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€544. A Convenient Solid for Calibration of the Gouy MagneticSusceptibility Apparatus. By B. N. FIGGIS and R. S. NYHOLM. THE measurement of magnetic susceptibilities by the Gouy method is a relative one, the apparatus being calibrated in terms of a substance of known susceptibility, for which water, nickel chloride solution, and powdered cupric sulphate pentahydrate or ferrous ammonium sulphate hexahydrate have been used. The low suceptibility of water is often inconvenient if small tubes are being calibrated. Nickel chloride solution requires accurate analysis before use and ferrous ammonium sulphate is often of questionable purity. This substance and copper sulphate do not pack well and several different values for the susceptibilities of both solids have been reported.The required properties for a calibrant are: (1) Readily available pure;(2) an accurately known and moderate susceptibility (xg = ;(3) stability in moist air;(4) xg must vary in a known and simple way, at least at room temperature; (5) easily and reproducibly packable into the Gouy tube. The complex mercury tetrathiocyanatocobaltate HgCo(CNS), offers some advantages and its susceptibility has therefore been accurately determined. at 20' 1 being used as reference, the gram susceptibility of the complex is 16.44 (-&O.OS) x at 20".As reported elsewhere,l it obeys the Curie-Weiss law, xg cc (T + lO)-l where T is expressed

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Constrained Hartree–Fock calculations have been performed to obtain wavefunctions that reproduce experimental X‐ray structure‐factor magnitudes for crystalline NH3 to within the limits of experimental error. Different model densities using both a single molecule and clusters of NH3 in the calculation of X‐ray structure‐factor magnitudes have been examined. The effects of the crystalline lattice on the experimental wavefunction of the NH3 unit can be reproducibly recovered. The construction of structure‐factor magnitudes based on normally distributed random perturbations of the experimental values has also been used to gauge the accuracy of integrated atomic properties obtained from the wavefunctions, the point at which the constraint procedure should be terminated, and the approximate error in the experimental values.

The room-temperature (295 K) crystal structures of potassium ferricyanide, K3[Fe(CN)6], have been determined for the simplest monoclinic (a reinvestigation) and orthorhombic polytypes by single- crystal X-ray diffraction. The monoclinic form is P21/c, a 7.047(3), b 10.400(3), c 8.384(3) Ǻ, β 107.29(3)°, Z 2. The iron atoms lie on special positions with symmetry 1. In the orthorhombic form, Pnca, a 13.422(6), b 10.396(4), 8.381(4) Ǻ, Z4, the iron atoms now lie on special positions with symmetry 2 (parallel to c). Residuals are 0.036 and 0.048 for 1232 and 855 'observed' reflections respectively.

A polarized neutron diffraction experiment on Cs2KCr(CN)6 yielded 437 unique magnetic structure factors, which were analyzed in terms of valence orbital or alternatively multipole populations on all atoms. Apart from a small population on the potassium atom, the spin density is described by a model corresponding to octahedral symmetry in the Cr(CN)63" ion with 3d and 4p orbitals on the chromium atom and (sp)" and pT orbitals on carbon and nitrogen atoms. The chromium configuration is t2g2- 30(3,eg0•12(4)4p°-83(7) with the 3d orbitals expanded 8% radially relative to the free ion. The ligand atom and orbital populations are C" = -0.044 (8), C" = -0.044 (5), N, = -0.034 (7), and N, = 0.087 (4). This complex spin density pattern is incompatible with molecular orbital theories of bonding that do not accommodate spin polarization effects, for example restricted Hartree-Fock. There is negative spin in the ligand framework and net positive spin in the orbitals and in the metal 3d orbitals, with the 3dr (t2g) population less than the free-ion value of 3.00. These features are compatible with a model involving back-bonding in the expected direction, which delocalizes spin from the t2g to ligand orbitals, together with spin polarization of about the same magnitude in the Cr-CN -bond. An approximate unrestricted calculation by the DVmethod shows fair agreement with our results, particularly in the cyanide ligand region. These results indicate that in this strongly covalent ion the electron-electron correlation effects on the spin distribution are of the same magnitude as the covalence effects.