Metal nitrosyl complex

Not to be confused with Nitro compound.

Sodium nitroprusside, a medicinally significant metal nitrosyl-pentacyanoferrate (Fe-III) compound, used to treat hypertension.

Metal nitrosyl complexes are complexes that contain nitric oxide, NO, bonded to a transition metal. ^[2] Many kinds of nitrosyl complexes are known, which vary both in structure and coligand.

Bonding and structure

(Top) the HOMO and LUMO of CO. (Middle) Sigma bond. (Bottom) Back-bond.

Most complexes containing the NO ligand can be viewed as derivatives of the nitrosyl cation, NO⁺. The nitrosyl cation is isoelectronic with carbon monoxide, thus the bonding between a nitrosyl ligand and a metal follows the same principles as the bonding in carbonyl complexes. The nitrosyl cation serves as a two-electron donor to the metal and accepts electrons from the metal via back-bonding. The compounds Co(NO)(CO)₃ and Ni(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 are isoelectronic and, incidentally, both obey the 18-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 a triple bond.

linear and bent M-NO bonds

Linear vs bent nitrosyl ligands

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 using infrared 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-O bond 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 as nitrosobenzene. A complex with a bent NO ligand is trans-[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 the Enemark-Feltham notation. ^[6] In their framework, the factor that determines the bent vs linear NO ligands is the electron count in the metal-N-O π 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 t_2g 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 neutral electron 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]

Bridging nitrosyl ligands

Nitric oxide can also serve as a bridging 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]

Isonitrosyl ligands

Structure of the isonitrosyl complex [Ru(Cl)(ON)(pyridine)₄] (color code: red (O), blue (N), gray (C), dark green (Ru), green (Cl)).

Usually only of transient existence, complexes of isonitrosyl ligands are known where the NO is coordinated by its oxygen atom. They can be generated by UV-irradiation of nitrosyl complexes. ^[7]

Representative classes of compounds

Homoleptic nitrosyl complexes

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.

Roussin red and black salts

One of the earliest examples of a nitrosyl complex to be synthesized is Roussin'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 two tetrahedra 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 complex cluster structure. The anion in this species has the formula [Fe₄(NO)₇S₃]⁻. It has C_3v symmetry. 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 threefold symmetry axis has a single nitrosyl group which also lies on that axis.

• 
  The anion in Roussin's red salt, [Fe₂S₂(NO)₄]²⁻.
• 
  The anion in Roussin's black salt, [Fe₄S₃(NO)₇]⁻.
• 
  The nitroprusside anion, [Fe(CN)₅NO]²⁻, an octahedral complex containing a "linear NO" ligand.
• 
  trans-[Co(en)₂(NO)Cl]⁺, an octahedral complex containing a "bent NO" ligand.

Preparation

Many nitrosyl complexes are quite stable, thus many methods can be used for their synthesis. ^[9]

From NO

Nitrosyl complexes are traditionally prepared by treating metal complexes with nitric oxide. The method is mainly used with reduced precursors. Illustrative is the nitrosylation of cobalt carbonyl to give cobalt tricarbonyl nitrosyl: ^[10]

Co₂(CO)₈ + 2 NO → 2 CoNO(CO)₃ + 2 CO

Alternatively, the cobalt may be reduced in situ:

2 CoX + Zn + 4 NO→ Co₂(NO)₄X₂ + ZnX₂

where X is Cl, Br, or I. ^[11]

From NO⁺ sources

Replacement of ligands by the nitrosyl cation may be accomplished using nitrosyl tetrafluoroborate. This reagent has been applied to the hexacarbonyls of molybdenum and tungsten: ^{[12] [13]}

M(CO)₆ + 4 MeCN + 2 NOBF₄ → [M(NO)₂(MeCN)₄](BF₄)₂

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 oxidize metal carbonyls to the corresponding nitrosyl, i.e.: ^[16]

Fe(CO)₅ + KNO₂ → K[Fe(CO)₃NO] + CO + CO₂

From hydroxylamine

Hydroxylamine is a source of nitric oxide anion via a disproportionation: ^[17]

K₂[Ni(CN)₄] + 2 NH₂OH + KOH → K₂[Ni(CN)₃)NO] + NH₃ + 2 H₂O + KCN

From nitric acid

Nitric acid is a source of nitric oxide complexes, although the details are obscure. Probably relevant is the conventional self-dehydration of nitric acid:

2 HNO₃ → NO₂⁺NO₃⁻ + H₂O

Nitric acid is used in some preparations of nitroprusside from ferrocyanide:

HNO₃ + [Fe(CN)₆]^4- → [Fe(CN)₅(NO)]^2- + OH⁻ + OCN⁻

From nitrite complexes

Some anionic nitrito complexes undergo acid-induced deoxygenation to give the linear nitrosyl complex.

