Lithium bis(trimethylsilyl)amide is a lithiated organosilicon compound with the formula LiN(SiMe3)2. It is commonly abbreviated as LiHMDS (lithium hexamethyldisilamide - a reference to its conjugate acid HMDS) and is primarily used as a strong non-nucleophilic base and as a ligand. Like many lithium reagents it has a tendency to aggregate and will form a cyclic trimer in the absence of coordinating species.
Monomer (does not exist)
|Preferred IUPAC name
Hexamethyldisilazane lithium salt
3D model (JSmol)
|Molar mass||167.326 g/mol|
|Density||0.86 g/cm3 at 25 °C|
|Melting point||71 to 72 °C (160 to 162 °F; 344 to 345 K)|
|Boiling point||80 to 84 °C (176 to 183 °F; 353 to 357 K) (0.001 mm Hg)|
|Solubility||Most aprotic solvents
THF, hexane, toluene
|Main hazards||flammable, corrosive|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
- HN(SiMe3)2 + C4H9Li → LiN(SiMe3)2 + C4H10
Reactions and applicationsEdit
As a baseEdit
LiHMDS is often used in organic chemistry as a strong non-nucleophilic base. Its conjugate acid has a pKa of ~26 making it is less basic that other lithium bases, such as LDA (pKa of conjugate acid ~36), but it is more sterically hindered and hence less nucleophilic. It can be used to form various organolithium compounds including acetylides, or lithium enolates.
As a ligandEdit
- MXx + x Li(hmds) → M(hmds)x + x LiX
- (X = Cl, Br, I and sometimes F)
Metal bis(trimethylsilyl)amide complexes are lipophilic due to the ligand and hence are soluble in a range of nonpolar organic solvents, this often makes them more reactive than the corresponding metal halides, which can be difficult to solubilise. The steric bulk of the ligands causes their complexes to be discrete and monomeric; further increasing their reactivity. Having a built-in base, these compounds conveniently react with protic ligand precursors to give other metal complexes and hence are important precursors to more complex coordination compounds.
Like many organolithium reagents, lithium bis(trimethylsilyl)amide can form aggregates in solution. The extent of aggregation depends on the solvent. In coordinating solvents such as ethers and amines the so-called monomer and dimer are prevalent. In the monomeric and dimeric state, one or two solvent molecules bind to lithium centers. With ammonia as donor base lithium bis(trimethylsilyl)amide forms a trisolvated monomer that is stabilized by intermolecular hydrogen bonds. In noncoordinating solvents, such as aromatics or pentane, the complex oligomers predominate, including the trimer. In the solid state structure is trimeric.
LiHMDS adduct with TMEDA.
THF solvated dimer: (LiHMDS)2•THF2
Trimer, solvent free: (LiHMDS)3
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