The synthesis, structure, and magnetic characterization of two solvates of bis(1,4,5,8-tetraazaphenanthrene)bis(thiocyanato)iron(II), [Fe(tap)2(NCS)2].nCH3CN with n = 1 (solvate A) and n = 1/2 (solvate B) are reported. A shows a continuous high-spin reversible low-spin conversion over the temperature range ca. 110-280 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 P1BAR with Z = 2 at both temperatures. The lattice constants are a = 8.920(3) angstrom, b = 9.372(3) angstrom, c = 16.838(4) angstrom, alpha = 96.32(2)-degrees, beta = 100.47(3)-degrees, gamma = 112.14(2)-degrees, and V = 1257.3 angstrom3 at 290 K and a = 8.742(2) angstrom, b = 9.265(2) angstrom, c = 16.535(3) angstrom, alpha = 96.56(2)-degrees, beta = 100.15(3)-degrees, gamma = 112.43(3)-degrees, and V = 1194.07 angstrom3 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 a = 22.636(4) angstrom, b = 16.810(3) angstrom, c = 18.528(3) angstrom, beta = 138.55 (3)-degrees, and V = 4666,90 angstrom3 at 290 K. The final reliability factor was R = 5.93%. Molecular structures for both solvates are very similar at room temperature where iron(II) lies in a distorted octahodron 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 metal-to-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 Fe-N(tap) and Fe-N(CS) distances decrease by 0.23 (mean value) and 0.12 (mean value) angstrom, 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-degrees along with a remarkable variation of the N-Fe-N angles. The gradual temperature dependence of chi(M)T for 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 1A1 reversible 5T2 Spin equilibrium. The enthalpy and entropy changes associated with the spin equilibrium were estimated as DELTAH = 15.5 kJ mol-1 and DELTAS = 92 J mol-1 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.