With help of quantum chemical MP2(full)/6-311+G** and B3LYP/6-311+G**
calculations we are studying the novel type of non-classical systems CO4M4
(M=Li, Na, K) containing hypercoordinated carbon. Energies, geometrical and
electronic characteristics of these non-classical structures 1 and 2 have been
compared with well-known classical systems like 3 and 4.

In classical 3 and 4 and other similar systems, carbonate anion form
usual classical bonds with surrounding ligands. At the same time, nonclassical
stereochemistry has been found in structures 1 and 2. According to Bader’s
topological and NBO analyses, central carbon form eight covalent bonds with
the four oxygens and four alkali metals. Thus carbon atom is octacoordinated in
these systems. The C-M (M=Li, Na, K) bond lengths in structures 3 correspond
to covalent values for all metals: ~1.9Å (M=Li), ~2.3Å (M=Na), ~2.6Å (M=K).
The metal cation-carbon anion interaction energies decrease in the row Li,
Na, K for all structural types. The charge on the carbon strongly increases in the
same row.
This work was supported by the Russian Foundation for Basic Research
(grant nos. 04-03-32538).