View source for Metal nitrosyl complex

{{Short description|Complex of a transition metal bonded to nitric oxide: Me–NO}}
{{Distinguish|Nitro compound}}
[[image:Sodium-nitroprusside-2D.png|thumb|right|[[Sodium nitroprusside]], a medicinally significant metal nitrosyl-pentacyanoferrate (Fe-III) compound, used to treat [[hypertension]].<ref>{{cite web|title=Sodium Nitroprusside|url=https://www.drugs.com/monograph/sodium-nitroprusside.html|publisher=The American Society of Health-System Pharmacists|website=www.drugs.com|access-date=21 October 2022}}</ref>]] 
'''Metal nitrosyl complexes''' are [[complex (chemistry)|complexes]] that contain [[nitric oxide]], NO, bonded to a [[transition metal]].<ref name=Legzdins>{{cite journal|last=Hayton|first=T. W.|author2=Legzdins, P. |author3=Sharp, W. B. |year=2002|title=Coordination and Organometallic Chemistry of Metal-NO Complexes|journal=Chem. Rev.|volume=102|issue=1|pages=935–991|doi=10.1021/cr000074t|pmid=11942784}}</ref> Many kinds of nitrosyl complexes are known, which vary both in structure and co[[ligand]].

==Bonding and structure==
[[File:Back bonding.png|thumb|2000px|(Top) the HOMO and LUMO of [[Carbon monoxide|CO]]. (Middle) Sigma bond. (Bottom) Back-bond.]]
Most complexes containing the NO ligand can be viewed as derivatives of the nitrosyl cation, NO<sup>+</sup>. 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 complex]]es. The nitrosyl cation serves as a two-electron donor to the metal and accepts electrons from the metal via [[back-bonding]]. The compounds [[Cobalt tricarbonyl nitrosyl|Co(NO)(CO)<sub>3</sub>]] and [[nickel carbonyl|Ni(CO)<sub>4</sub>]] illustrate the analogy between NO<sup>+</sup> 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)<sub>2</sub>(NO)<sub>2</sub> and [Ni(CO)<sub>4</sub>].<ref name=a>{{Greenwood&Earnshaw2nd|pages=447-453}}</ref> These complexes are [[isoelectronic]] and, incidentally, both obey the [[18-electron rule]]. The formal description of nitric oxide as NO<sup>+</sup> 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]].

[[Image:Metal-nitrosyl-coordination-modes-2D.png|thumb|left|100px|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&nbsp;cm<sup>−1</sup>, whereas bent nitrosyls absorb in the range 1525–1690&nbsp;cm<sup>−1</sup>.  The differing vibrational frequencies reflect the differing N-O [[bond order]]s for linear ([[triple bond]]) and bent NO ([[double bond]]).

The bent NO ligand is sometimes described as the anion, NO<sup>−</sup>. Prototypes for such compounds are the organic nitroso compounds, such as [[nitrosobenzene]]. A complex with a bent NO ligand is ''trans''-[Co([[ethylenediamine|en]])<sub>2</sub>(NO)Cl]<sup>+</sup>. The NO<sup>−</sup> is also common for alkali-metal or alkaline-earth metal-NO molecules. For example. LiNO and BeNO bear Li<sup>+</sup>NO<sup>−</sup> and Be<sup>+</sup>NO<sup>−</sup> ionic form.<ref>{{cite journal |last1=Ariyarathna |first1=Isuru R. |last2=Miliordos |first2=Evangelos |title=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=Journal of Computational Chemistry |date=15 July 2019 |volume=40 |issue=19 |pages=1740–1751 |doi=10.1002/jcc.25829|pmid=30920017 |s2cid=85546245 }}</ref><ref>{{cite web |last1=Ariyarathna |first1=Isuru |title=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 |url=https://etd.auburn.edu//handle/10415/7601 |language=en |date=1 March 2021}}</ref>

