Since for non-integral spin complexes, U ef f corresponds to |D|(S 2 -1/4), the D value of -109.13 cm -1 leads to U ef f of 218.26 cm -1 . The value is in nice agreement with the experimentally observed value of 226 cm -1 . Moreover, the D value of -108.55 cm -1 is obtained from MS-CASPT2 calculations, in nice agreement with that obtained from NEVPT2 For all the studied complexes, the SA-CASSCF/NEVPT2 and MS-CASPT2 computed D values are collected in Table . For complexes 2, 4 and 5, negative D value is obtained from NEVPT2 with moderate magnitude in the range -33 to -80 cm -1 . However, a positive D value of 40.30 cm -1 is observed for complex 3. CASPT2 also results in negative D values for complexes 2, 4 and 5 with comparable magnitude as obtained from NEVPT2 calculations. However, a discrepancy is observed between the computed D value for complex 3 for which NEVPT2 yields +45.34 cm -1 in contrast to the CASPT2 value of -25.84 cm -1 , although +34 cm -1 is reported by DDCI3 calculations by Meng et al. The value reported by CASPT2 in contrast to NEVPT2 and DDCI3 may be attributed to the issue of intruder states. Owing to large D value, complex 4 proffers itself as the second Fe(I) centred complex with large ZFS among other linear Fe(I) complexes reported so far with relatively low spin-reversal barrier.