For a molecule to possess uniaxial anisotropy with slow relaxation of magnetization under no applied field, apart from negative axial ZFS (D), the negligibly small rhombicity i.e., |E/D| ratio is required. The non-zero value of E allows mixing of ±M s levels, leading to relaxation of electron through quantum tunneling. The SA-CASSCF/NEVPT2 computed |E/D| ratio of all the complexes are collected in Table . Complexes 1 and 4 exhibit negligibly small rhombicity i.e., |E/D| <0.005 (Table ) indicating infinitesimal transition probability of relaxation through quantum tunneling of magnetization (QTM) within the ground-state Kramers doublet. Complexes 2 and 5 possess small |E/D| ∼0.04, implying partial relaxation through quantum tunneling of magnetization within the ground-state Kramers doublet. However, for complex 3, large |E/D| is observed indicating relaxation of magnetization through the ground-state Kramers doublet.