l) = 2.401 (3) Á) and the dioxygen molecule. The dioxygen molecule is tt bonded and a slight asymmetry in the rhodium-oxygen bond lengths (Rh-O(l) = 2.081 (8), Rh-0(2) = 2.005 (8) Á) is observed. One of the dichloromethanes of crystallization appears to be hydrogen bonded to the coordinated dioxygen.

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The compound bis(chloro(dioxygen)bis(triphenylphosphine)rhodium(I)), [RhC1(02)(P(C6H5)3)2.cHZC12]2, has been formed by the oxygenation of RhC1(P(C6H5)3)3. The crystal structure has been determined from three-dimensional x-ray data at room temperature. The compound crystallized in space group Pi with one molecule per unit cell (pobsd = 1.470, pcald = 1.469 g ~m -~) .The axial parameters are a = 13.889 (7), b = 13.678 (6), c = 11.433 (5) A; a = 105.73 (4), 0 = 115.74(3), y = 100.97 (4)". Least-squares refinement of absorption-and decomposition-corrected intensity data converged at a conventional R factor of 0.044 based on 1658 significant reflections. The structure may be described as a dimer having two trigonal-bipyramidal subunits, the bridge being formed by one oxygen atom of each subunit having a third bond to the second subunit. The dioxygen molecules thus have features similar to those of both ?r-bonded ligands and chelating peroxo groups.The crystal structure of p-hydroxo-bis[pentaaminechromium(III)] chloride dihydrate has been determined from threedimensional x-ray diffractometer data. The compound crystallizes in the monoclinic space group C2/c with a = 23.656 (9) A, b = 7.36 (1) A, c = 16.718 (8) A, and 0 = 128.1 (1)O. Least-s uares refinement of 1112 independent reflections led to a final R value of 0.091. The dimeric [(NH3)5CrOHCr(NH3)5] + complexes are oriented with their twofold axes parallel to the b axis of the unit cell. Single-crystal absorption spectra in the region of single pair excitations 4A2p4A2, -4A2,2E,, 4A21T1, have been measured down to liquid helium temperatures. The structural and spectroscopic informations are combined to determine the polarizations of the optical transitions with respect to the symmetry axes of the dinuclear complexes. There are a t least two intensity-gaining mechanisms active in this region. The most prominent absorption bands are shown to arise through an exchange-induced mechanism.9

The catalytic hydrogenation activity1 of chlorotris-(triphenylphosphine)rhodium(I) is considered to be due to either (a) a vacant site in the five-coordinate species RhH2Cl[P(C6Hd)3]2, or (b) a six-coordinate species RhH2Cl[P(C6H5)3]2S, where S is a weakly bound, and readily replaced, solvent molecule. Our attempts to produce suitable crystals of the methylene chloride adduct of this active hydrogenating species for an X-ray diffraction study have been unsuccessful. However, we decided to extend our investigation to other formally coordinatively unsaturated rhodium complexes where either chloroform or methylene chloride appears to be firmly retained in the solid state. The (1)

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