The presence of two low-frequency large-amplitude modes in the vibrational spectrum, experimental and computed data on B-B interatomic distances and the results of a topological analysis of the electron density distribution in the molecule of an 11-vertex monocarbaborate [2-CB10H11](-) lead to the conclusion that this polyhedron is not rigid, missing at least four double-center B-B bonds and thus being formed not only by triangular faces. Hence, this carbaborane does not have the closo-structure traditionally ascribed to it on formal [2n+2]-electron count grounds. (C) 2004 Elsevier B.V. All rights reserved.