Spectroscopic, structural and magnetic investigations of iron(II) complexes based on 1-isopropyl- and 1-isobutyl-substituted tetrazole ligands

► We synthesized two iron(II) spin-crossover complexes with isoalkyl-tetrazole ligands. ► We crystallised the homeotypic nickel(II) complexes. ► The isobutyl-tetrazole complex shows a complete but gradual spin transition at 220 K. ► The isopropyl-tetrazole complex has an incomplete spin transition. ► We support the magnetic data with Mößbauer, DSC, UV-Vis-NIR and IR data.