Metal Nitrosyl Complex
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Sodium nitroprusside, a medicinally significant metal nitrosyl-pentacyanoferrate (Fe-III) compound, used to treat hypertension">Sodium nitroprusside, a medicinally significant metal nitrosyl-pentacyanoferrate (Fe-III) compound, used to treat complexes that contain nitric oxide">hypertension. Metal nitrosyl complexes are complex (chemistry)">complexes that contain nitric oxide, NO, bonded to a transition metal. Many kinds of nitrosyl complexes are known, which vary both in structure and coligand.


Bonding and structure

Most complexes containing the NO ligand can be viewed as derivatives of the nitrosyl cation, NO+. The nitrosyl cation is
isoelectronic Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...
with
carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
, thus the bonding between a nitrosyl ligand and a metal follows the same principles as the bonding in
carbonyl complex Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe ch ...
es. 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 i(CO)4 These complexes are
isoelectronic Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...
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 A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond orde ...
.


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 Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or function ...
. 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 order In chemistry, bond order, as introduced by Linus Pauling, is defined as the difference between the number of bonds and anti-bonds. The bond order itself is the number of electron pairs ( covalent bonds) between two atoms. For example, in diat ...
s for linear (
triple bond A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond orde ...
) and bent NO (
double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...
). 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''- en)2(NO)Cl">ethylenediamine.html" ;"title="o(ethylenediamine">en)2(NO)Clsup>+. 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. The adoption of linear vs bent bonding can be analyzed with the Enemark-Feltham notation. In their framework, the factor that determines the bent vs linear NO ligands is the sum of electrons of pi-symmetry. Complexes with "pi-electrons" in excess of 6 tend to have bent NO ligands. Thus, en)2(NO)Cl">ethylenediamine.html" ;"title="o(ethylenediamine">en)2(NO)Clsup>+, with eight electrons of pi-symmetry (six in t2g orbitals and two on NO, 8), adopts a bent NO ligand, whereas e(CN)5(NO)sup>2−, with six electrons of pi-symmetry, 6), adopts a linear nitrosyl. In a further illustration, the d-electron count of the [Cr(CN)5NO]3− anion is shown. 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 is thus +2 (−3 = −5 + 2). Using the neutral
electron counting Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding. Many rules in chemistry rely on electron-counting: *Octet rule is used with Lewis structures for main group elemen ...
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 5. The results are the same if the nitrosyl ligand were considered NO+ or NO.


Bridging nitrosyl ligands

Nitric oxide can also serve as a
bridging ligand In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually ...
. In the compound n35C5H5)32-NO)33-NO) three NO groups bridge two metal centres and one NO group bridge to all three.


Isonitrosyl ligands

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.


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. 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 e2(NO)4S2sup>2−. The structure of the anion can be viewed as consisting of two
tetrahedra In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...
sharing an edge. Each iron atom is bonded linearly to two NO+ ligands and shares two bridging sulfido ligands with the other iron atom. Roussin's black salt has a more complex
cluster may refer to: Science and technology Astronomy * Cluster (spacecraft), constellation of four European Space Agency spacecraft * Asteroid cluster, a small asteroid family * Cluster II (spacecraft), a European Space Agency mission to study th ...
structure. The anion in this species has the formula e4(NO)7S3sup>−. 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 Rotational symmetry, also known as radial symmetry in geometry, is the property a shape has when it looks the same after some rotation by a partial turn. An object's degree of rotational symmetry is the number of distinct orientations in which i ...
has a single nitrosyl group which also lies on that axis.


Preparation

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


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: :Co2(CO)8 + 2NO → 2CoNO(CO)3 + 2CO


From NO+ and NOCl

Replacement of ligands by the nitrosyl cation may be accomplished using
nitrosyl tetrafluoroborate Nitrosonium tetrafluoroborate, also called nitrosyl tetrafluoroborate, is a chemical compound with the chemical formula NOBF4. This colourless solid is used in organic synthesis as a nitrosating agent. NOBF4 is the nitrosonium salt of fluorobor ...
. This reagent has been applied to the hexacarbonyls of molybdenum and tungsten: :M(CO)6 + 4MeCN + 2NOBF4(NO)2(MeCN)4BF4)2
Nitrosyl chloride Nitrosyl chloride is the chemical compound with the formula NOCl. It is a yellow gas that is commonly encountered as a component of aqua regia, a mixture of 3 parts concentrated hydrochloric acid and 1 part of concentrated nitric acid. It is a st ...
and molybdenum hexacarbonyl react to give o(NO)2Cl2sub>n. Diazald is also used as an NO source.


From hydroxylamine

Hydroxylamine Hydroxylamine is an inorganic compound with the formula . The material is a white crystalline, hygroscopic compound.Greenwood and Earnshaw. ''Chemistry of the Elements.'' 2nd Edition. Reed Educational and Professional Publishing Ltd. pp. 431–43 ...
is a source of nitric oxide anion via a disproportionation: : K2 i(CN)4+ 2NH2OH + KOH → K2 i(CN)3)NO+ NH3 + 2H2O + KCN


From nitric acid

Nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
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 Ferrocyanide is the name of the anion cyanide.html" ;"title="e(cyanide">CN)6sup>4−. Salts of this coordination complex give yellow solutions. It is usually available as the salt potassium ferrocyanide, which has the formula K4Fe(CN)6. e(CN)6s ...
: : HNO3 + e(CN)6sup>4- → e(CN)5(NO)sup>2- + OH + OCN


From nitrous acid

Some anionic nitrito complexes undergo acid-induced deoxygenation to give the linear nitrosyl complex. : nMNO2sup>− + H+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: :H2O + u(terpy)(bipy)(NH3)sup>+ →
u(terpy)(bipy)(NO) U or u, is the twenty-first and sixth-to-last letter and fifth vowel letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''u'' (pro ...
sup>2+ + 5H+ + 6e


Reactions

An important reaction is the acid/base equilibrium, yielding transition metal nitrite complexes: : ''n''MNOsup>2+ + 2OH L''n''MNO2 + 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. 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: : Cp2NbMe2 + NO → Cp2(Me)Nb(O)NMe : 2 Cp2(Me)Nb(O)NMe → 2 Cp2Nb(O)Me + ½MeN=NMe


Applications

Metal-nitrosyls are assumed to be intermediates in
catalytic converters A catalytic converter is an exhaust emission control device that converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction. Catalytic converters are usual ...
, which reduce the emission of from internal combustion engines. This application has been described as “one of the most successful stories in the development of catalysts.” Metal-catalyzed reactions of NO are not often useful in
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...
. 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, e(CN)5NOsup>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 haeme proteins, where it binds in the
bent geometry In chemistry, molecules with a non-collinear arrangement of two adjacent bonds have bent molecular geometry, also known as angular or V-shaped. Certain atoms, such as oxygen, will almost always set their two (or more) covalent bonds in non-colline ...
. Nitric oxide also attacks
iron-sulfur protein Iron–sulfur proteins (or iron–sulphur proteins in British spelling) are proteins characterized by the presence of iron–sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states. Iron–sulfur c ...
s giving
dinitrosyl iron complex In biochemistry, dinitrosyl iron complexes (DNIC's) are coordination complexes with the formula e(NO)2(SR)2sup>−. Together with Roussin esters (Fe2(NO)4(SR)2), they result from the degradation of iron-sulfur proteins by nitric oxide. Commonly ...
es.


Thionitrosyls

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


References

{{Nitric oxide signaling Oxides Nitrogen compounds Nitrosyl complexes Coordination chemistry