Bonded or Not?

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Science  30 Jan 2009:
Vol. 323, Issue 5914, pp. 563
DOI: 10.1126/science.323.5914.563a

Propellane molecules intrigue chemists for a number of reasons, not least because their carbon frameworks resemble the eponymous macroscopic propellers. The [1.1.1] variety comprises three triangles that share an edge at the center, and a persistent question has been whether this shared edge, or bridge, constitutes a strained bond between the atoms at either end of it, or whether these atoms keep an electron to themselves after sharing three others with the vertices. Most evidence points to a bond of some sort between these bridgehead carbons, and similarly between tin atoms in stannous analogs that have been prepared and structurally characterized. Nied et al. have synthesized and obtained the crystal structure of a germanium (Ge) analog by lithium naphthalenide reduction of chloride salts, with the vertices capped by bulky mesityl groups. In the solid state, the bridgehead atoms are roughly 20% farther apart than the length of a typical Ge-Ge single bond. The absence of an electron paramagnetic resonance signal suggests a singlet spin state. Nonetheless, the authors' calculations support a degree of biradical character in the interaction, which is bolstered by observation of facile trimethyltin hydride addition across the bridge. — JSY

Angew. Chem. Int. Ed. 48, 10.1002/anie.200805289 (2009).

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