The electronic structures of transition metal nitrosyls have long been the subject of lively interest, debate, and controversy. The crux of the problem is that NO, as a paradigmatic noninnocent ligand, does not allow a simple determination of the oxidation state of a metal center it is attached to. According to current chemical nomenclature, oxidation states are defined in terms of the ionic approximation (IA), whereby the two electrons of a heteronuclear bond, are both assigned to the more electronegative side. For NO complexes, the strongly covalent nature of metal(d)-NO(p*) interactions often interferes with the application of the IA.