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IUPAC name
Other names
Dimethylborane dimer
3D model (JSmol)
Molar mass 83.777
Appearance Colorless liquid
Odor Pungent;
Melting point −72.5 °C (−98.5 °F; 200.7 K)
Boiling point 68.6 °C (155.5 °F; 341.8 K)
NFPA 704
Related compounds
Related alkyl boranes
Related compounds
tetramethyl aluminium hydride
tetramethyl gallium hydride


Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N (what is ☑Y☒N ?)
Infobox references

Dimethylborane, (CH3)2BH is the simplest dialkylborane, consisting of a methyl group substituted for a hydrogen in borane. As for other boranes it normally exists in the form of a dimer called tetramethyldiborane or TMDB (CH3)2BH)2.[2] Other combinations of methylation occur on diborane, including monomethyldiborane, trimethyldiborane, 1,2-dimethylborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms.[3] The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.[4][5]


Dimethylborane is formed when lithium dimethylborohydride Li(CH3)2BH2 reacts with an acid.[6] The lithium dimethylborohydride can be made from a dimethylborinic ester and lithium monoethoxy aluminium hydride.[6]

Methylboranes are also formed by the reaction of diborane and trimethylborane. This reaction produces four different substitutions of methyl with hydrogen on diborane. Produced is 1-methyldiborane, 1,1-dimethyldborane, 1,1,2-trimethyldiborane and 1,1,2,2-tetramethyldiborane.[3] The latter is maximised when trimethylborane is six times the concentration of diborane.[7]

Other methods to form methyldiboranes include reacting hydrogen with trimethylborane between 80 and 200 °C under pressure, or reacting a metal borohydride with trimethylborane in the presence of hydrogen chloride, aluminium chloride or boron trichloride. If the borohydride is sodium borohydride, then methane is a side product. If the metal is lithium then no methane is produced.[4] dimethylchloroborane and methyldichloroborane are also produced as gaseous products.[4]

Atomic hydrogen converts trimethylborane on a graphene monolayer surface to dimethylborane which dimerises to tetramethyldiborane.[8]


Tetramethyldiborane has two boron atoms linked by a two hydrogen atom bridge, and each boron is linked to two methyl groups. A Tetramethyldiborane molecule belongs to the D2h point group. Its infrared spectrum shows a strong absorption band at 1602 cm−1 due to bridging hydrogen, a weak band at 1968 cm−1 and lines due to methyl between 900 and 1400 cm−1.[9] In the molecule the boron to hydrogen distance is 1.36 Å, the boron to boron distance is 1.84 Å; the boron to carbon distance is 1.590 Å; the angle of boron-boron to carbon is 120.0°; the boron-carbon-hydrogen angle is 112.0°.[10] The NMR J coupling between two boron-11 nuclei in tetramethyldiborane is 55 Hz.[11]

Tetramethyldiborane melts at -72.5 °C and boils at 68.6 °C.[12] Vapour pressure is approximated by Log P = 7.687-(1643/T).[12] Tetramethyldiborane has a vapour pressure of 48 mm Hg at 0 °C.[7] Heat of vapourisation was measured at 7.3 kcal/mol.[13] The predicted heat of formation for the liquid is ΔH0f=-65 kcal/mol, and for the gas -57 kcal/mol.[13]

A gas chromatograph can be used to determine the amounts of the methyl boranes in a mixture. The order they pass through are diborane, monomethyldiborane, trimethylborane, 1,1-dimethyldiborane, 1,2-dimethyldiborane, trimethyldiborane, and lastly tetramethyldiborane.[14]

The nuclear resonance shift for the bridge hydrogen is 8.90 ppm, compared to 10.49 for diborane.[15]


Dimethylborane reacts with alkenes with the highest yield in ether to produce a dimethylalkylborane.[6] The dimethylalkylboranes can then be converted to a tertiary alcohol by oxidative carbonylation. This requires heating to 150° with carbon monoxide under 50 bars of pressure, and then oxidation with hydrogen peroxide.[16]

Methylboranes such as tetramethyldiborane disproportionate in the gas phase to trimethylborane and diborane at room temperature.[3] The time period is on the order of a few hours, and disproportionation is faster the higher the temperature.[4] At 0 °C disproportionation takes about a day.[4] At -78.5 °C methyldiborane disproportionates slowly first to diborane and 1,1-dimethyldiborane.[17] In solution methylborane is more stable against disproportionation than dimethylborane.[6]

4(CH3)3B2H3 ⇌ (CH3)4B2H2 + B2H6 K=0.0067.[18]
3B2H2Me4 ⇌ 2 B2H3Me3 + 2 BMe3

Dimethylborane is hydrolyzed in water to Dimethylborinic acid (CH3)2BOH.[3]

Dimethyldiborane spontaneously inflames when exposed to air.[17]

Ammonia and tetramethyldiborane combine to form a white solid at -78 °C. The solid decomposes above 10 °C.[19] The structure of the solid is ionic [(CH3)2B(NH3)2]+ [(CH3)2BH2].[19][20] A simple adduct BHMe3.N3 is formed from tetramethyldiborane and ammonia in ether. This also forms during the thermal decomposition of the diammoniate.[21]

Acetonitrile reacts slowly with tetramethyldiborane at room temperature to form dimeric ethylideneaminodimethylborane (CH3CH=NB(CH3)2)2. This has a cis and a trans isomer, one melting at 76 °C and another at -5 °C.[22]

Tetramethyldiborane reacts with sodium in liquid ammonia to make a salt with formula Na2HB(CH3)2 called sodium dimethylboryl. The salt is white and stable to 90 °C.[21] With potassium K2HB(CH3)2 potassium dimethylboryl is formed.[23] Calcium metal react with tetramethyldiborane to make CaHB(CH3)2.NH3.[21]

Tetramethyldiborane combines with dimethylphosphine to yield an adduct of dimethylborane.[21]

Tetramethyldiborane reacts with organic borates to form methylboronic esters.

2 (CH3)4B2H2 + 4 B(OR)3 ⇌ 6 CH3(OR)2 + (CH3)2B2H4.[24]

Tetramethyldiborane acts as a catalyst to enable the same results from trimethylborane:

(CH3)3B + 2 B(OR)3 → 3 CH3(OR)2[24]


The tetramethylborate anion (CH3)4B only has one boron atom.[25]


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