A series of mononuclear square-based pyramidal complexes of iron containing two 1,2-diaryl-ethylene-1,2-dithiolate ligands in various oxidation levels has been synthesized. The reaction of the dinuclear species [Fe-2(III)(L-1 center dot)(2)(L-1)(2)](0), where (L-1)(2-) is the closed shell di-(4-tert-butylphenyl)-1,2-ethylenedithiolate dianion and (L-1 center dot)(1-) is its one-electron-oxidized pi-radical monoanion, with [N(n-Bu)(4)] CN in toluene yields dark green crystals of mononuclear [N(n-Bu)(4)][ Fe-II(L-1 center dot)(2)(CN)] (1). The oxidation of 1 with ferrocenium hexafluorophosphate yields blue [Fe-III(L-1 center dot)(2)(CN)] ((1)ox), and analogously, a reduction with [Cp2Co] yields [Cp2Co][N(n-Bu)(4)][Fe-II(L-1 center dot)(L-1)(CN)] ((1)red); oxidation of the neutral dimer with iodine gives [Fe-III(L-1 center dot)(2)I] (2). The dimer reacts with the phosphite P(OCH3)(3) to yield [Fe-II(L-1 center dot)(2){P(OCH3)(3)}] (3), and [Fe-2(III)(L-3)(2)(L-3)(2)] reacts with P(OC6H5)(3) to give [Fe-II(L-3 center dot)(2) {P(OC6H5)(3)}] (4), where (L-3)(2-) represents 1,2-diphenyl-1,2- ethylenedithiolate(2-). Both 3 and 4 were electrochemically one-electron oxidized to the monocations 3(ox) and 4(ox) and reduced to the monoanions 3(red) and 4(red). The structures of 1 and 4 have been determined by X-ray crystallography. All compounds have been studied by magnetic susceptibility measurements, X-band EPR, UV-vis, IR, and Mossbauer spectroscopies. The following five-coordinate chromophores have been identified: (a) [Fe-III(L center dot)(2)X](n), X = CN-, I- (n = 0) (1(ox), 2); X = P(OR)(3) (n)1+)) 3(ox), 4(ox)) with S-t = 1/2, S-Fe = 3/2; (b) [Fe-II(L center dot)(2)X](n), X = CN-, (n = 1-) (1); X = P(OR)(3) (n = 0) (3, 4) with S-t = S-Fe = 0; (c) [Fe-II(L center dot)(L)X](n) T <-> [Fe-II(L)(L center dot)X](n), X = CN- (n = 2-) (1red); X = P(OR)(3) (n = 1-) (3(red), 4(red)) with S-t = 1/2, S-Fe = 0 (or 1). Complex 1(ox) displays spin crossover behavior: St = 1/2 <-> S-t = 3/2 with intrinsic spin-state change S-Fe = 3/2 <-> S-Fe = 5/2. The electronic structures of 1 and 1(ox) have been established by density functional theoretical calculations: [Fe-II(L-1 center dot)(2)(CN)](1-) (S-Fe = 0, S-t = 0) and [Fe-III (L-1 center dot)(2)(CN)](0) (S-Fe) 3/2, S-t = 1/2).