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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 Mössbauer 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 σ 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 Mössbauer 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.

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The first coordination sphere influences the reactivity of metallo-β-lactamase monozinc model complexes.