Tuning of the exchange interaction and the Curie temperature by mixed crystal formation of the bridging anionic ligands.

Mixed crystals with the composition [Co(NCS)(x)(NCSe)(2-x)(pyridine)(2)](n) were prepared from solution and by annealing of Co(NCS)(x)(NCSe)(2-x)(pyridine)(4). With increasing selenocyanate content, an increase of the magnetic exchange constant and of the critical temperature (Curie temperature) is observed, which offers a rational route to control these parameters in detail, without changing the metal cations.