The iron(II) complexes of two structural isomers of 2-(1H-imidazol-2-yl)diazine reveal how ligand design can be a successful strategy to control the electronic and magnetic properties of complexes by fine-tuning their ligand field. The two isomers only differ in the position of a single diazinic nitrogen atom, having either a pyrazine (Z) or a pyrimidine (M) moiety. However, [Fe(M)(3)](ClO4)(2) is a spin-crossover complex with a spin transition at 241 K, whereas [Fe(Z)(3)](ClO4)(2) has a stable magnetic behavior between 2 and 300 K. This is corroborated by temperature-dependent Mossbauer spectra showing the presence of a quintet and a singlet state in equilibrium. The temperature-dependent single crystal X-ray diffraction results relate the spin-crossover observed in [Fe(M)(3)](ClO4)(2) to changes in the bond distances and angles of the coordination sphere of iron(II), hinting at a stronger sigma donation of ligand Z in comparison to ligand M. The UV/vis spectra of both complexes are solved by means of the multiconfigurational wave-function-based method CASPT2 and confirm their different spin multiplicities at room temperature, as observed in the Mossbauer spectra. Calculations show larger stabilization of the singlet state in [Fe(Z)(3)](2+) than in [Fe(M)(3)](2+), stemming from the slightly stronger ligand field of the former (506 cm(-1) in the singlet). This relatively weak effect is indeed capable of changing the spin multiplicity of the complexes and causes the appearance of the spin transition in the M complex.