Carbo-[3]oxocarbon and its isomers: evaluation of the stability and of the electron delocalization.

Various isomers of carbo-[3]oxocarbon C9O3 have been characterized on the singlet and triplet spin state potential energy surfaces. Despite its localized structure, the ring carbo-mer of [3]oxocarbon is thermodynamically protected from subsequent isomerization and stable versus dissociation into C3O. It therefore appears as a reasonable synthetic target. In contrast, the less stable tetracyclic isomer exhibits electron delocalization both in the central six-membered ring and in the external three-membered rings, as evidenced from structural, magnetic and electron localization function (ELF) analysis. Another monocyclic isomer may be considered as a planar C6(CO)3 "carbon-complex", a carbon version of the related iron carbonyl complex Fe(PH3)2(CO)3. C6 and Fe(PH3)2 are indeed isolobal on the basis of frontier orbital analysis.