Synthesis of a Stable Compound with Fivefold Bonding Between Two Chromium(I) Centers

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([micro]-Cl)Ar'}₂ [where Ar' indicates C₆H₃-2,6(C₆H₃-2,6-Pr[superscript 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 3d⁵ chromium(I) ions.