The adoption of linear vs bent bonding can be analyzed with the '''Enemark-Feltham notation'''.<ref name=enemark-feltham>{{cite journal | last1 = Enemark | first1 = J. H. | last2 = Feltham | first2 = R. D.  | title = Principles of structure, bonding, and reactivity for metal nitrosyl complexes | journal = Coord. Chem. Rev. | volume = 1974 | issue = 13| pages = 339–406 | doi = 10.1016/S0010-8545(00)80259-3 | year = 1974 }}</ref>  In their framework, the factor that determines the bent vs linear NO ligands is the electron count in the metal-N-O [[conjugated system|π system]].  Complexes more than 6 electrons in the system tend to have bent geometries at N.  Thus, [Co([[ethylenediamine|en]])<sub>2</sub>(NO)Cl]<sup>+</sup>, with eight electrons of pi-symmetry (six in t<sub>2g</sub> orbitals and two on NO, {CoNO}<sup>8</sup>), adopts a bent NO ligand, whereas [Fe(CN)<sub>5</sub>(NO)]<sup>2−</sup>, with six electrons of pi-symmetry, {FeNO}<sup>6</sup>), adopts a linear nitrosyl.  In a further illustration, consider the {MNO} d-electron count of the [Cr(CN)<sub>5</sub>NO]<sup>3&minus;</sup> anion. In this example, the cyanide ligands are "innocent", i.e., they have a charge of &minus;1 each, &minus;5 total. To balance the fragment's overall charge, the charge on {CrNO} is thus +2 (&minus;3 = &minus;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}<sup>5</sup>, and the nitrosyl is linear. The results are the same if the nitrosyl ligand were considered NO<sup>+</sup> or NO<sup>&minus;</sup>.<ref name=enemark-feltham/>

===Bridging nitrosyl ligands===
Nitric oxide can also serve as a [[bridging ligand]]. In the compound [Mn<sub>3</sub>(η<sup>5</sup>C<sub>5</sub>H<sub>5</sub>)<sub>3</sub> (μ<sub>2</sub>-NO)<sub>3</sub> (μ<sub>3</sub>-NO)], three NO groups bridge two metal centres and one NO group bridge to all three.<ref name=a/>

===Isonitrosyl ligands===
[[File:Structure of Ru(Cl)(ON)(pyridine)4+ (9RUKQOE02).png|thumb|Structure of the isonitrosyl complex [Ru(Cl)(ON)(pyridine)<sub>4</sub>]<sup>+</sup> (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.<ref>{{cite journal |doi=10.1002/chem.201901205|title=Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes|year=2019|last1=Mikhailov|first1=Artem A.|last2=Wenger|first2=Emmanuel|last3=Kostin|first3=Gennadiy A.|last4=Schaniel|first4=Dominik|journal=Chemistry – A European Journal|volume=25|issue=31|pages=7569–7574|pmid=30957917|s2cid=102349334 |url=https://hal.univ-lorraine.fr/hal-02319723/file/Mikhailov-ChemEurJ_final_HAL.pdf}}</ref>

==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)<sub>4</sub>.<ref>{{cite journal | doi = 10.1002/anie.197210921 | volume=11 | title=Tetranitrosylchromium [Cr(NO)4] | year=1972 | journal=Angewandte Chemie International Edition in English | pages=1092–1094 | author=Herberhold Max| issue=12 }}</ref> 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 [Fe<sub>2</sub>(NO)<sub>4</sub>S<sub>2</sub>]<sup>2−</sup>. The structure of the anion can be viewed as consisting of two [[tetrahedron|tetrahedra]] sharing an edge. Each iron atom is bonded linearly to two NO<sup>+</sup> ligands and shares two bridging sulfidi ligands with the other iron atom. [[Roussin's black salt]] has a more complex [[metal cluster|cluster]] structure. The anion in this species has the formula [Fe<sub>4</sub>(NO)<sub>7</sub>S<sub>3</sub>]<sup>−</sup>. It has ''C''<sub>3v</sub> [[molecular point group|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.

