By reacting monomethyldiborane with ether, dimethylether borine is formed [(CH3)2O].BH3 leaving methylborane which rapidly dimerises to 1,2-dimethyldiborane.

[8] 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.

[4] When Cp2Zr(CH3)2 reacts with borane dissolved in tetrahydrofuran, a borohydro group inserts into the zirconium carbon bond, and methyl diboranes are produced.

The molecule is expected to have a Cs point group due to rapid rotation of the methyls.

The infrared spectrum of trimethyldiborane has a strong absorption band at 2509 cm−1 due to the non-bridge boron-hydrogen bond.

[14] The nuclear resonance shift for the bridge hydrogen is 9.27 ppm, compared to 10.49 for diborane.

[15] Trimethyldiborane partially disproportionates over a period of hours at room temperature to yield tetramethyldiborane and 1,2-dimethyldiborane.

[18] Trimethyldiborane reacts with liquid ammonia initially forming methylborohydride anions and (CH3)2B(N3)2+ cations.