Metal nitrido complexes are
coordination compound
A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...
s and
metal clusters 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 a chemical compound of nitrogen. Nitrides can be inorganic or organic, ionic or covalent. The nitride anion, N3−, is very elusive but compounds of nitride are numerous, although rarely naturally occurring. Some nitr ...
materials that are useful in
materials science
Materials science is an interdisciplinary field of researching and discovering materials. Materials engineering is an engineering field of finding uses for materials in other fields and industries.
The intellectual origins of materials sci ...
. 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 dinitrogen () is converted into ammonia (). It occurs both biologically and abiological nitrogen fixation, abiologically in chemical industry, chemical industries. Biological nitrogen ...
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>−, 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 above is prepared from
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent any group, typically organyl functional group, groups or hydrogen at ...
(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
nitrile
In organic chemistry, a nitrile is any organic compound that has a functional group. The name of the compound is composed of a base, which includes the carbon of the , suffixed with "nitrile", so for example is called " propionitrile" (or pr ...
s 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), usually denoted by H+, to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brø ...
and N-
alkylation Alkylation is a chemical reaction that entails transfer of an alkyl group. 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 effecting al ...
. 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 r ...
, several metal clusters are known to contain nitride ligands at their center. Such nitrido ligands are termed
interstitial. 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
*
Transition metal dinitrogen complex
General references
{{coordination complexes
Inorganic chemistry
Coordination complexes