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An S = ³/₂, high-anisotropy building unit, trans-[ReCl₄(CN)₂]²⁻, representing the first paramagnetic complex with a mixture of just cyanide and halide ligands, has been synthesized through the reaction of (Bu₄N)CN with ReCl₄(THF)₂. This species is characterized in detail and employed in directing the formation of a series of one-dimensional coordination solids of formula (DMF)₄MReCl₄(CN)₂ (M = Mn (2), Fe (3), Co (4), Ni (5)). Variable-temperature dc magnetic susceptibility measurements demonstrate the presence of intrachain antiferromagnetic (2) and ferromagnetic (3−5) exchange coupling within these solids. In addition, probing the ac magnetic susceptibility as a function of both temperature and frequency reveals that all of the chain compounds exhibit slow relaxation of the magnetization. The relaxation time is shown to be thermally activated, with energy barriers to relaxation of Δ_τ = 31, 56, 17, and 20 cm⁻¹ …