Five on Five

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Science  12 Feb 2010:
Vol. 327, Issue 5967, pp. 762
DOI: 10.1126/science.327.5967.762-c
CREDIT: KANO ET AL., NAT. CHEM. 2, 112 (2010)

In most molecular contexts, silicon behaves like carbon in forming covalent bonds to four neighboring atoms. Kano et al. have now coaxed the element into a more crowded motif. Specifically, by using lithium as an electron source, the authors reduced two four-coordinate silicon centers and brought them together to form a bond. The resulting dianion, characterized in solution as well as the solid state, proved remarkably stable, even persisting for days in boiling water. Protonation with acid liberated a substituent at each silicon, forming a product with more conventional four-coordinate centers, but the process was efficiently reversed on treatment with a strong base. The authors attribute the stability of the unusual five-coordinate bonding arrangement partly to the electron-withdrawing character of the surrounding substituents (benzyl alcohol derivatives bearing trifluoromethyl groups). Theoretical calculations supported a bond order between silicons approaching 1 and suggested that the silicon centers themselves bore positive charges, despite the overall dual negative charge of the complex.

Nat. Chem. 2, 112 (2010).

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