Magnetic ionic liquids (MILs) show clear superiority over conventional ionic liquids suitably nested in many applications, particularly in medicine and drug delivery engineering. The easy way of collecting them using an external magnet and separating them from the reaction mixture is a favorable and unique technique. A magnetic imidazolium-based IL, including iron coordinated with -NO and chloride ligands ([BMIm][Fe(NO)2Cl2]) (BMIm = 1-n-butyl-3-methyl-imidazolium), has been studied using density functional theory. Such dinitrosyl iron compounds are significant as NO-saving sources and carriers because of their longer physiological lifetime compared with molecular NO. The dependability of the calculations was examined and elucidated at three different methods (M06-2X, B3LYP, and B3LYP-D3) to unravel the importance of noncovalent interactions, including dispersion and H-bonding. Also, the effect of a large basis set on different features of this MIL was assessed. This research is a pioneer in theoretically characterizing the type of -NO moiety of this open-shell dinitrosyl iron compound. The complicated structure of the dinitrosyliron unit was determined by the geometrical parameters, stretching frequencies, and magnetic moment calculation. Based on these fingerprint data, the predominant form of two NO's in this MIL is the nitroxyl anion NO-, rather than ṄO or NO+. Unraveling that one of the NO ligands structurally is dangling enhances the application of this MIL as a NO-saving and source compound. Consequently, Fe3+ is identified as the major oxidation state of the iron atom, leading to the MIL with a strong magnetic moment (of 5.22μB).