A Boron Bridge

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Science  25 Mar 2005:
Vol. 307, Issue 5717, pp. 1843
DOI: 10.1126/science.307.5717.1843a

Boron compounds have been of continued fundamental interest because of their tendency to adopt unusual electron-deficient bonding. Unlike carbon, boron can form so-called 3-center, 2-electron bonds with two other atoms. Braunschweig et al. have now coaxed boron into a different arrangement, which resembles that of the central carbon in allene. They prepared two compounds in which a lone B atom bridges two transition metal centers: a pentamethylcyclopentadienyl iron dicarbonyl on one side, and either iron tetracarbonyl or chromium pentacarbonyl on the other. X-ray crystallography confirmed an essentially linear bridge structure in both compounds. Density functional theory suggests that the boron forms a traditional 2-electron σ bond with each metal, as well as a partial π bond. Similar compounds have been prepared with the heavier group 13 elements (gallium and thallium), but in those cases π bonding is absent. — JSY

Angew. Chem. Int. Ed. 44, 1658 (2005).

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