This paper examines the influence of transition metal cations with empty nd0 cores on the Eu3+ asymmetry ratio R=I(5D0−7F2)I(5D0−7F1) and the magnitude of crystal field splitting of the 7F1 manifold (ΔΕ; 7F1). Two families of compounds that crystallize with the pyrochlore and zircon structures are considered. The pyrochlores under consideration have the general formula La2M2O7 (M = Hf, Zr, Sn). Comparison of the Eu3+ optical properties indicate that R increases in order La2Sn2O7 < La2M2O7 (M = Hf, Zr) while (ΔΕ; 7F1) varies as La2M2O7 (M = Hf, Zr) < La2Sn2O7. The strength of M-O covalent bonding is responsible for these trends. The M-cations in La2M2O7 (M = Hf, Zr) have empty nd0 orbitals that participate strongly in covalent bonding with oxide anions, while the covalent contribution to the Sn4+-O2- bonding is weak due to filled Sn4+ 4d10 core. Mulliken charge calculations of the cations and anions in isostructural pyrochlore compounds are interpreted to indicate the presence of a stronger low-symmetry crystal field component that increases the asymmetry ratio in compounds with cations having empty ndo cores. This stronger covalent bonding is also responsible for decreasing (ΔΕ; 7F1). The second family of compounds under consideration crystallize in the zircon structure with the general formula YZO4 (Z = P, V). Comparison of Mulliken charges of cations and anions in these compounds indicates strong covalent bonding in the vanadate because it contains V5+ transition metal cation with empty 3d0 orbital. It leads to R(YPO4) < R(YVO4) and [YVO4: (ΔΕ; 7F1)] < [YPO4: (ΔΕ; 7F1)]. It is concluded that in isostructural family of compounds, the covalent bonding within the crystal framework determines R and (ΔΕ; 7F1).