:Transition metal imido complex
File:TiImide.png, CMe3 = tert-butyl)Hazari, N.; Mountford, P., "Reactions and Applications of Titanium Imido Complexes", Acc. Chem. Res. 2005, 38, 839-849. {{doi|10.1021/ar030244z}}]]
In coordination chemistry and organometallic chemistry, transition metal imido complexes is a coordination compound containing an imido ligand. Imido ligands can be terminal or bridging ligands. The parent imido ligand has the formula NH, but most imido ligands have alkyl or aryl groups in place of H. The imido ligand is generally viewed as a dianion, akin to oxide.
Structural classes
=Complexes with terminal imido ligands=
In some terminal imido complexes, the M=N−C angle is 180° but often the angle is decidedly bent. Complexes of the type M=NH are assumed to be intermediates in nitrogen fixation by synthetic catalysts.Nugent, W. A.; Mayer, J. M., "Metal-Ligand Multiple Bonds," J. Wiley: New York, 1988.
image:SchrockCatalyst.svg catalyst features imides as spectator ligands.]]
=Complexes with bridging imido ligands=
Imido ligands are observed as doubly and, less often, triply bridging ligands.
Synthesis
=From metal oxo complexes=
Commonly metal-imido complexes are generated from metal oxo complexes. They arise by condensation of amines and metal oxides and metal halides:
:LnMO + H2NR → LnMNR + H2O
This approach is illustrated by the conversion of MoO2Cl2 to the diimido derivative MoCl2(NAr)2(dimethoxyethane), precursors to the Schrock carbenes of the type Mo(OR)2(NAr)(CH-t-Bu).{{cite journal|author=Schrock, R. R. |title=Recent Advances in High Oxidation State Mo and W Imido Alkylidene Chemistry|journal=Chemical Reviews|year=2009|volume=109|issue=8 |pages=3211–3226|doi=10.1021/cr800502p|pmid=19284732 |pmc=2726908}}
:LnMCl2 + 3 H2NR → LnMNR + 2 RNH3Cl
Aryl isocyanates react with metal oxides concomitant with decarboxylation:
:LnMO + O=C=NR → LnMNR + CO2
=Alternative routes=
Some are generated from the reaction of low-valence metal complexes with azides:
:LnM + N3R → LnMNR + N2
A few imido complexes have been generated by the alkylation of metal nitride complexes:
:LnMN− + RX → LnMNR + X−
Utility
Metal imido complexes are mainly of academic interest. They are however assumed to be intermediates in ammoxidation catalysis, in the Sharpless oxyamination, and in nitrogen fixation.
=In nitrogen fixation=
A molybdenum imido complex appears in a common nitrogen fixation cycle:
:{{awrap|Mo•N2 →}} {{awrap|Mo-N=N− →}} {{awrap|Mo-N{{=}}NH (diazenido) →}} {{awrap|Mo-N{{=}}NH2+ →}} {{awrap|Mo{{=}}N-NH2 (hydrazido) →}} {{awrap|Mo{{=}}N-NH3+ (hydrazidium) →}} {{awrap|Mo≡N (nitrido) + NH3 →}} {{awrap|Mo≡NH+ →}} {{awrap|Mo{{=}}NH (imido) →}} {{awrap|Mo{{=}}NH2+ →}} {{awrap|Mo-NH2 (amido) →}} {{awrap|Mo-NH3+ →}} Mo•NH3 (ammine);
with the oxidation state of molybdenum varying to accommodate the number bonds from nitrogen.{{cite thesis |last1=Stringer |first1=Damien |title=Synthetic and Theoretical Studies of Lanthanide Imide and Alkene Complexes |url=https://eprints.utas.edu.au/22237/1/whole_StringerDamienNoel2009_thesis.pdf |access-date=10 November 2020 |location=University of Tasmania |date=July 2009}}