{{gallery
|width=190
|Image:Roussin's-red-salt-anion-3D-balls.png|The anion in [[Roussin's red salt]], [Fe<sub>2</sub>S<sub>2</sub>(NO)<sub>4</sub>]<sup>2−</sup>.
|Image:Roussin's-black-salt-anion-3D-balls.png|The anion in [[Roussin's black salt]], [Fe<sub>4</sub>S<sub>3</sub>(NO)<sub>7</sub>]<sup>−</sup>.
|File:Nitroprusside-anion-from-xtal-3D-balls.png|The nitroprusside anion, [Fe(CN)<sub>5</sub>NO]<sup>2−</sup>, an octahedral complex containing  a "linear NO" ligand.
|Image:CoCl(en)2NO-3D-balls.png|''trans''-[Co(en)<sub>2</sub>(NO)Cl]<sup>+</sup>, an octahedral complex containing  a "bent NO" ligand.
}}

==Preparation==
Many nitrosyl complexes are quite stable, thus many methods can be used for their synthesis.<ref>{{Greenwood&Earnshaw2nd|page=449}}</ref>

===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]]:<ref>{{cite book |author=Paul Gilmont |author2=Arthur A. Blanchard  
|title = Inorganic Syntheses |year=1946| volume = 2 | page = 238 | chapter = Dicobalt Octacarbonyl, Cobalt Nitrosyl Tricarbonyl, and Cobalt Tetracarbomyl Hydride|doi = 10.1002/9780470132333.ch76|isbn = 978-0-470-13233-3}}</ref>
:Co<sub>2</sub>(CO)<sub>8</sub> + 2{{nbsp}}NO → 2{{nbsp}}CoNO(CO)<sub>3</sub> + 2{{nbsp}}CO
Alternatively, the cobalt may be reduced ''in situ'':
:{{chem2|2 CoX + Zn + 4 NO→ Co2(NO)4X2 + ZnX2}}
where X is [[chlorine|Cl]], [[bromine|Br]], or [[iodine|I]].{{sfn|Seel|1965|pp=1761-1762}}

===From NO<sup>+</sup> 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:<ref>{{cite book |author=Richard R. Thomas |author2=Ayusman Sen |title = Inorganic Syntheses |year=1990| volume = 28 | pages = 63–67 | chapter = Acetonitrile Complexes of Selected Transition Metal Cations|doi = 10.1002/9780470132593.ch14 |isbn = 978-0-470-13259-3 }}</ref><ref>{{cite book |author=Francine Agbossou |author2=Edward J. O'Connor |author3=Charles M. Garner |author4=N. Quirós Méndez |author5=Jesús M. Fernández |author6=Alan T. Patton |author7=James A. Ramsden |author8=J. A. Gladysz |author9=Joseph M. O'Connor |author10=Tracy Tajima |author11=Kevin P. Gable |title = Inorganic Syntheses | year = 1992| volume = 29 | pages = 211–225 | chapter = Cyclopentadienyl Rhenium Complexes|doi = 10.1002/9780470132609.ch51|isbn = 978-0-470-13260-9 }}</ref>
:M(CO)<sub>6</sub> + 4{{nbsp}}MeCN + 2{{nbsp}}NOBF<sub>4</sub> → [M(NO)<sub>2</sub>(MeCN)<sub>4</sub>](BF<sub>4</sub>)<sub>2</sub>

[[Nitrosyl chloride]] and molybdenum hexacarbonyl react to give [Mo(NO)<sub>2</sub>Cl<sub>2</sub>]<sub>n</sub>.<ref>{{cite book |author=B. F. G. Johnson |author2=K. H. Al-Obadi |title = Inorganic Syntheses |year=1970| volume = 12| pages = 264–266 | chapter = Dihalogenodinitrosylmolybdenum and Dihalogenodinitrosyltungsten|doi = 10.1002/9780470132432.ch47|isbn = 978-0-470-13243-2 }}</ref>  [[Diazald]] is also used as an NO source.<ref>{{cite book |author=James K. Hoyano |author2=Peter Legzdins |author3=John T. Malito |title = Inorganic Syntheses |chapter = (η <sup>5</sup> -Cyclopentadienydnitrosyl Complexes of Chromium, Molybdenum, and Jungsten |year=1978| volume = 13| pages = 126–131|doi = 10.1002/9780470132494.ch21|isbn = 978-0-470-13249-4}}</ref>

