Nitrosyl–metal bonding relies on the two interactions between the pair of N–O‐π* and two of the metal's d orbitals. These (back)bonds are largely covalent, which makes their allocation in the course of an oxidation‐state determination ambiguous. However, apart from M‐N‐O‐angle or net‐charge considerations, IUPAC′s “ionic approximation” is a useful tool to reliably classify nitrosyl metal complexes in an orbital‐centered approach.

Among the isoelectronic ligands CN − , CO, and NO + , an oblique bonding to the metal is well-established for the nitrosyl ligand, with M−N−O angles down to ≈120°. In the last decades, the nitrosyl community got into the habit of addressing a bent-bonded nitrosyl ligand as 1 NO − . Thus, because various redox forms of a nitrosyl ligand seem to exist, the ligand is considered to be "noninnocent" because of the obvious ambiguity of an oxidation state (OS) assignment of the ligand and metal. Among the bent-bonded species, the low-spin {CoNO} 8 class is prototypic. From this class, some 20 new nitrosyl compounds, the X-ray structure determinations of which comply with strict quality criteria, were analyzed with respect to the OS issue. As a result, the effective OS method shows a low-spin d 8 Co I −NO + couple instead of a negative OS of the ligand at the BP86/def2-TZVP (+D3, +CPCM with infinite permittivity) level of theory. The same holds for some new members of the linear subclass of {CoNO} 8 compounds. For all compounds, a largely invariable "real" charge of ≈ −0.3 e was obtained from population analyses. All of these electron-rich d 8 species strive to manage Pauli repulsion between the metal electrons and the lone pair at the nitrosyl's nitrogen atom, with the bending of the CoNO unit as the most frequent escape.

Pyridinyl amide ion pairs carrying various electron-withdrawing substituents were synthesized with selected ammonium or phosphonium counterions. Compared to neutral pyridine-based organocatalysts, these new ion pair Lewis bases display superior catalytic reactivity in the reaction of isocyanates with alcohols and the aza-Morita−Baylis−Hillman reaction of hindered electrophiles. The high catalytic activity of ion pair catalysts appears to be due to their high Lewis basicities toward neutral electrophiles as quantified through quantum chemically calculated affinity data.

Die Bindung eines Nitrosyl‐Liganden an ein Zentralmetallatom wird durch die Wechselwirkung zwischen den beiden N−O‐π*‐ und zwei Metall‐d‐Orbitalen bestimmt. Diese beiden (Rück‐)Bindungen sind weitgehend kovalent, wodurch deren Zuordnung bei der Oxidationsstufenbestimmung erschwert ist. Dabei erweist sich IUPACs “ionische Näherung” als wirksames Werkzeug, um auf der Grundlage der beteiligten Orbitale eine schlüssige Zuordnung zu erreichen – und zwar ohne den M‐N‐O‐Winkel oder Nettoladungen heranzuziehen.