The first heterobimetallic sandwich-type aggregate formed by bowl-shaped corannulene trianion-radicals, C20H10c3-, has been synthesized using mixed-metal reduction of C20H10. The product was crystallographically characterized to reveal the self-assembly of [Cs+//(C20H10.3-)/4K+/(C20H10 3-)//Cs+], in which two triply-charged corannulene decks encapsulate a rectangle of four potassium ions (the K/K separations are 4.212(4) and 5.185(4) °A), with the exterior concave bowl cavities being selectively filled by one cesium ion each. In order to provide insights into the geometrical features and electronic structure of this novel mixed-metal organometallic self-assembly, an in-depth theoretical investigation has been carried out...