The coordination of acetonitrile and water to the Hg atom in [Hg(closo-1-CB11F11)(2)](2-) (1) reveals the Lewis acidity of the Hg-II center, which is unprecedented, since 1 is a dianion. Both coordination compounds were characterized by single-crystal X-ray diffraction, vibrational spectroscopy, and differential scanning calorimetry (DSC). In contrast, the Hg atom in [PhHg(closo-1-CB11F11)](-) (2) does not coordinate to CH3CN and H2O, although it has only a single negative charge.