Structure refinement with block diagonal least squares

The structure was refined with block diagonal least squares. In cases of pseudo-symmetry, least squares refinement is usually troublesome due to the high correlations between atoms related by false symmetry operations. Because of the poor quality of the data, only those reflections not suffering from the effects of decomposition were used in the refinement. With all non-hydrogen atoms refined with isotropic thermal parameters and hydrogen atoms included at fixed positions, the final R and R values were 0.142 and 0.190, respectively. Refinement with anisotropic thermal parameters resulted in slightly more attractive R values, but the much lower data to parameter ratio did not justify it. 

The structure was refined by block-diagonal least squares in which carbon and oxygen atoms were modeled with isotropic and then anisotropic thermal parameters. Although many of the hydrogen atom positions were available from difference electron density maps, they were all placed in ideal locations. Final refinement with all hydrogen atoms fixed converged at crystallographic residuals of R=0.061 and R =0.075. 

The positional and thermal parameters, and a scale factor, were refined by block-diagonal least-squares methods. After six cycles with isotropic thermal parameters R was 0-136. Five further cycles with anisotropic parameters for the gold atom and isotropic parameters for the fluorines reduced R to 0-099, and t ee cycles with all atoms anisotropic completed the refinement, the final R being 0-090. Measured and calculated structure factors are listed in Table 1. 

Crystals of the compound of empirical formula FiiPtXe are orthorhombic with unit-cell dimensions a = 8-16, h = 16-81. c = 5-73 K, V = 785-4 A . The unit cell volume is consistent with Z = 4, since with 44 fluorine atoms in the unit cell the volume per fluorine atom has its usual value of 18 A. Successful refinement of the structure is proceeding in space group Pmnb (No. 62). Three-dimensional intensity data were collected with Mo-radiation on a G.E. spectrogoniometer equipped with a scintillation counter. For the subsequent structure analysis 565 observed reflexions were used. The platinum and xenon positions were determined from a three-dimensional Patterson map, and the fluorine atom positions from subsequent electron-density maps. Block diagonal least-squares refinement has led to an f -value of 0-15. Further refinements which take account of imaginary terms in the anomalous dispersion corrections are in progress. 

X-ray Structure Determination. A three-dimensional single-crystal structure was determined on crystals obtained by recrystallization of NiZn(BAA)2en from pyridine. The crystals were monoclinic I 2/c with lattice parameters of a = 28.403(6) A, b = 8.465(3) A, c = 30.220(9) A, == 105.86(2)°, and Z = 8. Intensity data were collected by the 6 20 scan technique with graphite-monochromated Mo-Ka radiation on a Syntex P2i diffractometer. Of the 5055 data with sin 0/ < 0.54, 2559 had I > 3(t(I) and these were used in the solution and reflnement of the structure. The structure was solved by Patterson-Fourier methods and reflned by full-matrix (isotropic) and block-diagonal (anisotropic) least squares reflnement. The conventional discrepancy factors at the present state of refinement are R = 0.056 and Rw = 0.071. 

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