In this Letter, a comprehensive ab initio post-HF study was conducted on the geometry and vibrational spectrum of cyclic, hyper-coordinate carbon containing (B6C)(-2) dianion. The analysis reveals extraordinary sensitivity of out-of-plane bending mode of this molecule to the diffuse functions of Pople-type basis sets. Accordingly, this molecule seems to be another example of the set of recently described planar molecules that exhibit spurious (but artificial) 'imaginary' out-of-plane bending frequencies at post-HF levels. The analysis eventually comes to the conclusion that this anion is indeed stable with regard to nuclear distortions, but unstable in regard to electron auto-detachment. (c) 2007 Elsevier B.V. All rights reserved.