Theoretical observation of hexaatomic molecules containing pentacoordinate planar carbon

The smallest molecules up to date containing a D _5h pentacoordinate planar carbon (PPC) atom, CBe5 and CBe₅ ⁴⁻, are presented by means of ab initio calculations. To gain a better understanding about which electronic factors contribute to their stabilization, natural bond orbital (NBO) analysis and the nucleus independent chemical shifts (NICS) were calculated. The data reported here suggest that D _5h CBe₅ is σ aromaticity in nature, while in D _5h CBe₅ ⁴⁻ π aromaticity is dominating. The classical octet rule is well satisfied in both molecules, and is one of the fundamental reasons to understand the stability of the pentagon structures. The Be₅ ring serves as σ donor in D _5h CBe₅, and π-acceptor in D _5h CBe₅ ⁴⁻. The D _5h CBe₅ ⁴⁻ possessing 18 valence electrons with a closed-shell electron configuration is the most plausible candidate for experimental detection. Supported by the Research Fund for the Doctoral Program of Higher Education (Grant No. 20070533142), the China Postdoctoral Science Foundation (Grant No. 20070410139), the 111 Project of China (Grant No. B07012) and the National Natural Science Foundation of China (Grant No. 20802093)