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[1,2-Bis(diphenylphosphino)ethane]tetracarbonylchromium, Cr(CO)4(diphos), where diphos= (C6Hs)zPCHzCHzP(C6Hs)z, prepared by the reaction of diphos with Cr(CO)6 crystallizes in the orthorhombic system with unit-cell dimensions, at 24 + 1 °C, of a = 16.73 + 0.06, b = 14.56_+ 0"04, c = 22.32 + 0"06 A. Cell dimensions at 2+2°C are: a= 16.663+0.009, b= 14.519+0.006, c=22.277+0.009 A. There are eight molecules per unit cell; dealt (240) = 1"374 g.cm-3, dob., = 1"371 g.cm-3. Systematic absences uniquely identify the space group as Pbca. Using nickel-filtered Cu K~ radiation, 2707 independent reflections within the sphere bounded by sin 0]2=0.550 were collected using an automated counter diffractometer operated at 2_+2°C. Using the 2163 reflections whose intensities were deemed much greater than zero, the structure was solved by conventional Patterson and electron density functions and was refined by full-matrix least-squares procedures to final unweighted and weighted residuals of 0.066 and 0.075, respectively. All non-hydrogen atoms were refined with anisotropic thermal parameters. The structure comprises Cr(CO)4(diphos) molecules in which each chromium atom is surrounded by a distorted octahedral array of two cis phosphorus atoms and four carbon atoms. The Cr-P distances are equal at 2"360+0.002 A and the P-Cr-P angle is 83.41 +_0"08 °. The Cr-C bonds trans to Cr-P bonds have a mean length of 1.831 +0.007 A, while the other two Cr-C bonds have a mean length of 1-884_+ 0-007 A. All Cr-C-O groups are slightly but significantly bent, the angles being in the range 176.9_+0.8 to 177.8+0.8 °. The C-O bond lengths range from 1.141 +0.012 to 1-165+ 0.012 ,~. The molecular structure is qualitatively consistent with views on metal-to-ligand n bonding previously deduced from vibrational spectra.

Chemical structure and reactivity of a silicon single crystal surface fluorinated by xenon fluorideThe 19F shielding tensors of the two aromatic flourines of potassium tetrafluorophthalate (KTFP) are reported. The results indicate that the most shielded elements of the tensors lie perpendicular to the carbon ring in agreement with previous powder studies and, of the two in· plane components, the more shielded is along the CF bond. The results also show that the "ortho effect" of Karplus and Das is not simply additive in the case of KTFP. Such additivity has been assumed by Nehring and Saupe in their liquid crystal study of fluorine shielding tensors in multifluorobenzenes.