Carbones

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Carbones are a family of unusual carbon-containing molecules where a central carbon atom is in oxidation state zero and carries two unshared pairs of electrons. The molecule has the general formula CL2, with L being a very strong electron-donating ligand. The common L groups are phosphines (forming carbodiphosphoranes) or N-heterocyclic carbenes (carbodicarbenes). These ligands attach to the carbon and stabilize it through donor-acceptor bonds, keeping the carbon in a highly reactive, yet controlled, state.

Because of this arrangement, the central carbon has high-energy orbitals and behaves as a very strong Lewis base and nucleophile. It can accept protons and can also act as a powerful donor to metals, forming a wide range of metal complexes. The two lone pairs on carbon are key to this behavior, enabling back-and-forth electron sharing with attached ligands or bound metals.

There are two main families of carbones. Carbodiphosphoranes have two phosphine ligands, while carbodicarbenes have two N-heterocyclic carbene ligands. In both cases, the central carbon is bent rather than linear, and the carbon atom hosts two lone pairs that engage in donor-acceptor bonding with the ligands. This unusual bonding pattern is what gives carbones their distinctive reactivity.

The history of their synthesis shows how chemists learned to stabilize this unique carbon center. The first carbodiphosphorane was prepared in 1961 by reducing a suitable phosphorus-containing precursor. Over the years, researchers developed additional methods to make a wider range of carbodiphosphoranes. The first carbodicarbenes appeared in 2008, with later improvements allowing asymmetric versions and easier routes to the compounds.

In reactivity, carbones form strong and diverse complexes with many metals, including tungsten, nickel, copper, silver, and gold. A notable gold complex provided experimental support for the carbon(0) view, showing two lone pairs on carbon donating to gold atoms. Carbodicarbenes also coordinate to metals such as rhodium and palladium and can drive catalytic reactions, including olefin reduction and various cross-coupling processes.

Carbones also shine in main-group chemistry. Their powerful donating ability helps stabilize reactive species, leading to the formation of novel boron-containing ions, hydridoboron compounds, and even stable carbon–bismuth species. They have also supported advances in beryllium chemistry, such as ring systems created through C–H activation.

In short, carbones feature a carbon(0) center with two lone pairs that acts as a exceptionally strong electron donor. This makes them flexible ligands for a wide range of metal and main-group chemistry, enabling new catalysts, unusual bonded species, and exciting applications in modern chemical research.