Zero Tolerance

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Science  17 Nov 2006:
Vol. 314, Issue 5802, pp. 1050
DOI: 10.1126/science.314.5802.1050d

In carbenes, a carbon atom engages only two of its valence electrons in bonds to other atoms, leaving the remaining two electrons free to react. Over 40 years ago, a different class of divalent carbon compound was prepared, termed a carbodiphosphorane (CDP), in which the central C was bound to two phosphines in a motif that has often been represented as cumulated double bonds: R3P=C=PR3, where R is a halide, amide, or hydrocarbon substituent. More recently, several stable free and complexed CDPs have been characterized, prompting Tonner et al. to explore the valence structure more thoroughly. Using quantum chemical calculations to analyze reported as well as model compounds, they find that unlike carbenes, CDPs are best described as donor-acceptor complexes: each phosphine datively donates two electrons to a central carbon, in the zero oxidation state, that has two essentially nonbonding lone pairs. The basicity of these lone pairs is borne out in a new compound, synthesized by the authors, that links two protonated CDP moieties to a silver cation. — JSY

Angew. Chem. Int. Ed. 45, 10.1002/anie.200602552 (2006).

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