Metal nitrido complexes are
coordination compounds and
metal cluster
In chemistry, an atom cluster (or simply cluster) is an ensemble of bound atoms or molecules that is intermediate in size between a simple molecule and a nanoparticle; that is, up to a few nanometers (nm) in diameter. The term ''microcluster ...
s that contain an atom of nitrogen bound only to transition metals. These compounds are ''molecular'', i.e. discrete in contrast to the polymeric, dense
nitride
In chemistry, a nitride is an inorganic compound of nitrogen. The "nitride" anion, N3- ion, is very elusive but compounds of nitride are numerous, although rarely naturally occuring. Some nitrides have a find applications, such as wear-resistant ...
materials that are useful in
materials science. The distinction between the molecular and solid-state polymers is not always very clear as illustrated by the materials Li
6MoN
4 and more condensed derivatives such as Na
3MoN
3. Transition metal nitrido complexes have attracted interest in part because it is assumed that
nitrogen fixation
Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...
proceeds via nitrido intermediates. Nitrido complexes have long been known, the first example being salts of
sO3Nsup>−, described in the 19th century.
Structural trends
Mononuclear complexes feature terminal nitride ligands, typically with short M-N distances consistent with
metal ligand multiple bonds. For example, in the anion in
PPh4 oNCl4 the Mo-N distance is 163.7 pm. The occurrence of terminal nitrido ligands follow the patterns seen for oxo complexes: they are more common for early and heavier metals. Many bi- and polynuclear complexes are known with bridging nitrido ligands. More exotic metal nitrido complexes are also possible, such as a recently reported compound containing a terminal uranium nitride (-U≡N) bond.
File:OsNO3_anion.png, sNO3sup>−, which is isoelectronic with osmium tetroxide
Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound with the formula OsO4. The compound is noteworthy for its many uses, despite its toxicity and the rarity of osmium. It also has a number of unusual properties, one being that the ...
.
File:MoNCl4_anion.png, oNCl4sup>−, a square pyramidal Mo(VI) complex.
File:W2NCl10_anion.png, 2(μ-N)Cl10sup>−, containing two W(VI) centres bridged by a nitrido ligand.
File:Ir3N(SO4)6aq3.png, r3N(SO4)6(H2O)3sup>4−, structurally related to basic iron acetate.
File:Uranium_nitride_anion.png, A uranium nitrido complex.
File:Cp"2Fe2N2.png, ((t-Bu)3C5H2)2Fe2N2.
Preparative routes
Metal nitrides are produced using a variety of nitrogen sources. The first example was prepared from
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it i ...
(NH
2−) as the N
3− source:
:OsO
4 + KNH
2 → KOsO
3N + H
2O
Most commonly however, nitrido complexes are produced by decomposition of
azido complexes. The driving force for these reactions is the great stability of N
2.
Nitrogen trichloride is an effective reagent to give chloro-nitrido complexes. In some cases, even N
2 and
nitriles can serve as sources of nitride ligands.
Reactions of nitrido ligands
The nitride ligand can be both electrophilic and nucleophilic. Terminal nitrides of early metals tend to be basic and oxidizable, whereas nitrides of the later metals tend to be oxidizing and electrophilic. The former behavior is illustrated by their N-
protonation
In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...
and N-
alkylation
Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecti ...
. Ru and Os nitrido complexes often add organo
phosphine
Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...
s to give iminophosphine derivatives containing the R
3PN
− ligand.
Interstitial nitrides
Owing to the ability of nitrido ligands to 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 ...
, several metal clusters are known to contain nitride ligands at their center. Such nitrido ligands are termed
interstitial
An interstitial space or interstice is a space between structures or objects.
In particular, interstitial may refer to:
Biology
* Interstitial cell tumor
* Interstitial cell, any cell that lies between other cells
* Interstitial collagenase ...
. In some cases, the nitride is completely encased in the center of six or more metals and cannot undergo reactions, although it contributes to the intermetallic bonding.
See also
*
Abiological nitrogen fixation
Abiological nitrogen fixation describes chemical processes that fix (react with) N2, usually with the goal of generating ammonia. The dominant technology for abiological nitrogen fixation is the Haber process, which uses an iron-based heterogeneou ...
*
Transition metal dinitrogen complex
General references
{{coordination complexes
Inorganic chemistry
Coordination complexes