Metal nitrosyl
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Metal nitrosyls are compounds derived from the transition metals that contain NO as a ligand. Metal carbonyls are more common but are related electronically.[1]
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[edit] 18-electron rule perspective
In the language of the 18-electron rule, NO is a three-electron ligand when the M-N-O angle is close to 180. This bonding mode is more common, but some nitrosyl complexes have M-N-O is strongly bent, approaching often 130. Such species are related to classical organic and main group nitrosyl compounds, e.g. nitrosobenzene and nitrosyl chloride. In these cases, the NO is considered to be a one-electron ligand. Because of the two different bonding modes, metal nitrosyl compounds confuse many students. For transition metals, if in doubt, one can usually assume that the nitrosyl is linear.
[edit] Examples
Two of the earliest nitrosyl complexes to be reported were "Roussin's Red and Black Salts". The red salt has the formula Na2Fe2(NO)4. The anion, Fe2 S2(NO)4]2-, can be viewed as an edge-shared bitetrahedron. Each Fe is surrounded by two (linear) NO ligands and shares a pair of sulfido ligands with the other iron. The Black salt is more complex cluster with the formula NaFe3S4(NO)7, with C3v point group symmetry. This molecule adopts an incomplete cubane structure.
Simpler nitrosyl complexes include Co(NO)(CO)3 and Mn(NO)(CO)4.
A medicinally important nitrosyl is the nitroprusside anion, [Fe(CN)5NO]2-.
[edit] Preparation
Most metal nitrosyls are prepared by treatment of a metal with NO gas. Others have been prepared by oxidation of hydroxylamine complexes. A third method is by treatment of metal nitrito complexes with protic acids.
[edit] Reactions
Since nitrogen is more electronegative than carbon, metal-nitrosyl complexes tend to be more electrophilic than related metal carbonyl complexes. Nucleophiles often add to the nitrogen.[2]
The NO ligand also exhibits many reactions. The most important is the acid/base equilibrium:
- [LnMNO]2+ + OH-
LnMNO2 + H+
This reaction illustrates the fact that linear nitrosyls can be viewed a an acid anhydride.
In rare but noteworthy cases, NO is cleaved by metal centers:
- Cp2NbMe2 + NO → Cp2(Me)Nb(O)NMe
- 2Cp2(Me)Nb(O)NMe → 2 Cp2Nb(O)Me + ½MeN=NMe
The nitrogen atom in bent metal nitrosyls is basic, thus can be oxidized, alkylated, and protonated, e.g.:
- (Ph3P)2(CO)ClOsNO + HCl → (Ph3P)2(CO)ClOsN(H)O
[edit] References
- ^ Hayton, T. W.; Legzdins, P.; Sharp, W. B. "Coordination and Organometallic Chemistry of Metal-NO Complexes" Chemical Reviews 2002, volume 102, pages 935-991; Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford University Press: New York, 1992
- ^ Richter-Addo, G. B. and Legzdins, P., Metal Nitrosyls, Oxford University Press: New York, 1992
