There are less than two dozen structurally characterized transition-metal hyponitrite complexes reported in the literature. Yet, despite this small sample size, the hyponitrite ligand has been shown to exhibit a rich variety of binding modes.
Herein, we review the preparation and coordination chemistry of cis and trans isomers of hyponitrite, [N2O2]2–. Hyponitrite is known to bind to metals via a variety of bonding modes. In fact, at least eight different bonding modes have been observed, which is remarkable for such a simple ligand. More importantly, it is apparent that the cis isomer of hyponitrite is more reactive than the trans isomer because the barrier of N2O elimination from cis-hyponitrite is lower than that of trans-hyponitrite. This observation may have important mechanistic implications for both heterogeneous NOx reduction catalysts and NO reductase. However, our understanding of the hyponitrite ligand has been limited by the lack of a general route to this fragment, and most instances of its formation have been serendipitous.