A rare class of multiply bonded trigonal-planar pnictogen complexes: Rational syntheses, versatile reactivities, and unique semiconducting properties

The main group-containing transition metal carbonyl complexes with multiple bond characters have attracted broad interest for synthetic chemists due to their reactive nature that can be applied in a wide range of potential applications. This short account will review the recent progress regarding the chemistry of two multiply bonded trigonal-planar chromium carbonyl pnictogen complexes, [PnCr₃(CO)₁₅]⁻ (Pn = Bi, 1a; Sb, 1b), in which their oxidation states of central pnictogens were well-defined as Bi(III) and Sb(0). Complexes 1a and 1b were found to exhibit pronounced Lewis acidity on the low-valent pnictogen centers, showing the unique color change in common organic solvents and high affinities toward various inorganic or organic nucleophiles to form functionalized or heterometallic tetrahedral adducts. In addition, the unique mixed Cr–Fe trigonal-planar intermediates [PnCr₂Fe(CO)₁₄]⁻ (Pn = Bi, 4a; Sb, 4b) were also produced from the selective demetallation of [BiCr₃Fe(CO)₁₉]³⁻ (2) and hydride abstraction of [HSbCr₂Fe(CO)₁₄]²⁻ (3), which permitted us to produce a series of heterometallic Bi₂ or Sb₂ complexes. Finally, trigonal-planar complexes 1a and 1b exhibited semiconducting properties with low energy gaps and good electrical conductivities. The efficient electron transport was proved to arise from various secondary interactions through the supramolecular anionic 2D plane or 1D zig-zag chain, which were studied in detail with the aid of DFT calculations.