:Metal nitrido complex
{{Short description|Coordination compounds and metal clusters containing a nitrogen atom bound to a transition metal}}
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.{{Greenwood&Earnshaw}} 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.{{cite journal | last1 = Fritzsche | first1 = J. | last2 = Struve | first2 = H. | year = 1847 | title = Ueber die Osman-Osmiumsäure | url = https://zenodo.org/record/1427802| journal = Journal für Praktische Chemie | volume = 41 | issue = 1| pages = 97–113 | doi = 10.1002/prac.18470410113 }}
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.{{cite journal | last1 = Dehnicke | first1 = Kurt | last2 = Strähle | first2 = Joachim | year = 1992 | title = Nitrido Complexes of Transition Metals | journal = Angewandte Chemie International Edition in English | volume = 31 | issue = 8| pages = 955–978 | doi = 10.1002/anie.199209551 }} More exotic metal nitrido complexes are also possible, such as a recently reported compound containing a terminal uranium nitride (-U≡N) bond.{{cite journal | last1 = King | first1 = David M. | last2 = Tuna | first2 = Floriana | last3 = McInnes | first3 = Eric J. L. | last4 = McMaster | first4 = Jonathan | last5 = Lewis | first5 = William | last6 = Blake | first6 = Alexander J. | last7 = Liddle | first7 = Stephen T. | year = 2012 | title = Synthesis and Structure of a Terminal Uranium Nitride Complex | journal = Science | volume = 337 | issue = 6095| pages = 717–720 | doi = 10.1126/science.1223488 | pmid = 22745250 | bibcode = 2012Sci...337..717K | doi-access = free }}
File:OsNO3_anion.png|[OsNO3]−, isoelectronic with osmium tetroxide.
File:MoNCl4_anion.png|[MoNCl4]−, a square pyramidal Mo(VI) complex.
File:W2NCl10_anion.png|[W2(μ-N)Cl10]−, containing two W(VI) centres bridged by a nitrido ligand.
File:Ir3N(SO4)6aq3.png|[Ir3N(SO4)6(H2O)3]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 (NH2−) as the N3− source:{{cite encyclopedia|entry=The platinum metals: Potassium osmiamate|page=1605|entry-url=https://archive.org/details/Handbook_of_Preparative_Inorganic_Chemistry_1_2_Brauer/page/n1651/|via=the Internet Archive|first=H. L.|last=Grube|translator-first=Paul G.|translator-last=Stecher|year=1956|publisher=Academic Press|location=NY, NY|lccn=63-14307|encyclopedia=Handbook of Preparative Inorganic Chemistry|edition=2nd|volume=2|editor-first=Georg|editor-last=Brauer}}
:OsO4 + KNH2 → KOsO3N + H2O
Most commonly however, nitrido complexes are produced by decomposition of azido complexes.{{cite journal|doi=10.1039/C7SC00570A|title=Reactivity studies on [Cp′Fe(μ-I)]2: Nitrido-, sulfido- and diselenide iron complexes derived from pseudohalide activation|year=2017|last1=Reiners|first1=Matthias|last2=Maekawa|first2=Miyuki|last3=Daniliuc|first3=Constantin G.|last4=Freytag|first4=Matthias|last5=Jones|first5=Peter G.|last6=White|first6=Peter S.|last7=Hohenberger|first7=Johannes|last8=Sutter|first8=Jörg|last9=Meyer|first9=Karsten|last10=Maron|first10=Laurent|last11=Walter|first11=Marc D.|journal=Chemical Science|volume=8|issue=5|pages=4108–4122|pmid=30155215|pmc=6099922}} 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.{{cite journal |doi=10.1021/ja960574x|title=Dinitrogen Cleavage by Three-Coordinate Molybdenum(III) Complexes: Mechanistic and Structural Data1|year=1996|last1=Laplaza|first1=Catalina E.|last2=Johnson|first2=Marc J. A.|last3=Peters|first3=Jonas C.|last4=Odom|first4=Aaron L.|last5=Kim|first5=Esther|last6=Cummins|first6=Christopher C.|last7=George|first7=Graham N.|last8=Pickering|first8=Ingrid J.|journal=Journal of the American Chemical Society|volume=118|issue=36|pages=8623–8638}}
Reactions of nitrido ligands
The nitride ligand can be both electrophilic and nucleophilic.{{cite journal | last1 = Dehnicke | first1 = Kurt | last2 = Weller | first2 = Frank | last3 = Strähle | first3 = Joachim | year = 2001 | title = Nitrido bridges between transition metals and main group elements illustrated by the series [M]NNa to [M]NCl | journal = Chem. Soc. Rev. | volume = 30 | issue = 2| pages = 125–135 | doi = 10.1039/a802635a }}{{cite book | last1 = Smith | first1 = Jeremy M. | title = Progress in Inorganic Chemistry Volume 58 | year = 2014 | chapter = Reactive Transition Metal Nitride Complexes | journal = Progress in Inorganic Chemistry | volume = 58 | pages = 417–470 | doi = 10.1002/9781118792797.ch06 | isbn = 9781118792797 }} 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.{{cite book | last1 = Gladfelter | first1 = Wayne L. | year = 1985 | title = Organometallic Metal Clusters Containing Nitrosyl and Nitrido Ligands | journal = Advances in Organometallic Chemistry | volume = 24 | pages = 41–86 | doi=10.1016/S0065-3055(08)60413-X| isbn = 9780120311248 }}
