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.

The Front Cover shows, pictorially as the collapse of a house of cards, the repeal of the customary correlation of a nitrosyl ligand's charge and the strength of its N−O bond. Isomeric couples comprising both a linear and a bent MNO moiety show, in fact, a weaker bond of the bent form in terms of a markedly red‐shifted NO stretch. However, the charge of the NO group is not a function of the bond strength. Hence, the assignment of an 1NO− to a bent‐bonded nitrosyl and NO+ to a linear‐bonded one is unjustified as unraveled by local‐mode and QTAIM analysis. More information can be found in the Research Article by J. Popp and P. Klüfers.

A nitrosyl ligand is bonded to a central metal mainly via a mostly covalent normal bond and a coordinative metal‐to‐NO π‐backbond. A recent analysis had unravelled similar bonding characteristics of both linear and bent CoNO moieties in terms of ligand charge and antibond occupation. Thus, there should be no justification for the usual assignment of an NO+ ligand to a linear MNO unit and a singlet‐NO− ligand to a bent one. This claim seems to contradict that bending an MNO unit weakens the N−O bond with a marked red‐shift of the N−O stretch as one indicator. In this work, the failure of Dewar‐Chatt‐Duncanson‐derived conclusions is demonstrated for linear/bent isomer couples by the analysis of M−N and N−O bond strengths. Instead of DCD behavior, lateral electrostatic influence on NO and other diatomic ligands modulates the intraligand bond strength in a similar way as has been shown in former work for polar interaction of a charge with CO in the ‘non‐classical’ carbonyls. Methodologically, local‐mode analysis is used to determine bond strengths. Oxidation states are determined by the effective‐oxidation‐state (EOS) method.

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.