The reaction of the normal isomer of eicosaborohydride anion (I) with potassium tert-butyl thiocarbonate (II) in refluxing THF produced the unexpected apical-apical isomer of the thiocarbonate borane (III). Acid cleavage of the thiocarbonate group afforded the target thiol derivative, which was isolated as the tetrasodium salt after treatment with an ion-exchange resin.