Consecutive thermochromic lattice distortional and spin crossover equilibria in solid-state Ni(detu)(4)Cl-2 (detu = N,N'-diethylthiourea) are investigated by variable-temperature X-ray crystallography (173-333 K), DFT calculations, and differential scanning calorimetry. Thermochromism and anomalous magnetism were reported previously (S. L. Holt, Jr., et al. J. Am. Chem. Soc. 1964, 86, 519-520); the latter was attributed to equilibration of a singlet ground state and a thermally accessible triplet state, but structural data were not obtained. A crystal structure at 173(2) K revealed [Ni(detu)(4)](2+) centers with distorted planar ligation of nickel(II) to the four sulfur atoms, with an average Ni S bond length of 2.226(3) angstrom. The nickel ion was displaced out-of-plane by 0.334 angstrom toward a proximal apical chloride at a nonbonding distance of 3.134(1) angstrom. Asymmetry in the trans S-Ni-S angles was coupled to a monoclinic <-> tetragonal lattice distortion (T-1/2 = 254 +/- 11 K), resulting in thermochromism. Spin crossover occurs by tetragonal modulation of nickel(II) with approach of the proximal chloride at higher temperatures (T-1/2 = 383 +/- 18 R), which is consistent with a contraction of -0.096(4) angstrom in the Ni center dot center dot center dot Cl separation observed at 293 K. A high-spin (S = 1) square-pyramidal [Ni(dmtu)(4)Cl](+) model (dmtu = N,N'-dimethylthiourea) was optimized by DFT calculations, which estimated limiting equatorial Ni-S bond lengths of 2.45 angstrom and an apical Ni-Cl bond of 2.43 angstrom. Electronic spectra of the spin isomers were calculated by TD-DFT methods. Assignment of the FTIR spectrum was assisted by frequency calculations and isotope substitution.