Complex 1 was fully characterized by solid-state IR spectroscopy ranging from 400 K to 200 K (Figures 2, left and S3 -S5). IR spectroscopy has been applied as an important technique to characterize the oxidation state and spin-state of metal centers, particularly for those metalcoordinated ligands containing N≡C and C=O units. The stretching frequency of these units is affected by the electronic and spin-state of the metal center. IR spectrum of 1 at room temperature shows a strong and a very weak absorption peak at around 1650 and 1634 cm -1 respectively, suggesting the characteristic stretching vibration of (C=O) of metal coordinated bik* ligand (Figure ). Iron(II) complexes containing bik* ligand are less in number, while most of them belong in LS state at room temperature, showing the (C=O) at around 1635 cm -1 . In complex 1, the strong bands at about 1650 cm -1 suggests the presence of HS state of the complex while the latter bands at around 1635 cm -1 are consistent with LS state of iron(II) center at room temperature. Overall, these peaks suggest the presence of an almost HS state of 1 at room temperature with partial LS contribution. Further, the IR spectrum of 1 at room temperature shows strong bands at 730 and 705 cm -1 , suggesting the presence of BPh4 -anion (Figure ).