A method for the least‐squares rigid‐body refinement of a general electron density model is described. The present formulation is different from a previously reported one in the computation of the derivatives of the calculated Fourier coefficients, which are derived analytically here. This, together with a judicious choice of the Fourier transform search arrays, makes the procedure extremely fast and sufficiently accurate. Although originally designed simply to optimize the values of the positional parameters obtained by Patterson search techniques, the method proved to be extremely efficient as an aid for evaluation of the correctness of potential molecular‐replacement solutions.

The crystal structure of [Zn(sac)2(py)2] has been de termined by single-crystal X-ray diffractometry. It crystallizes in the monoclinic space group C2/c with Z = 4. We propose and discuss an assignment of its IR and Raman spectra, compatible with the structural characteristics. Its thermal behavior was investigated by means of TG and DTA methods. Some comparisons with related compounds are made.

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The single crystal and anion structure of Mn[Fe(CN)sNO'2H~_O, obtained by slow interdiffusion of reactant solutions through a TMS gel, was solved by X-ray diffraction methods and refined to RI = 0.036. Spatial group: orthorhombic, Prima, a = 14.069(2), b = 7.538(1), c = 10.543(1),~,, Z = 4. The Mn(ll) ion and the water molecules are sited on mirror planes, which bisect the nitroprusside ions. One of the water molecules is coordinated to Mn(lI) and the other, strongly hydrogen (as acceptor) bonded to the first molecule. The IR spectrum confirms the bonding of the water molecules and TGA results are in accordance with the dihydrate character of the substance and its dehydration in two successive steps. DTA results and the Raman spectrum agree with other results and the comparison between IR and Raman vNO wavenumbers confirms the expected strong vibrational interaction between the closely packed antiparallel (eclipsed) NO groups. There is a topotactic relationship between the dihydrate and the trihydrate, which crystallizes in the space subgroup P2Jn.