The structure, bonding, and reactivity of the monocarbonyl derivatives of closo-borate anions [B_nH_n–1CO]^– (n = 6, 10, 12) have been analyzed. The B–B, B–H, B–C and C–O bonding interactions in such anions have been theoretically examined using the Quantum Theory of Atoms in Molecules (QTAIM). Several local and integral topological properties of the electron density involved in these interactions have been computed, and different chemical reactivity descriptors have been calculated via conceptual density functional theory (DFT).