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 useful in materials science.^[1] The distinction between the molecular and solid-state polymers is not always very clear as illustrated by the materials Li₆MoN₄ and more condensed derivatives such as Na₃MoN₃. Transition metal nitrido complexes have attracted interest in part because it is assumed that nitrogen fixation proceeds via nitrido intermediates. Nitrido complexes have long been known, the first example being salts of [OsO₃N]⁻, described in the 19th century.^[2]

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[MoNCl₄], 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.^[3] More exotic metal nitrido complexes are also possible, such as a recently reported compound containing a terminal uranium nitride (-U≡N) bond.^[4]

• Example metal nitrido complexes
• 
  [OsNO₃]⁻, isoelectronic with osmium tetroxide.
• 
  [MoNCl₄]⁻, a square pyramidal Mo(VI) complex.
• 
  [W₂(μ-N)Cl₁₀]⁻, containing two W(VI) centres bridged by a nitrido ligand.
• 
  [Ir₃N(SO₄)₆(H₂O)₃]⁴⁻, structurally related to basic iron acetate.
• 
  A uranium nitrido complex.
• 
  ((t-Bu)₃C₅H₂)₂Fe₂N₂.

Preparative routes

Metal nitrides are produced using a variety of nitrogen sources. The first example above is prepared from amide (NH₂⁻) as the N³⁻ source:^[5]

OsO₄ + KNH₂ → KOsO₃N + H₂O

Most commonly however, nitrido complexes are produced by decomposition of azido complexes.^[6] The driving force for these reactions is the great stability of N₂. Nitrogen trichloride is an effective reagent to give chloro-nitrido complexes. In some cases, even N₂ and nitriles can serve as sources of nitride ligands.^[7]

Reactions of nitrido ligands

The nitride ligand can be both electrophilic and nucleophilic.^[8][9] 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 and N-alkylation. Ru and Os nitrido complexes often add organophosphines to give iminophosphine derivatives containing the R₃PN⁻ ligand.

Interstitial nitrides

Owing to the ability of nitrido ligands to serve as a bridging ligand, 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.^[10]