The crystal structure of the complex [Cun(mnt)2]1 2'[MB+]2-(acetone) [CuC4oN10S6H36'C3H60] is reported, where mnt = maleonitriledithiolato (C4N2S2)2" and MB+ = the methylene blue cation = 3,9-bis(dimethylamino)phenazothionium.The space group is PI, with a = 10.346 (3) A, b = 14.522 (3) A, c = 15.524 (5) A, a = 93.69 (3)°, ß = 90.69 (2)°, y = 105.87 (3)°, and with Z = 2. The residuals are R = 4.4% and Pw = 4.4% for 4235 independent reflections collected with a diffractometer using Mo Ka radiation. The MB+ cations are stacked along the c axis. The [Cu(mnt)2]2" anions are paired, these pairs being stacked along the c axis as well. The anions have approximately D2 symmetry with a dihedral angle of 47.4°between the planes of the ligands. The copper-sulfur distances range from 2.240 (2) to 2.261 (2) A. Susceptibility measurements reveal an antiferromagnetic exchange coupling. An exchange J of -2.6 cm"1 was derived from a fit of a singlet-triplet model to the experimental data. Single-crystal EPR spectra are in accord with a triplet state of two coupled Cu(II) ions. The experimental g, copper hyperfine, and zero-field splitting tensors are compared with tensors which are calculated from semiemperical molecular orbital data.

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e-Melibiose is a disaccharide of e-D-glucose and e-D-galactose, linked e-1 ~ 6, with glucose as the reducing unit. The crystal data are a= 8.878 (5), b= 10.920 (6), c= 15.730 (10) ~; space group P2t2~21; Z=4, D,,,= 1.552, D~= 1.570 g cm -3. The structure analysis was performed with 1921 independent reflexions, collected on a Nonius CAD-4 diffractometer, by applying the tangent formula. The H atoms were located on difference syntheses. Final difference syntheses showed a partial (19.6%) substitution of e-melibiose by the fl-anomer. After anisotropic least-squares refinement for non-hydrogen and isotropic refinement for H atoms the final R was 0.046. Both rings have the usual 4C~ chair conformation. The conformation of the e-1 --+ 6 linkage is similar to that in the trisaccharide raffinose, which contains the melibiose disaccharide moiety. All hydroxyl groups are involved in intermolecular hydrogen bonds. With the exception of the cO(l) anomeric hydroxyl group all other groups function as hydrogen-bond acceptors. The ring O atom of the glucose residue takes part in hydrogen bonding; in the/~-anomer the ring O of the galactose residue also acts as an acceptor. The melibiose molecules are linked by an intricate three-dimensional network of hydrogen bonds, in which the water molecule plays a central part by donating and accepting two hydrogen bonds.

The probability density of phase angles for structures with one or more atoms on known positions [Sim (1959). A cta Cryst. 12,[813][814][815][816][817][818] is expressed in terms of normalized difference-structure factors, and used to calculate standard deviations of phases and expectation values for amplitudes (IE~ I) of the normalized difference-structure factors. Numerical results are tabulated for various values of I Ell and I E 21, i.e. the minimum and maximum I Erl value a given reflection can have. Applications to the DIRDIF procedures [van den Hark, Prick & Beurskens (1976). Acta Cryst. A32, 816-821] are described. New applications are the calculation of the a priori probability density function for levi values, and a statistical rriethod for the detection of a centre of symmetry in the remaining part of the structure. For the statistical method, a centricity parameter, X c, which is unity for centric and zero for acentric distributions, is defined; for the difference structure, X c is calculated by an iterative procedure, extrapolating towards zero contribution of the known part of the structure. Numerical results for 13 test structures are given.

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