Among lanthanide-based single-molecule magnets (SMMs), erbium(III) is a Kramers ion, apart from dysprosium(III), which provides magnetic bistability in the presence of a suitable coordination environment. However, Er-based SMMs exhibit significantly less magnetic anisotropy than Dy because their prolate electronic density necessitates equatorially correlated ligands to minimize the charge contact with the Er atom. Here, in this work, we have computationally investigated the heteroleptic organometallic complexes with an Er(III) atom sandwiched between two distinct cyclic rings (five- and eight-membered) with the aim of tuning the magnetic anisotropy via exploiting the ligand field. The ligand field is manipulated by substituting one of the C atoms from the five-membered ring with heteroatoms (groups 14 and 15), while the other (eight-membered) ring remains intact. The electronic and magnetic properties have been investigated using first-principles-based ab initio approaches. The distortion in the planarity of the five-membered ring generated by the larger heteroatom affects the bonding with magnetic Er and consequently the electronic structure. This is observed to modify the ligand field and the magnetic axis, thereby improving the magnetic relaxation barrier.