The crystal structure of [Fe(bt)(2)(NCS)(2)] (A) was determined by X-ray diffraction at 293 and at 150 K in order to analyze the structural changes associated with the spin transition. The space group is P (1) over bar with Z = 2 at both temperatures. Lattice constants are as follows: a = 8.5240(4), b = 11.0730(6), c = 12.5300(8) at 293 K and a = 8.1490(4), b = 11.4390(5), c = 12.1270(6) at 150 K. The iron(II) atom lies at the center of a distorted octahedron [FeN6] defined by two bt ligands arranged in a cis conformation. The two remaining coordination positions are occupied by two isothiocyanate anions. The average bond lengths of 2.159(4) angstrom (293 K) and 1.951(2) angstrom (150 K) clearly indicate the change in spin configuration. The trigonal distortion parameter phi has a value of 9.6 degrees and 5.5 degrees at 293 and 150 K, respectively. For A, Delta V = Delta V-SCO = 28 angstrom(3) per formula unit and is accompanied by a hysteresis of 10 K. chi T-M vs T curves at atmospheric pressure for A show an abrupt spin transition with T-c(down arrow) = 176 K and T-c(up arrow) = 187 K. The thermodynamic parameters associated with the spin transition are Delta H = 8.4 +/- 0.4 kJ mol(-1) and Delta S = 46.5 +/-3 J K mol(-1). The thermal dependence of the magnetic susceptibility at different pressures, 0.1-0.91 GPa, points out an unusual behavior, which can only be understood in terms of a crystallographic phase transition or a change in the bulk modulus of the complex. Polymorph B crystallizes in the C2/c space group with an average Fe-N bond length of 2.168(2) angstrom and phi = 14.7 degrees at 293 K. B remains in the HS configuration even at pressures of 1.06 GPa.