Although in principle transition metals can form bonds with six shared electron pairs, only quadruply bonded compounds can be isolated as stable species at room temperature. Here we show that the reduction of {Cr(μ-Cl)Ar′} ₂ [where Ar′ indicates C ₆ H ₃ -2,6(C ₆ H ₃ -2,6-Pr ⁱ ₂ ) ₂ and Pr indicates isopropyl] with a slight excess of potassium graphite has produced a stable compound with fivefold chromium-chromium (Cr–Cr) bonding. The very air- and moisture-sensitive dark red crystals of Ar′CrCrAr′ were isolated with greater than 40% yield. X-ray diffraction revealed a Cr–Cr bond length of 1.8351(4) angstroms (where the number in parentheses indicates the standard deviation) and a planar transbent core geometry. These data, the structure's temperature-independent paramagnetism, and computational studies support the sharing of five electron pairs in five bonding molecular orbitals between two 3d ⁵ chromium(I) ions.

Ammonia borane (H₃N-BH₃, AB) is a lightweight material containing a high density of hydrogen (H₂) that can be readily liberated for use in fuel cell–powered applications. However, in the absence of a straightforward, efficient method for regenerating AB from dehydrogenated polymeric spent fuel, its full potential as a viable H₂ storage material will not be realized. We demonstrate that the spent fuel type derived from the removal of greater than two equivalents of H₂ per molecule of AB (i.e., polyborazylene, PB) can be converted back to AB nearly quantitatively by 24-hour treatment with hydrazine (N₂H₄) in liquid ammonia (NH₃) at 40°C in a sealed pressure vessel.