The title compd. (I), prepd. in two steps from pentaethyl-1,5-dicarba-closo-pentaborane(5), is characterized at 103 K as the low-temp. solid phase by a single-crystal x-ray structure anal. I reacts with alkali metals (K, Cs) to form the C4B6Et10 dianion I2-, which is easily oxidized to I (e.g., with I2, FeCl2, HgCl2). Ab initio MO and IGLO calcns. of I were carried out.