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Abstract Nitrous acid (HONO) plays pivotal roles in various metal‐free as well as metal‐mediated routes relevant to biogeochemistry, atmospheric chemistry, and mammalian physiology. While the metastable nature of HONO hinders the detailed investigations into its reactivity at a transition metal site, this report herein utilizes a heteroditopic copper(II) cryptate [oC]Cu^IIfeaturing a proton‐responsive second‐coordination‐sphere located at a suitable distance from a [Cu^II](ONO) core, thereby enabling isolation of a [Cu^II](κ¹‐ONO⋅⋅⋅H⁺) complex (2H‐NO₂). A set of complementary analytical studies (UV‐vis,¹⁴N/¹⁵N FTIR,¹⁵N NMR, HRMS, EPR, and CHN) on2H‐NO₂and its¹⁵N‐isotopomer (2H‐¹⁵NO₂) reveals the formulation of2H‐NO₂as {[oCH]Cu^II(κ¹‐ONO)}(ClO₄)₂. Non‐covalent interaction index (NCI) based on reduced density gradient (RDG) analysis on {[oCH]Cu^II(κ¹‐ONO)}²⁺discloses a H‐bonding interaction between the apical 3° ammonium site and the nitrite anion bound to the copper(II) site. The FTIR spectra of [Cu^II](κ¹‐ONO⋅⋅⋅H⁺) species (2H‐NO₂) shows a shift of ammonium NH vibrational feature to a lower wavenumber due to the H‐bonding interaction with nitrite. The reactivity profile of [Cu^II](κ¹‐ONO⋅⋅⋅H⁺) species (2H‐NO₂) towards anaerobic nitration of substituted phenol (2,4‐DTBP) is distinctly different relative to that of the closely related tripodal [Cu^II]‐nitrite complexes (1‐NO₂/3‐NO₂/4‐NO₂).