The Enemark-Feltham formalism at 50: An interview with John Enemark and a personal reflection

In an interview, one of us (JHE) recounts the state of NO chemistry around the middle of the twentieth century and the events that led to the development of Enemark and Feltham's {MNO}ⁿ notation. A personal perspective by one of us (AG) underscores the continued role of the notation as a source of electro-structural correlations. Interestingly, although recent ab initio calculations have on occasion resulted in somewhat different NO oxidation states relative to earlier, classic studies, the Enemark-Feltham notation remains as relevant as ever. Thus, for iron nitrosyls, there appears to be a one-to-one mapping between the Enemark-Feltham count and the NO oxidation state. Whether an analogous mapping exists for other transition metals has not been resolved at this point.