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The crystal structure of trans-[Rh(CO) (PPh,),(OCIO,)] has been determined, and it has been established that the perchlorate group is easily displaced by substituted hydrazines to give trans-[Rh(CO) (PPh,),L] -CIO, ( L = NH,NH,Jx; x = 1 or 2; R = M e or Ph). X-Ray crystallography shows that for all these complexes the unsymmetrically substituted hydrazine co-ordinates to rhodium via the N H, group.

Single-crystal Laue diffraction patterns have not hitherto been used for the precise determination of unit-cell dimensions, although axial ratios and interaxial angles have frequently been derived. Here, we use an attenuator such as 0.1 mm palladium foil in the white synchrotron-radiation beam incident on the crystal; the absorption edge (e.9. palladium at 0.509 A) then sharply defines the minimum wavelength present. With gnomonic projections in the manner described by Carr, Cruickshank & Harding [J. Appl. Cryst. (1992), 25, 294-308], the unit-cell dimensions on an absolute scale can be derived. Three tests show that these dimensions are correct to circa 0.5%.

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