metalnitrosyls

Metal nitrosyls are coordination compounds in which nitric oxide (NO) is bound to a metal center. They arise in inorganic and organometallic chemistry as NO can act as an electron donor, a redox mediator, and a ligand with flexible bonding modes, including terminal and bridging arrangements.

Binding modes include terminal nitrosyls, where NO attaches end-on to a single metal, often with a linear M–N–O arrangement, and bent nitrosyls with M–N–O angles well below 180°. Bridging nitrosyls link two or more metals. The M–N–O geometry correlates with NO character: linear nitrosyls tend to have NO+ character and higher N–O stretching frequencies (roughly 1800–1900 cm−1), while bent nitrosyls show more NO−/neutral character and lower frequencies (roughly 1600–1700 cm−1).

Electron counting is commonly described by the Enemark–Feltham notation {MNO}^n, which sums the metal d electrons and the electrons in the NO fragment. This framework groups redox-ambiguous nitrosyls independent of formal oxidation state and helps predict geometry and reactivity.

Synthesis and occurrence: metal nitrosyls are prepared by direct nitrosylation of metal precursors with NO or NO donors, and by transnitrosylation or redox chemistry. They are known for many transition metals, including iron, cobalt, nickel, and copper, in both discrete complexes and clusters. They also serve as models for NO interactions in biology and as NO storage or delivery reagents.

Characterization and applications: infrared spectroscopy is a primary tool, with the N–O stretch serving as a diagnostic. Metal nitrosyls display rich redox chemistry, photochemical behavior, and insights into metal–ligand bonding, with relevance to catalysis and NO-handling in chemistry and biology.