Transition metal azide complexes are

coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

es containing one or more

azide In chemistry, azide is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant ...

(N₃⁻)

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...

Structure and bonding

Azide is a

pseudohalide Pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds. Pseudohalogens occur in pseudohalogen molecules, inorganic ...

but more

nucleophilic In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they a ...

than chloride. As a

monodentate ligand In coordination chemistry, denticity () refers to the number of donor groups in a given ligand that bind to the central metal atom in a coordination complex. In many cases, only one atom in the ligand binds to the metal, so the denticity equals ...

, azide binds through one of the two terminal nitrogen atoms, i.e. M-N=N=N. The N₃ unit is linear or nearly so. The M-N-N angles are quite bent. Azide functions 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 ...

via two bonding modes. Commonly the metals share the same nitrogen ("N-diazonium" mode). Less common is the motif M-N=N=N-M, illustrated by u(N₃)(PPh₃)₂sub>2.

Homoleptic complexes

Many homoleptic complexes (with only one kind of ligand) are known. Coordination numbers range from 2 (e.g., u(N₃)₂sup>−) to 7 (e.g., (N₃)₇sup>−). Many homoleptic complexes are octahedral anions of the type (N₃)₆sup>n-: *dianions for tetravalent metals V, Pt, Ti, Zr, Hf *trianions for trivalent metals Cr, Fe, Ru, Rh, Ir *tetraanions for the divalent Ni For some metals, homoleptic complexes exist in two oxidation states: u(N₃)₂sup>− vs u(N₃)₄sup>− and t(N₃)₆sup>4- vs t(N₃)₄sup>2-.

Mixed ligand complexes

Azide forms myriad mixed ligand complexes. Examples include Zn(N₃)₂(NH₃)₂ and (C₅H₅)₂Ti(N₃)₂.

Synthesis and reactions

Traditionally, metal azide complexes are prepared by

salt metathesis A salt metathesis reaction, sometimes called a double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding a ...

, e.g. the reaction of metal chlorides with sodium azide. In some cases,

trimethylsilyl azide Trimethylsilyl azide ((CH3)3SiN3) is a chemical compound used as a reagent in organic chemistry. Preparation Trimethylsilyl azide is commercially available. It may be prepared by the reaction of trimethylsilyl chloride and sodium azide: : TMSC ...

is employed as the azide source. Heating and, in some cases, UV radiation often cause azide complexes to release nitrogen gas. This behavior is the basis of their frequent explosive properties (see

lead azide Lead(II) azide is an inorganic compound. More so than other azides, is explosive. It is used in detonators to initiate secondary explosives. In a commercially usable form, it is a white to buff powder. Preparation and handling Lead(II) azide ...

). With suitable coligands, the resulting

metal nitrido complex Metal nitrido complexes are coordination compounds 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 materials that are ...

can be isolated. Azide ligands are react with nitrosonium to give

nitrous oxide Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has ...

. This reaction is used to generate coordinatively unsaturated complexes. : o(NH₃)₅N₃sup>2+ + NO⁺ + H₂O → o(NH₃)₅(H₂O)sup>3+ + N₂O + N₂ Metal complexes of

organic azide An organic azide is an organic compound that contains an azide (–) functional group. Because of the hazards associated with their use, few azides are used commercially although they exhibit interesting reactivity for researchers. Low molecular w ...

s are intermediates in the

azide-alkyne Huisgen cycloaddition The azide-alkyne Huisgen cycloaddition is a 1,3-dipolar cycloaddition between an azide and a terminal or internal alkyne to give a 1,2,3-triazole. Rolf Huisgen was the first to understand the scope of this organic reaction. American chemist Karl ...

, the basis of

Click chemistry In chemical synthesis, click chemistry is a class of biocompatible small molecule reactions commonly used in bioconjugation, allowing the joining of substrates of choice with specific biomolecules. Click chemistry is not a single specific reactio ...

References

{{Coordination complexes Ligands Azides