Four‐Coordinate Copper Halonitrosyl {CuNO} 10 Complexes

While copper nitrosyl complexes are implicated in numerous biological systems, isolable examples remain limited. In this report, we show that [Cl 3 CuNO] − , with a {CuNO} 10 electron configuration, can be generated by nitrite reduction at a copper(I) dichloride anion or by nitric oxide addition to a copper(II) trichloride precursor. The bromide analogue, [Br 3 CuNO] − was synthesized analogously, and both copper halonitrosyl complexes were characterized by X‐ray diffraction and a variety of spectroscopic methods. Experimental data and multireference (CASSCF/NEVPT2) calculations provide strong evidence for a Cu II –NO . ground state. Both [Cl 3 CuNO] − and [Br 3 CuNO] − release and recapture NO . reversibly, and exhibit nitrosative reactivities toward a wide range of biological nucleophiles, such as amines, alcohols, and thiols.