An unprecedented redox-neutral S-nitrosation of thiol has been achieved at dicopper(I,I) center. Treatment of dicopper (I,I) complex with excess NO• and thiol generates a dicopper (I,I) di-S-nitrosothiol complex [CuICuI(RSNO)2]2+ or dicopper (I,I) mono-S-nitrosothiol complex [CuICuI(RSNO)]2+, which readily release RSNO in 88-94% yield. The S-nitrosation reaction proceeds through a mixed-valence [CuIICuIII(m-O)(m-NO)]2+ species, which deprotonates RS-H at the basic m-O site and nitrosates the

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... at the m-NO site. The [CuIICuIII(m-O)(m-NO)]2+ complex is also competent for O-nitrosation of MeOH, which is isoelectronic to thiol. In this case, a rare [CuIICuII(m-NO)(OMe)]2+ intermediate has been isolated and fully characterized, suggesting the S-nitrosation proceeds through the intermediary of analogous [CuIICuII(m-NO)(SR)]2+ species. The redox- and proton-neutral S-nitrosation process reported here represents the first functional model of ceruloplasmin in mediating S-nitrosation of external thiols, adding further implications for biological copper sites in the interconversion of NO•/RSNO.