Volume 86 Issue 10 | p. 6 | Letters
Issue Date: March 10, 2008

Hazardous Reductions With Metal Hydrides

Department: Letters

A fire caused by the production of silane gas during the synthesis of silicon nanoparticles by reduction of SiCl4 by LiAlH4 was described in a safety letter several years ago (C&EN, Dec. 19, 2005, page 4). The letter points out serious hazards associated with a preparation described in the chemical literature and has been valuable to the chemistry community.

As a result of this letter, two errata have been published to correct the published procedure and warn of the release of silane (Phys.Rev. B 2007, 76, 199903; Appl. Phys. Lett. 2008, 92, 029902-1). A third erratum has been published warning of the production of germane (GeH4) in the analogous reduction using GeCl4 (Phys. Rev. B 2007, 76, 199904).

I would like to expand the warning issued in the previous letter to include attempts to reduce silicon, germanium, and tin halides with metal hydrides. Metal hydrides are not just powerful reducing agents, they are also capable of participating in halide/hydride metathesis reactions. The above-mentioned example of the metathesis reaction with SiCl4 to form silane gas has been reported to occur with as high as 99% yield (J. Am. Chem. Soc. 1947, 69, 2692). GeCl4 and SnCl4 can similarly be converted by LiAlH4 to germane and stannane with yields reported at 27.7% and 20.4%, respectively.

These reactions should not be conducted without planning for the handling of the hazardous gases that may be generated, nor should they be reported in the literature without mention of these hazardous by-products. The hazards in all cases are very serious: Silane is pyrophoric, germane is pyrophoric and extremely toxic, and stannane is flammable and extremely toxic. While the production of these gases is best documented in the case of LiAlH4, it is advisable to assume that other metal hydrides will similarly produce these gases unless it has been definitively shown that they do not.

Dale L. Huber
Albuquerque, N.M.

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