The complex of formula [Co(dmvi)(phen)(2)]ClO4.3H(2)O (dmvi = C6H6N3O4, dimethylviolurate anion, and phen = C12H8N2, 1,10-phenanthroline) has been synthesized, and its structure has been determined by X-ray crystallographic methods at two different temperatures, 294 and 92 K. Crystal data : triclinic system, space group PI, with a = 11.613(5) Angstrom, b = 12.390(4) Angstrom, c = 13.041(6) Angstrom, alpha = 71.61(3)degrees, beta = 83.67(3)degrees, gamma = 79.09(3)degrees, and Z = 2 at 294 K and a = 11.29(1) Angstrom, b = 12.26(1) Angstrom, c = 13.08(2) Angstrom, alpha = 72.26(9)degrees, beta = 80.63(8)degrees, gamma = 77.99(8)degrees, and Z = 2 at 92 K. The structure was solved by direct methods. The perchlorate anion and the water molecules are disordered, and the latter could be located only in the low temperature structure. The structures were refined by the full-matrix least-squares method. The refinements, including 3374 (294 K) and 2244 (92 K) reflections converged at R = 0.083 and 0.107, respectively. The structure consists of [Co(dmvi)(phen)2](+) cations well separated from the ClO4- counterions and water of hydration. The metal ion exhibits a N5O distorted-octahedral surrounding at room temperature. One phenanthroline ligand occupies two equatorial positions, With identical bond lengths to Co(II) of 2.00 Angstrom, whereas the other is coordinated in a very asymmetric way, with one short equatorial (1.99 Angstrom) and one long axial (2.11 Angstrom) bonds. The dmvi anion also acts as a very asymmetrical chelating ligand, Co-N and Co-O bond lengths being 1.93 and 2.16 Angstrom, respectively. The coordination sphere of the metal ion is very similar in the low temperature structure, the bond lengths being somewhat shorter. Average Co-N(equatorial) and Co-X(axial) (X = O, N) distances are 1.977 and 2.137 Angstrom in the room temperature structure, while the corresponding values in the low temperature structure are 1.94 and 2.12 Angstrom. Variable-temperature magnetic susceptibility measurements and EPR spectra, reveal that the complex exhibits a smooth and incomplete spin transition between S = 3/2 and S = 1/2 states which are separated by ca. 150 cm(-1).