A series of neutral Ir(III) complexes that possess cyclometalated ligands (C^N) and different ancillary ligands, N-heterocyclic carbene (NHC) (R₁ = H and R₂ = H for complex 1, R₁ = NO₂ and R₂ = H for complex 2, R₁ = N(CH₃)₂ and R₂ = H for complex 3, R₁ = H and R₂ = NO₂ for complex 4, and R₁ = H and R₂ = N(CH₃)₂ for complex 5) and their ionic complexes 1^+/−–5^+/− have been investigated using density functional theory. The effects caused by different electron-withdrawing/donating substituents and electronic valence states on their geometries, electronic structures, UV-vis absorption spectra, and second-order nonlinear optical (NLO) properties were explored. The results show that the geometries of these complexes are slightly affected by the variation of the ancillary ligand and the electronic valence state. A significant impact on absorption spectra and second-order NLO properties is observed during the redox process of the neutral complexes. The absorption spectrum has a remarkable red-shift for ionic complexes 1^+/−–5^+/− compared with their neutral complexes. The obtained β_tot values of complexes 1^+/−–5^+/− range from 7289.2 to 20753.9 a.u., while the value is only 1395.9 a.u. for neutral complex 1. The dramatic enhancement of NLO response is mainly ascribed to the charge redistribution and the variation of transition energy caused by the loss/gain of an electron. Among five neutral complexes, complex 4 presents a high β_tot value, 7019.3 a.u. This illustrates that the introduction of a strong electron-withdrawing group in R₂ of ancillary ligands promotes the second-order NLO response for this type of cyclometalated Ir(III) complex. This work will be beneficial for designing high performance and excellent switchable NLO materials.