Spin-orbit Mott insulators with the $t_{2g}^5$ electron configuration are promising platforms for the Kitaev spin liquid, yet fine-tuning of their crystal structures is essential to suppress non-Kitaev interactions. Here, we investigate the local electronic structures of the ilmenite iridates $A{\mathrm{IrO}}_3$ ($A=\mathrm{Mg},\mathrm{Zn},\mathrm{Cd}$) and the hyperhoneycomb $\beta \text{−}{\mathrm{ZnIrO}}_3$ using Ir $L_3$-edge resonant inelastic x-ray scattering (RIXS). Multiplet analysis of the RIXS spectra reveals a systematic evolution of the crystal field and intraionic interaction parameters upon chemical substitution at the $A$ site. We observe an enhancement of the trigonal distortion with increasing $A$-site ionic radius, providing a microscopic explanation for the deviation from the ideal $J=1/2$ state and the antiferromagnetic interactions identified in ${\mathrm{CdIrO}}_3$. Furthermore, the local multiplet parameters of ilmenite ${\mathrm{ZnIrO}}_3$ and hyperhoneycomb $\beta \text{−}{\mathrm{ZnIrO}}_3$ are found to be nearly identical, demonstrating that their different magnetic ground states are primarily governed by their distinct lattice structures rather than by single-ion properties. These findings establish a solid foundation for understanding how local crystal-field distortions control the magnetic Hamiltonian in Kitaev-candidate materials.

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• Iridates
• Strongly correlated systems
• Transition metal oxides
• Molecular orbital theory
• Resonant inelastic x-ray scattering