THE COORDINATION CHEMISTRY OF HNO: FROM WARREN ROPER TO HEMOGLOBIN

The coordination chemistry of the one-electron reduced form of nitric oxide, termed a nitroxyl or nitrosyl hydride (NO- or HNO), is described anecdotally through the author's discovery of a stable HNO adduct of myoglobin, HNO-Mb, as an illustration of how different areas of chemistry, such as organometallic and bioinorganic, overlap and influence one another in unexpected ways. HNO is short-lived in solution, and many fundamental characteristics of HNO, such as the widely cited values of its oxidation potential and pKa, have been significantly revised through collaborations of theoreticians, biologists, and physical chemists. The very first HNO metal complex was discovered in Warren Ropers lab in 1970, with some 27 years before the next fully characterized example was reported by Hillhouse. Hillhouse in turn helped guide our characterization of HNO-Mb. As with oxymyoglobin, there are several possible descriptions of bonding for HNO-adducts, we present an analogy to the π-bonding interactions of a Fischer carbene. Roper's initial work demonstrated the importance of redox and protonation equilibria in these species, a current focus of ongoing work.