Why is borane often supplied as a complex with tetrahydrofuran?

Borane (${\text{BH}}_3$) is supplied as a complex with tetrahydrofuran (THF) because free ${\text{BH}}_3$ is highly unstable and cannot be isolated in its monomeric form.1

Borane is an electron-deficient compound that readily reacts with electron-donor molecules, known as Lewis bases, to form stable adducts. By forming a complex with THF—which acts as a Lewis base by donating an unshared pair of electrons to the boron atom—the boron atom achieves a stable, four-coordinated tetrahedral structure. This stabilization allows for the convenient storage and handling of the reagent, which is widely used in chemical synthesis, particularly for the hydroboration of unsaturated organic compounds like alkenes and alkynes.12