Simple nitrite salts also oxidize [[metal carbonyl]]s to the corresponding nitrosyl, i.e.:<ref>{{cite encyclopedia|chapter=Carbonyl and nitrosyl compounds|pp=1759-1761|first=F.|last=Seel|encyclopedia=Handbook of Preparative Inorganic Chemistry|volume=2 (3 in German original)|edition=2|editor-first=Georg|editor-last=Brauer|translator-first=Paul&nbsp;G.|translator-last=Stecher|year=1965|publisher=Academic Press / Scripta Technica|location=New York, NY|lccn=63-14307}}</ref>
:Fe(CO)<sub>5</sub>&nbsp;+ KNO<sub>2</sub>&nbsp;→ K[Fe(CO)<sub>3</sub>NO]&nbsp;+ CO&nbsp;+ CO<sub>2</sub>

===From hydroxylamine===
[[Hydroxylamine]] is a source of nitric oxide anion via a disproportionation:<ref>{{Greenwood&Earnshaw1st|p=516}}</ref>
: K<sub>2</sub>[Ni(CN)<sub>4</sub>] + 2{{nbsp}}NH<sub>2</sub>OH + KOH → K<sub>2</sub>[Ni(CN)<sub>3</sub>)NO] + NH<sub>3</sub> + 2{{nbsp}}H<sub>2</sub>O + 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 HNO<sub>3</sub> → NO<sub>2</sub><sup>+</sup>NO<sub>3</sub><sup>−</sup> + H<sub>2</sub>O
Nitric acid is used in some preparations of [[nitroprusside]] from [[ferrocyanide]]:
: HNO<sub>3</sub> + [Fe(CN)<sub>6</sub>]<sup>4-</sup> → [Fe(CN)<sub>5</sub>(NO)]<sup>2-</sup> + OH<sup>−</sup> + OCN<sup>−</sup>

===From nitrite complexes===
Some anionic nitrito complexes undergo acid-induced deoxygenation to give the linear nitrosyl complex.
:[L<sub>n</sub>MNO<sub>2</sub>]<sup>−</sup> + H<sup>+</sup> → [L<sub>n</sub>MNO] + OH<sup>−</sup>
The reaction is reversible in some cases.

===Oxidation of ammine complexes===
In some [[metal-ammine complex]]es, the ammonia ligand can be oxidized to nitrosyl:<ref name=RMB>{{cite journal |doi=10.1021/jacs.0c08269|title=Oxidation of Ammonia with Molecular Complexes|year=2020|last1=Dunn|first1=Peter L.|last2=Cook|first2=Brian J.|last3=Johnson|first3=Samantha I.|last4=Appel|first4=Aaron M.|last5=Bullock|first5=R. Morris|journal=Journal of the American Chemical Society|volume=142|issue=42|pages=17845–17858|pmid=32977718|osti=1706682|s2cid=221938378}}</ref>
:H<sub>2</sub>O + [Ru(terpy)(bipy)(NH<sub>3</sub>)]<sup>+</sup> → [Ru(terpy)(bipy)(NO)]<sup>2+</sup> + 5{{nbsp}}H<sup>+</sup> + 6{{nbsp}}e<sup>−</sup>

==Reactions==
An important reaction is the acid/base equilibrium, yielding [[transition metal nitrite complex]]es:
:[L<sub>''n''</sub>MNO]<sup>2+</sup> + 2OH<sup>−</sup> {{eqm}} L<sub>''n''</sub>MNO<sub>2</sub> + H<sub>2</sub>O
This equilibrium serves to confirm that the linear nitrosyl ligand is, formally, NO<sup>+</sup>, with nitrogen in the oxidation state +3
:NO<sup>+</sup> + 2 OH<sup>−</sup> {{eqm}} NO<sub>2</sub><sup>−</sup> + H<sub>2</sub>O

