Single-ion magnets (SIMs) are the centrepiece of numerous areas of research with promising applications in quantum computing, 1,2 molecular spintronics, 3 classical data storage 4 etc. These are distinct molecules displaying slow relaxation of magnetization that is characterized by an energy barrier (U ef f ) for reversal of total molecular spin. The energy barrier further depends on the total spin of the system and is given by U ef f = S 2 |D| and (S 2 -1/4)|D| (for integer and non-integer spin states respectively), where D is the axial zero field splitting parameter. Complexes based on lanthanides (Ln) have gained much popularity as propitious candidates for SIMs since the report of phthalocyanine Tb(III) complex, TbPc 2 in 2003 owing to huge magnetic anisotropy and large spin ground state. Followed by this, a plethora of Ln-based complexes have been synthesized and characterized with high anisotropic energy barriers. They exhibit large unquenched angular momentum and strong spin-orbit coupling, which are solely responsible for their magnetic behaviors. Transition metal (TM) complexes have also entered the spotlight in recent years, offering tantalizing alternatives to be utilized in prospective applications. In this regard, in mononuclear complexes containing 3d-metal ions, it is essential to maintain the first-order orbital angular momentum to accomplish large magnetic anisotropy on a level comparable with Ln based complexes. Controlling various chemical modifications in the coordination environment, such as the coordination number, geometry of the complex and nature of ligand atoms directly bonded to the metal center, assists in the conservation of the first-order orbital angular momentum. In TM complexes with coordination number greater than 4, the ligand field largely quenches the orbital angular momentum as a result of Jahn Teller distortion. However, the complexes with high axial symmetry, show signs of unquenched angular momentum and hence moderate magnetic anisotropy, but the molecules with high local symmetry are quite scarce. The breaking of symmetry significantly lowers the magnitude of magnetic anisotropy as elegantly reported by Feng et al. where the Fe(III) complex i.e. [(PMe 3 ) 2 FeCl 3 ] with appropriate local symmetry results in D = -50 cm -1 which lowers to -17 cm -1 in [(PMe 2 Ph) 2 FeCl 3 ] complex with the broken symmetry. Prompted by the high D value for the symmetrical Fe(III) TBP complex, we also modelled 14 complexes by varying the axial ligands with Group 15 elements and equatorial ligands with halides on the reported symmetrical complex [(PMe 3 ) 2 FeCl 3 ] and obtained significant D values in the range -40 to -60 cm -1 for 8 complexes. To overcome these ligand field effects, the low-coordinate complexes (coordination number <4) renewed the interest of researchers, since, they favor degenerate ground states resulting in minimal quenching of orbital angular momentum. To this end, linear or quasi-linear two-coordinate complexes emerge as the choicest complexes for mitigating these effects and eventually resulting in large anisotropic energy barriers. The stability and isolation of these low coordinated complexes necessitate sterically encumbered ligands. A copious number of two-and three-coordinate complexes featuring Fe(II) center are already reported with intriguing magnetic properties. Nearly, all these complexes anchorage sterically bulky ligands. However, in Fe(II) based complexes, the slow relaxation of magnetization is observed only in the presence of d.c. field owing to Fe(II) being a non-Kramer ion. Therefore, taking advantage of Kramer's theorem and thus, attenuating quantum tunneling of magnetization (QTM) within ground doublet, Zadrozny et al. reported the first prominent example of two coordinate complex of Fe(I) i.e. [Fe(C(SiMe 3 ) 3 ) 2 ] -with an exceptionally high energy barrier of 226 cm -1 under zero applied field. Later on, Werncke et al. reported another Fe(I) complex, [Fe(N(SiMe 3 ) 2 ) 2 ] -, but with a very small energy barrier of 64 cm -1 which is attributed to the reduced symmetry of the complex. Another Fe(I) complex [Fe(N(SiMe =1,3-diiso-propyl-4,5-dimethylimidazolin-2-ylidene) has been synthesized and characterized with spin-reversal barrier of 64 cm -1 by Chakraborty et al. In recent years, although two-coordinate Fe(I) complexes with sterically bulky ligands have been synthesized, but a thorough study to underpin the origin of magnetic anisotropy in these complexes has not been done yet. The present work aims to gain an in-depth understanding of magnetic anisotropy in a series of linear two-coordinate Fe(I) complexes, employing state-of-the-art ab initio calculations. To this end, we have chosen 5 experimentally reported Fe(I) complexes, namely, [Fe{C(SiMe 3 )} 3 ] -1 (1), [Fe(cyIDep