Linkage isomers of reduced metal-nitrosyl complexes serve as key species in nitric oxide (NO) reduction at monometallic sites to produce nitrous oxide (N 2 O), a potent greenhouse gas. While factors leading to extremely rare side-on nitrosyls are unclear, we describe a pair of nickel-nitrosyl linkage isomers through controlled tuning of noncovalent interactions between the nitrosyl ligands and differently encapsulated potassium cations. Furthermore, these reduced metal-nitrosyl species with N-centered spin density undergo radical coupling with free NO and provide a N-N coupled cis-hyponitrite intermediate whose protonation triggers the release of N 2 O. This report outlines a stepwise molecular mechanism of NO reduction to form N 2 O at a mononuclear metal site that provides insight into the related biological reduction of NO to N 2 O.Nitrous oxide (N 2 O) is a long-lived (ca. 114 years) greenhouse gas with a global warming potential 298 times that of CO 2 on a molecular basis. 1 Enhanced through feeding of crops with nitrogen-rich fertilizers, 2 global emission of N 2 O is mainly attributed to the microbial and fungal denitrification processes mediated by metalloenzymes. 3 The most critical step for N 2 O generation is N-N bond formation that occurs via the reductive coupling of two nitric *

A series of β-diketiminate Ni–NO complexes with a range of NO binding modes and oxidation states were studied by X-ray emission spectroscopy (XES). The results demonstrate that XES can directly probe and distinguish end-on vs side-on NO coordination modes as well as one-electron NO reduction. Density functional theory (DFT) calculations show that the transition from the NO 2s2s σ* orbital has higher intensity for end-on NO coordination than for side-on NO coordination, whereas the 2s2s σ orbital has lower intensity. XES calculations in which the Ni–N–O bond angle was fixed over the range from 80° to 176° suggest that differences in NO coordination angles of ∼10° could be experimentally distinguished. Calculations of Cu nitrite reductase (NiR) demonstrate the utility of XES for characterizing NO intermediates in metalloenzymes. This work shows the capability of XES to distinguish NO coordination modes and oxidation states at Ni and highlights applications in quantifying small molecule activation in enzymes.

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