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.<ref name=Legzdins/> The nitrogen atom in bent metal nitrosyls is basic, thus can be oxidized, alkylated, and protonated, e.g.:
:(Ph<sub>3</sub>P)<sub>2</sub>(CO)ClOsNO + HCl → (Ph<sub>3</sub>P)<sub>2</sub>(CO)ClOsN(H)O

In rare cases, NO is cleaved by metal centers:
: [[cyclopentadienyl|Cp]]<sub>2</sub>NbMe<sub>2</sub> + NO → Cp<sub>2</sub>(Me)Nb(O)NMe
: 2 Cp<sub>2</sub>(Me)Nb(O)NMe → 2 Cp<sub>2</sub>Nb(O)Me + {{sfrac|1|2}}MeN=NMe
[[File:Fe-NO-porph.png|thumb|312px|Nitrosylation of a heme-thiolate, steps in cell signaling by nitric oxide (porphyrin is depicted as the square).<ref name=FAW>{{cite journal|author=Walker, F. A.|title=Nitric Oxide Interaction with Insect Nitrophorins and Thoughts on the Electron Configuration of the FeNO<sup>6</sup> Complex|journal=J. Inorg. Biochem.|year=2005|volume=99|issue=1 |pages=216–236|doi=10.1016/j.jinorgbio.2004.10.009|pmid=15598503}}</ref>]]

==Applications==
Metal-nitrosyls are assumed to be intermediates in [[Catalytic converter#Three-way|catalytic converters]], which reduce the emission of {{NOx}} from internal combustion engines.  This application has been described as "one of the most successful stories in the development of catalysts".<ref>{{cite journal|title=Automotive Catalytic Converters: Current Status and Some Perspectives|author1=Kaspar, Jan |author2=Fornasiero, Paolo |author3=Hickey, Neal |journal=Catalysis Today|year=2003|volume=77|issue=4 |pages=419–449|doi=10.1016/S0920-5861(02)00384-X}}</ref>
[[File:DNIC structure.svg|thumb|left|200px|Structure of a [[dinitrosyl iron complex]] (DNIC).<ref>{{cite journal|title=Synthetic Modeling Chemistry of Iron–Sulfur Clusters in Nitric Oxide Signaling |author=Jessica Fitzpatrick |author2=Eunsuk Kim |journal=Acc. Chem. Res.|year=2015|volume=48|issue=8|pages=2453–2461|doi=10.1021/acs.accounts.5b00246|pmid=26197209}}</ref>]]

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)<sub>5</sub>NO]<sup>2−</sup>, a mixed nitrosyl cyano complex, has pharmaceutical applications as a slow release agent for NO.  The [[Cell signaling|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 protein]]s giving [[dinitrosyl iron complex]]es.

==Thionitrosyls==
Several complexes are known with NS ligands.  Like nitrosyls, thionitrosyls exist as both linear and bent geometries.<ref>{{cite journal |doi=10.1039/C5DT02513C|title=Ruthenium chalcogenonitrosyl and bridged nitrido complexes containing chelating sulfur and oxygen ligands|year=2015|last1=Ng|first1=Ho-Yuen|last2=Cheung|first2=Wai-Man|last3=Kwan Huang|first3=Enrique|last4=Wong|first4=Kang-Long|last5=Sung|first5=Herman H.-Y.|last6=Williams|first6=Ian D.|last7=Leung|first7=Wa-Hung|journal=Dalton Transactions|volume=44|issue=42|pages=18459–18468|pmid=26442594}}</ref>

==References==
{{Reflist|2}}{{Coordination complexes}}{{Nitric oxide signaling}}

[[Category:Oxides]]
[[Category:Nitrogen compounds]]
[[Category:Nitrosyl complexes]]
[[Category:Coordination chemistry]]