Transition metal imido complex

tert-butyl)^[1]

In

. 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.^[3]

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

. They arise by condensation of amines and metal oxides and metal halides:

L_nMO + H₂NR → L_nMNR + H₂O

This approach is illustrated by the conversion of

L_nMCl₂ + 3 H₂NR → L_nMNR + 2 RNH₃Cl

Aryl isocyanates react with metal oxides concomitant with decarboxylation:

L_nMO + O=C=NR → L_nMNR + CO₂

Alternative routes

Some are generated from the reaction of low-valence metal complexes with azides:

L_nM + N₃R → L_nMNR + N₂

A few imido complexes have been generated by the alkylation of

:

L_nMN⁻ + RX → L_nMNR + 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:

Mo•N₂ →

ammine

);

with the oxidation state of molybdenum varying to accommodate the number bonds from nitrogen.^[6]

References