2-(3′,5′-Dimethylphenyl)-5,7-dimethylstibindolyl potassium–pmdeta (1) (pmdeta = pentamethyldiethylenetriamine) is obtained from trimesitylantimony, potassium and the amine ligand in tetrahydrofuran; crystals of 1 consist of stacks of (pmdeta)K⁺ ions and stibindolyl anions with alternating Sb–K and η⁵-SbC₄–K interactions.