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The synthetic chemistry of carbon is mostly concerned with tetravalent carbon compounds where all four valence electrons are used for chemical bonding. The divalent chemistry of carbene compounds played only a minor role until in 1991 Arduengo introduced imidazolin-2-ylidenes (Arduengo carbenes) as synthetically useful stable molecules. Carbodiphosphoranes were discovered in 1961 and give rise to the question concerning their electronic structure and reactivity. We want to present theoretical and experimental evidence suggesting that carbodiphosphoranes and analogous compounds are actually carbon(0) compounds where the valence electrons of carbon remain as two electron-lone pairs which are not engaged in chemical bonding in the free molecules. The chemistry of the latter compounds exhibits novel features which distinguish them from carbenes as shown for the example of N-heterocyclic carbenes. |
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