A series of isomorphic salts [CN-BzPy][Cu_xNi_1−x(mnt)₂] (x = 0–1; CN-BzPy⁺ = 1-(4′-cyanobenzyl)pyridinium, mnt²⁻ = maleonitriledithiolate) were prepared and characterized by microanalysis (C, H and N elements), IR and Raman spectroscopy, TG, and powder X-ray diffraction techniques. All salts crystallize in the monoclinic space group P2₁/n with similar cell parameters, and both [M(mnt)₂]⁻ (M = Ni or Cu) anions and CN-BzPy⁺ cations form segregated and regular stacks. The thermal stabilities of the salts are enhanced with reducing cell volume in [CN-BzPy][Cu_xNi_1−x(mnt)₂] (x = 0–1). Intriguingly, a spin-Peierls-type transition occurs at T_C ≈ 191 K in the parent salt [CN-BzPy][Ni(mnt)₂]; however, this vanishes in solid solutions even at x = 0.005, indicating that it is quite sensitive to being doped by a nonmagnetic impurity. Both the overlap integral of magnetic orbitals and spin density distribution analyses demonstrate that the antiferromagnetic couplings in a [Ni(mnt)₂]⁻ stack are realized mainly through spatial dipole–dipole type interactions between the NiS₄ cores of adjacent [Ni(mnt)₂]⁻ anions, which are strongly related to the intermolecular Ni⋯Ni, Ni⋯S and S⋯S separations. The functions of the shortest intermolecular Ni⋯Ni, Ni⋯S and S⋯S distances as x changes indicate that a sharp increase occurs in a solid solution even at too low a concentration of nonmagnetic impurity (x = 0.005), suggesting that the spin-Peierls-state collapsing in solid solution is relevant to the sudden increase of intermolecular Ni⋯Ni, Ni⋯S and S⋯S distances between neighboring [Ni(mnt)₂]⁻ anions, leading to a rapid alteration of magnetic dipole–dipole interactions.