Phase-pure solid solutions of [Co(terpy)(2)][BF(4)](2) and [M(bpp)(2)][BF(4)](2) (M = Fe or Ru, bpp = 2,6-dipyrazol-1-ylpyridine) have been prepared. The spin-state of the [Co(terpy)(2)](2+) dopant is modulated by the spin-state of the [Fe(bpp)(2)](2+) centres in the iron-containing materials.