[L_nMNO₂]⁻ + H⁺ → [L_nMNO] + OH⁻

The reaction is reversible in some cases.

Oxidation of ammine complexes

In some metal-ammine complexes, the ammonia ligand can be oxidized to nitrosyl: ^[18]

H₂O + [Ru(terpy)(bipy)(NH₃)]⁺ → [Ru(terpy)(bipy)(NO)]²⁺ + 5 H⁺ + 6 e⁻

Reactions

An important reaction is the acid/base equilibrium, yielding transition metal nitrite complexes:

[L_nMNO]²⁺ + 2OH⁻⇌ L_nMNO₂ + H₂O

This equilibrium serves to confirm that the linear nitrosyl ligand is, formally, NO⁺, with nitrogen in the oxidation state +3

NO⁺ + 2 OH⁻⇌ NO₂⁻ + H₂O

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.:

(Ph₃P)₂(CO)ClOsNO + HCl → (Ph₃P)₂(CO)ClOsN(H)O

In rare cases, NO is cleaved by metal centers:

Cp ₂NbMe₂ + NO → Cp₂(Me)Nb(O)NMe

2 Cp₂(Me)Nb(O)NMe → 2 Cp₂Nb(O)Me + ⁠1/2⁠MeN=NMe

Nitrosylation of a heme-thiolate, steps in cell signaling by nitric oxide (porphyrin is depicted as the square).

Applications

Metal-nitrosyls are assumed to be intermediates in catalytic converters, which reduce the emission of NO_x from internal combustion engines. This application has been described as "one of the most successful stories in the development of catalysts". ^[20]

Structure of a dinitrosyl iron complex (DNIC).

Metal-catalyzed reactions of NO are not often useful in organic 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. The nitroprusside anion, [Fe(CN)₅NO]²⁻, a mixed nitrosyl cyano complex, has pharmaceutical applications as a slow release agent for NO. The signalling function of NO is effected via its complexation to haem proteins, where it binds in the bent geometry. Nitric oxide also attacks iron-sulfur proteins giving dinitrosyl iron complexes.

Thionitrosyls

Several complexes are known with NS ligands. Like nitrosyls, thionitrosyls exist as both linear and bent geometries. ^[22]

