The compound [Fe(tvp)(2)(NCS)(2)] . CH(3)OH, where tvp is 1,2-di-(4-pyridyl)-ethylene, has been synthesized and characterized by x-ray single-crystal diffraction. It consists of two perpendicular, two-dimensional networks organized in parallel stacks of sheets made up of edge-shared [Fe(II)](4) rhombuses. The fully interlocked networks define large square channels in the [001] direction. Variable-temperature magnetic susceptibility measurements and Mössbauer studies reveal that this compound shows low-spin to high-spin crossover behavior in the temperature range from 100 to 250 kelvin. The combined structural and magnetic characterization of this kind of compound is fundamental for the interpretation of the mechanism leading to the spin crossover, which is important in the development of electronic devices such as molecular switches.

The synthesis, structure, and magnetic characterization of two solvates of bis( 1,4,5&tetraazaphenanthrene)bis- (thiocyanato)iron(II), [Fe(tap)2(NCS)z].nCH3CN with n = 1 (solvate A) and n = '/z (solvate B) are reported. A shows a continuous high-spin-low-spin conversion over the temperature rangeca. 1 10-280 K, while B is paramagnetic over the temperature range 4.2-290 K. The X-ray structure for A was solved at 290 and 135 K. It crystallizes in the triclinic space group Pi with 2 = 2 at both temperatures. The lattice constants are u = 8.920 (3) A, b = 9.372(3) A, c = 16.838(4) A, a = 96.32(2)", B = 100.47(3)O, y = 112.14(2)", and V = 1257.3 A3 at 290 K and u = 8.742(2) A, b = 9.265(2) A, c = 16.535(3) A, a = 96.56(2)', j3 = 100.15(3)", y = 112.43(3)O, and V = 1194.07 A3 at 135 K. The data were refined to R = 5.67 (290 K) and 7.57% (135 K). B crystallizes in the monoclinic space group C2/c, with u = 22.636(4) A, b = 16.810(3) A, c = 18.528(3) A, 0 = 138.55 (3)", and V = 4666,90 A3 at 290 K. The final reliability factor was R = 5.93%. Molecular structures for both solvates are very similar at room temperature where iron(I1) lies in a distorted octahedron with NCS-ligands in the cis position. The most significant structural features which could account for the different magnetic behavior of A and B are found to be the metalto-ligand bond distances and trigonal distortion. Structural modifications associated with the spin change in A mainly consist of a large reorganization of the metal environment: the FeN(tap) and Fe-N(CS) distances decreaseby 0.23 (mean value) and 0.12 (mean value) A, respectively, when the temperature is lowered from 290 to 135 K, and a more regular shape of the [FeN6] octahedron is achieved through a modification of the trigonal deformation from 8 to 3' along with a remarkable variation of the N -F e N angles. The gradual temperature dependence of XMTfor A was considered as a Boltzmann distribution of molecules in the low-spin ground state and in the thermally accessible high-spin excited state reflecting the 'Al -5T2 spin equilibrium. The enthalpy and entropy changes associated with the spin equilibrium were estimated as AH = 15.5 kJ mol-' and A S = 92 J mol-' K-1. Analysis of the magnetic data versus temperature for B by using the zero-field-splitting spin Hamiltonian for S = 2 leads to D = 7.4 cm-1 and g = 2.09.

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