Metal nitrosyl complexes arecomplexes that containnitric oxide, NO, bonded to atransition metal.[2] Many kinds of nitrosyl complexes are known, which vary both in structure and coligand.
Most complexes containing the NO ligand can be viewed as derivatives of the nitrosyl cation, NO+. The nitrosyl cation isisoelectronic withcarbon monoxide, thus the bonding between a nitrosyl ligand and a metal follows the same principles as the bonding incarbonyl complexes. The nitrosyl cation serves as a two-electron donor to the metal and accepts electrons from the metal viaback-bonding. The compoundsCo(NO)(CO)3 andNi(CO)4 illustrate the analogy between NO+ and CO. In an electron-counting sense, two linear NO ligands are equivalent to three CO groups. This trend is illustrated by the isoelectronic pair Fe(CO)2(NO)2 and [Ni(CO)4].[3] These complexes areisoelectronic and, incidentally, both obey the18-electron rule. The formal description of nitric oxide as NO+ does not match certain measureable and calculated properties. In an alternative description, nitric oxide serves as a 3-electron donor, and the metal-nitrogen interaction is atriple bond.
The M-N-O unit in nitrosyl complexes is usually linear, or no more than 15° from linear. In some complexes, however, especially when back-bonding is less important, the M-N-O angle can strongly deviate from 180°. Linear and bent NO ligands can be distinguished usinginfrared spectroscopy. Linear M-N-O groups absorb in the range 1650–1900 cm−1, whereas bent nitrosyls absorb in the range 1525–1690 cm−1. The differing vibrational frequencies reflect the differing N-Obond orders for linear (triple bond) and bent NO (double bond).
The bent NO ligand is sometimes described as the anion, NO−. Prototypes for such compounds are the organic nitroso compounds, such asnitrosobenzene. A complex with a bent NO ligand istrans-[Co(en)2(NO)Cl]+. The NO− is also common for alkali-metal or alkaline-earth metal-NO molecules. For example. LiNO and BeNO bear Li+NO− and Be+NO− ionic form.[4][5]
The adoption of linear vs bent bonding can be analyzed with theEnemark-Feltham notation.[6] In their framework, the factor that determines the bent vs linear NO ligands is the electron count in the metal-N-Oπ system. Complexes more than 6 electrons in the system tend to have bent geometries at N. Thus, [Co(en)2(NO)Cl]+, with eight electrons of pi-symmetry (six in t2g orbitals and two on NO, {CoNO}8), adopts a bent NO ligand, whereas [Fe(CN)5(NO)]2−, with six electrons of pi-symmetry, {FeNO}6), adopts a linear nitrosyl. In a further illustration, consider the {MNO} d-electron count of the [Cr(CN)5NO]3− anion. In this example, the cyanide ligands are "innocent", i.e., they have a charge of −1 each, −5 total. To balance the fragment's overall charge, the charge on {CrNO} is thus +2 (−3 = −5 + 2). Using the neutralelectron counting scheme, Cr has 6 d electrons and NO· has one electron for a total of 7. Two electrons are subtracted to take into account that fragment's overall charge of +2, to give 5. Written in the Enemark-Feltham notation, the d electron count is {CrNO}5, and the nitrosyl is linear. The results are the same if the nitrosyl ligand were considered NO+ or NO−.[6]
Nitric oxide can also serve as abridging ligand. In the compound [Mn3(η5C5H5)3 (μ2-NO)3 (μ3-NO)], three NO groups bridge two metal centres and one NO group bridge to all three.[3]
(ON)(pyridine)4%252B_(9RUKQOE02).png)
Usually only of transient existence, complexes ofisonitrosyl ligands are known where the NO is coordinated by its oxygen atom. They can be generated by UV-irradiation of nitrosyl complexes.[7]
Metal complexes containing only nitrosyl ligands are called isoleptic nitrosyls. They are rare, the premier member being Cr(NO)4.[8] Even trinitrosyl complexes are uncommon, whereas polycarbonyl complexes are routine.
One of the earliest examples of a nitrosyl complex to be synthesized isRoussin's red salt, which is a sodium salt of the anion [Fe2(NO)4S2]2−. The structure of the anion can be viewed as consisting of twotetrahedra sharing an edge. Each iron atom is bonded linearly to two NO+ ligands and shares two bridging sulfidi ligands with the other iron atom.Roussin's black salt has a more complexcluster structure. The anion in this species has the formula [Fe4(NO)7S3]−. It hasC3vsymmetry. It consists of a tetrahedron of iron atoms with sulfide ions on three faces of the tetrahedron. Three iron atoms are bonded to two nitrosyl groups. The iron atom on the threefoldsymmetry axis has a single nitrosyl group which also lies on that axis.
2NO-3D-balls.png)
Many nitrosyl complexes are quite stable, thus many methods can be used for their synthesis.[9]
Nitrosyl complexes are traditionally prepared by treating metal complexes with nitric oxide. The method is mainly used with reduced precursors. Illustrative is the nitrosylation ofcobalt carbonyl to givecobalt tricarbonyl nitrosyl:[10]
Alternatively, the cobalt may be reducedin situ:
Replacement of ligands by the nitrosyl cation may be accomplished usingnitrosyl tetrafluoroborate. This reagent has been applied to the hexacarbonyls of molybdenum and tungsten:[12][13]
Nitrosyl chloride and molybdenum hexacarbonyl react to give [Mo(NO)2Cl2]n.[14]Diazald is also used as an NO source.[15]
Simple nitrite salts also oxidizemetal carbonyls to the corresponding nitrosyl, i.e.:[16]
Hydroxylamine is a source of nitric oxide anion via a disproportionation:[17]
Nitric acid is a source of nitric oxide complexes, although the details are obscure. Probably relevant is the conventional self-dehydration of nitric acid:
Nitric acid is used in some preparations ofnitroprusside fromferrocyanide:
Some anionic nitrito complexes undergo acid-induced deoxygenation to give the linear nitrosyl complex.
The reaction is reversible in some cases.
In somemetal-ammine complexes, the ammonia ligand can be oxidized to nitrosyl:[18]
An important reaction is the acid/base equilibrium, yieldingtransition metal nitrite complexes:
This equilibrium serves to confirm that the linear nitrosyl ligand is, formally, NO+, with nitrogen in the oxidation state +3
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 nitrogen atom in bent metal nitrosyls is basic, thus can be oxidized, alkylated, and protonated, e.g.:
In rare cases, NO is cleaved by metal centers:
Metal-nitrosyls are assumed to be intermediates incatalytic converters, which reduce the emission ofNOx from internal combustion engines. This application has been described as "one of the most successful stories in the development of catalysts".[20]
Metal-catalyzed reactions of NO are not often useful inorganic chemistry. In biology and medicine, nitric oxide is however an important signalling molecule in nature and this fact is the basis of the most important applications of metal nitrosyls. Thenitroprusside anion, [Fe(CN)5NO]2−, a mixed nitrosyl cyano complex, has pharmaceutical applications as a slow release agent for NO. Thesignalling function of NO is effected via its complexation tohaem proteins, where it binds in thebent geometry. Nitric oxide also attacksiron-sulfur proteins givingdinitrosyl iron complexes.
Several complexes are known with NS ligands. Like nitrosyls, thionitrosyls exist as both linear and bent geometries.[22]