We report the synthesis, structure, and physical properties of (BEDT-TTF)[Ni(tdas)(2)] [BEDT-TTF, or ET, is bis-(ethylenedithio)tetrathiafulvalene; tdas is 1,2,5-thiadiazole-3,4-dithiolate], which is the first example of a salt containing monomeric [Ni(tdas)(2)](-) monoanions. This salt, which crystallizes in the monoclinic space group P2(1)/c with a = 17.2324(6) Angstrom, b = 13.2740(5) Angstrom, c = 10.9467(4) Angstrom, beta = 96.974(2)degrees, and V = 2485.5(2) Angstrom(3), forms a layered structure. One layer contains dimerized BEDT-TTF electron donor molecules and isolated [Ni(tdas)(2)](-) monoanions, while the second layer contains chains of [Ni(tdas)(2)](-) monoanions. Conductivity measurements show that (BEDT-TTF)[Ni(tdas)(2)] has a semiconductor-to-semiconductor transition near 200 K, while magnetic measurements indicate that it is an S = 1/2 paramagnet with weak antiferromagnetic coupling. Reflectance spectra reveal bands in the near-infrared region (6.6 x 10(3) and 10.6 x 10(3) cm(-1)) which are typical of (BEDT-TTF)(2)(2+) dimers. From these data, we can conclude that the unpaired electron lies on the [Ni(tdas)(2)](-) anions. Tight-binding band structure calculations were used to analyze the electronic structure of this salt.