Metal amides

Metal amides (systematic name metal azanides) are a class of coordination compounds composed of a metal center with amide ligands of the form NR₂⁻. Amido complexes of the parent amido ligand NH₂⁻ are rare compared to complexes with diorganylamido ligand, such as dimethylamido. Amide ligands have two electron pairs available for bonding.

• 
  Tris(dimethylamino)aluminium dimer^[1]
• 
  Tetrakis(dimethylamino)titanium
• 
  Pentakis(dimethylamido)tantalum

Geometry and structure

In principle, the M-NX₂ group could be pyramidal or planar. The pyramidal geometry is not observed.

In many complexes, the amido is a bridging ligand. Some examples have both bridging and terminal amido ligands. Bulky amide ligands have a lesser tendency to bridge. Amide ligands may participate in metal-ligand π-bonding giving a complex with the metal center being co-planar with the nitrogen and substituents. Metal bis(trimethylsilyl)amides form a significant subcategory of metal amide compounds. These compounds tend to be discrete and soluble in organic solvents.

Alkali metal amides

Main articles: lithium amide, sodium amide, and potassium amide

Lithium amides are the most important amides. They are prepared from n-butyllithium and the appropriate amine

R₂NH + BuLi → R₂NLi + BuH

The lithium amides are more common and more soluble than the other alkali metal analogs. Potassium amides are prepared by transmetallation of lithium amides with potassium t-butoxide (see also Schlosser base) or by reaction of the amine with potassium, potassium hydride, n-butylpotassium, or benzylpotassium.^[2]

The alkali metal amides, MNH₂ (M = Li, Na, K) are commercially available. Sodium amide (also known as sodamide) is synthesized from sodium metal and ammonia with ferric nitrate catalyst.^[3][4] The sodium compound is white, but the presence of metallic iron turns the commercial material gray.

2 Na + 2 NH₃ → 2 NaNH₂ + H₂

Lithium diisopropylamide is a popular non-nucleophilic base used in organic synthesis. Unlike many other bases, the steric bulk prevents this base from acting as a nucleophile. It is commercially available, usually as a solution in hexane. It may be readily prepared from n-butyllithium and diisopropylamine.

Main group amido complexes

Amido derivatives of main group elements are well developed.^[5]

Transition metal complexes

Early transition metal amides may be prepared by treating anhydrous metal chloride with alkali amide reagents. In some cases, two equivalents of a secondary amine can be used, one equivalent serving as a base:^[6]

MCl_n + n LiNR₂ → M(NR₂)_n + n LiCl

MCl_n + 2n HNR₂ → M(NR₂)_n + n HNR₂·HCl

Transition metal amide complexes have been prepared by these methods:^[6]

• treating a halide complex with an alkali amide
• deprotonation of a coordinated amine
• oxidative addition of an amine

Structure of the nitride-amido complex NMo(N(t-Bu)(C₆H₃Me₂)₃.^[7]

Amido-ammine complexes

Highly cationic metal ammine complexes such as [Pt(NH3)6]4+ spontaneously convert to the amido derivative:

[Pt(NH₃)₆]⁴⁺ ↔ [Pt(NH₃)₅(NH₂)]³⁺ + H⁺

Transition metal amides are intermediates in the base-induced substitution of transition metal ammine complexes. Thus, the Sn1CB mechanism for the displacement of chloride from chloropentamminecobalt chloride by hydroxide proceeds via an amido intermediate:^[8]

[Co(NH₃)₅Cl]²⁺ + OH⁻ → [Co(NH₃)₄(NH₂)]²⁺ + H₂O + Cl⁻

[Co(NH₃)₄NH₂]²⁺ + H₂O → [Co(NH₃)₅OH]²⁺

See also

• Inorganic imide

References

1. Ouzounis, K.; Riffel, H.; Hess, H.; Kohler, U.; Weidlein, J. (1983). "Dimethylaminoalane, H_3−nAl[N(CH₃)₂]_n, n = 1, 2, 3 Kristallstrukturen und Molekülspektren". Zeitschrift für anorganische und allgemeine Chemie. 504 (9): 67–76. doi:10.1002/zaac.19835040909.
2. Michael Lappert, Andrey Protchenko, Philip Power, Alexandra Seeber (2009). "2. Alkali Metal Amides". Metal Amide Chemistry. John Wiley & Sons. ISBN 978-0-470-74037-8.{{cite book}}: CS1 maint: multiple names: authors list (link)
3. Bergstrom, F. W. (1955). "Sodium Amide". Organic Syntheses; Collected Volumes, vol. 3, p. 778.
4. Greenlee, K. W.; Henne, A. L.; Fernelius, W. Conard (1946). "Sodium Amide". Inorganic Syntheses. Vol. 2. pp. 128–135. doi:10.1002/9780470132333.ch38. ISBN 978-0-470-13233-3. {{cite book}}: |journal= ignored (help)
5. Waggoner, K.M.; Olmstead, M.M.; Power, P.P. (1990). "Structural and spectroscopic characterization of the compounds [Al(NMe2)3]2, [Ga(NMe2)3]2, [(Me2N)2Al{μ-N(H)1-Ad}]2 (1-Ad = 1-adamantanyl) and [{Me(μ-NPh2)Al}2NPh(μ-C6H4)]". Polyhedron. 9 (2–3): 257–263. doi:10.1016/S0277-5387(00)80578-1.
6. John F. Hartwig (2009). "4. Covalent (X-Type) Ligands Bound Through Metal-Heteroatom Bonds". Organotransition Metal Chemistry: From Bonding to Catalysis. University Science Books. ISBN 978-1-891389-53-5.
7. Curley, J. J.; Cook, T. R.; Reece, S. Y.; Müller, P.; Cummins, C. C. (2008). "Shining Light on Dinitrogen Cleavage: Structural Features, Redox Chemistry, and Photochemistry of the Key Intermediate Bridging Dinitrogen Complex". Journal of the American Chemical Society. 130 (29): 9394–9405. doi:10.1021/ja8002638. PMID 18576632.
8. G. L. Miessler and D. A. Tarr "Inorganic Chemistry" 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6.