The　development　of　nonlinear　optical　materials　with　large　nonlinear　optical　susceptibilities,　wide　transmission　ranges,　and　comprehensive　full-wavelength　phase　matching　capabilities　remains　a　significant　challenge.　In　this　study,　we　synthesized　a　novel　potassium　tetraiodatoiodate(III)　iodate(V)　via　an　in-situ　reduction　and　complexation　reaction.　The　resulting　compound,　K2[IIII(IVO3)4]IVO3,　exhibits　exceptional　second-harmonic　generation　responses　(21.6KH2PO4　at　1064 nm,　1.0KTiOPO4　at　2050 nm),　a　wide　transparency　range　from　the　visible　to　the　mid-infrared　wavelengths　(0.37–6.05　and　7.03–13.40 µm),　and　a　high　birefringence　(0.358　at　543 nm)　to　achieve　full-wavelength　phase　matching.　These　properties　are　primarily　ascribed　to　the　synergistic　substantial　hyperpolarizability　and　polarizability　anisotropy　exhibited　by　the　[IIII(IVO3)4]−　complex　ions,　which　comprise　square-planar　coordinated　trivalent　iodine　and　optimally　arranged　iodate　ligands.　This　study　demonstrates　the　pivotal　role　of　nonmetallic　cation　centered　coordination　entities　in　influencing　linear　and　nonlinear　optical　properties,　a　discovery　that　has　significant　implications　for　the　development　of　innovative　inorganic　functional　materials.　©　2025　Wiley-VCH　GmbH.