References

1. "Sodium Nitroprusside". www.drugs.com. The American Society of Health-System Pharmacists. Retrieved 21 October 2022.
2. Hayton, T. W.; Legzdins, P.; Sharp, W. B. (2002). "Coordination and Organometallic Chemistry of Metal-NO Complexes". Chem. Rev. 102 (1): 935–991. doi:10.1021/cr000074t. PMID   11942784.
3. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 447–453. ISBN   978-0-08-037941-8.
4. Ariyarathna, Isuru R.; Miliordos, Evangelos (15 July 2019). "Electronic and geometric structure analysis of neutral and anionic metal nitric chalcogens: The case of MNX series (M=Li, Na, Be and X=O, S, Se, Te)". Journal of Computational Chemistry. 40 (19): 1740–1751. doi:10.1002/jcc.25829. PMID   30920017. S2CID   85546245.
5. Ariyarathna, Isuru (1 March 2021). "First Principle Studies on Ground and Excited Electronic States: Chemical Bonding in Main-Group Molecules, Molecular Systems with Diffuse Electrons, and Water Activation using Transition Metal Monoxides".
6. Enemark, J. H.; Feltham, R. D. (1974). "Principles of structure, bonding, and reactivity for metal nitrosyl complexes". Coord. Chem. Rev. 1974 (13): 339–406. doi:10.1016/S0010-8545(00)80259-3.
7. Mikhailov, Artem A.; Wenger, Emmanuel; Kostin, Gennadiy A.; Schaniel, Dominik (2019). "Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes" (PDF). Chemistry – A European Journal. 25 (31): 7569–7574. doi:10.1002/chem.201901205. PMID   30957917. S2CID   102349334.
8. Herberhold Max (1972). "Tetranitrosylchromium [Cr(NO)4]". Angewandte Chemie International Edition in English. 11 (12): 1092–1094. doi:10.1002/anie.197210921.
9. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 449. ISBN   978-0-08-037941-8.
10. Paul Gilmont; Arthur A. Blanchard (1946). "Dicobalt Octacarbonyl, Cobalt Nitrosyl Tricarbonyl, and Cobalt Tetracarbomyl Hydride". Inorganic Syntheses. Vol. 2. p. 238. doi:10.1002/9780470132333.ch76. ISBN   978-0-470-13233-3.
11. Seel 1965, pp. 1761–1762.
12. Richard R. Thomas; Ayusman Sen (1990). "Acetonitrile Complexes of Selected Transition Metal Cations". Inorganic Syntheses. Vol. 28. pp. 63–67. doi:10.1002/9780470132593.ch14. ISBN   978-0-470-13259-3.
13. Francine Agbossou; Edward J. O'Connor; Charles M. Garner; N. Quirós Méndez; Jesús M. Fernández; Alan T. Patton; James A. Ramsden; J. A. Gladysz; Joseph M. O'Connor; Tracy Tajima; Kevin P. Gable (1992). "Cyclopentadienyl Rhenium Complexes". Inorganic Syntheses. Vol. 29. pp. 211–225. doi:10.1002/9780470132609.ch51. ISBN   978-0-470-13260-9.
14. B. F. G. Johnson; K. H. Al-Obadi (1970). "Dihalogenodinitrosylmolybdenum and Dihalogenodinitrosyltungsten". Inorganic Syntheses. Vol. 12. pp. 264–266. doi:10.1002/9780470132432.ch47. ISBN   978-0-470-13243-2.
15. James K. Hoyano; Peter Legzdins; John T. Malito (1978). "(η ⁵ -Cyclopentadienydnitrosyl Complexes of Chromium, Molybdenum, and Jungsten". Inorganic Syntheses. Vol. 13. pp. 126–131. doi:10.1002/9780470132494.ch21. ISBN   978-0-470-13249-4.
16. Seel, F. (1965). "Carbonyl and nitrosyl compounds". In Brauer, Georg (ed.). Handbook of Preparative Inorganic Chemistry. Vol. 2 (3 in German original). Translated by Stecher, Paul G. (2 ed.). New York, NY: Academic Press / Scripta Technica. pp. 1759–1761. LCCN   63-14307.
17. Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. p. 516. ISBN   978-0-08-022057-4.
18. Dunn, Peter L.; Cook, Brian J.; Johnson, Samantha I.; Appel, Aaron M.; Bullock, R. Morris (2020). "Oxidation of Ammonia with Molecular Complexes". Journal of the American Chemical Society. 142 (42): 17845–17858. doi:10.1021/jacs.0c08269. OSTI   1706682. PMID   32977718. S2CID   221938378.
19. Walker, F. A. (2005). "Nitric Oxide Interaction with Insect Nitrophorins and Thoughts on the Electron Configuration of the FeNO⁶ Complex". J. Inorg. Biochem. 99 (1): 216–236. doi:10.1016/j.jinorgbio.2004.10.009. PMID   15598503.
20. Kaspar, Jan; Fornasiero, Paolo; Hickey, Neal (2003). "Automotive Catalytic Converters: Current Status and Some Perspectives". Catalysis Today. 77 (4): 419–449. doi:10.1016/S0920-5861(02)00384-X.
21. Jessica Fitzpatrick; Eunsuk Kim (2015). "Synthetic Modeling Chemistry of Iron–Sulfur Clusters in Nitric Oxide Signaling". Acc. Chem. Res. 48 (8): 2453–2461. doi:10.1021/acs.accounts.5b00246. PMID   26197209.
22. Ng, Ho-Yuen; Cheung, Wai-Man; Kwan Huang, Enrique; Wong, Kang-Long; Sung, Herman H.-Y.; Williams, Ian D.; Leung, Wa-Hung (2015). "Ruthenium chalcogenonitrosyl and bridged nitrido complexes containing chelating sulfur and oxygen ligands". Dalton Transactions. 44 (42): 18459–18468. doi:10.1039/C5DT02513C. PMID   26442594.