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C &EN reports that "the gas-phase hydride affinity of AlH3 to form AlH4- ... is the same as that for BH3 to form BH4-. This means the two anions should have the same reducing capabilities in the gas phase" and "the difference in reactivities of LiAlH4 and NaBH4 in solution is probably related to the cations and/or solvent properties rather than to the properties of the hydride anions" (C&EN, Aug. 29, 2005, page 27). These conclusions were attributed to Joseph S. Francisco, Paul G. Wenthold, and others (J. Am. Chem. Soc. 2005, 127, 11684).
Although in their otherwise excellent publication these authors do not explicitly state this, it is inferred by the last sentence of their conclusion: "Gas-phase studies find similar thermochemical properties for BH4- and AlH4-, highlighting the role of the media in determining their condensed-phase properties and reactivity."
The condensation of a full paper into a one-paragraph newsworthy highlight may have led to misconceptions, prompting the following commentary. The reactivity of a compound, provided the free-energy change ΔG of the involved reaction is sufficiently exothermic, is not determined by the magnitude of ΔG but by the height of its activation energy barrier ΔE. The BH4- and AlH4- anions are isoelectronic with CH4 and SiH4, respectively and, therefore, the two isoelectronic pairs can be expected to exhibit comparable reactivity trends. Whereas CH4 is coordination-wise saturated, silicon in SiH4 is not and prefers a coordination number of 6. Consequently, CH4 reactions exhibit very high activation energy barriers because they involve the complete breakage of a strong C-H bond as the first step.
By contrast, SiH4 is extremely reactive due to the availability of free coordination sites on Si allowing attack by a reagent without prior breakage of a Si-H bond. For example, CH4 can be mixed with oxygen or air requiring ignition to start a reaction, whereas SiH4 reacts spontaneously with air. This analogy should also hold for BH4- and AlH4- and explains the experimentally well-known higher reactivity of the latter.
Based on these arguments, the increased reactivity of AlH4- compared with that of BH4- is primarily related to the anions and not, as implied in the above publication and highlight, to cation or solvent effects.
Karl O. Christe
Los Angeles
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