Tetraphenylphosphonium salts of Co and Fe complexes with alkyl-substituted, tert-butyl (tb), and isopropyl (dp) 2,3-thiophenedithiolate (α-tpdt) ligands, namely, TPP[Co(α-tb-tpdt)2] (3), TPP2[Fe(α-tb-tpdt)2]2 (4a-b), TPP[Co(α-dp-tpdt)2] (5), and TPP[Fe(α-dp-tpdt)2] (6) were prepared and characterized by cyclic voltammetry, single crystal X-ray diffraction, magnetic susceptibility measurements, and 57Fe Mössbauer spectroscopy. Compound 3 and 5 are isostructural with their Au and Ni analogues with a square-planar coordination geometry. Compound 4 presents two polymorphs (4a-b) both showing a Fe(III) bisdithiolene dimerization. The magnetic susceptibility of 3 and 5 exhibits behavior dominated by antiferromagnetic interactions, with room-temperature magnetic moments of 3.40 and 3.36 μB, respectively, indicating that these square-planar Co(III) complexes assume an intermediate spin electronic configuration (S = 1) as supported by multiconfigurational and DFT calculations.