New carbide clusters in the cobalt subgroup. Part 13. Synthesis and chemical characterization of the anions [Co6C(CO)14]–, [Co6C(CO)15]2–, and [Co8C(CO)18]2–, and crystal structure of µ6-carbido-ennea-µ-carbonyl-hexacarbonyl-polyhedro-hexacobaltate(2–) as its benzyltrimethylammonium salt; a comparison with isostructural species

Abstract

The anions [Co₆C(CO)₁₅]^2– and [Co₈C(CO)₁₈]^2– have been prepared starting from compounds such as [Co₃C(CO)₉Cl], Na[Co(CO)₄], and [Co₄(CO)₁₂], and [Co₆C(CO)₁₄]^– has been obtained from [Co₆C(CO)₁₅]^2– by oxidation with FeCl₃ or iodine. The chemistry and relationships between these anions are discussed. The molecular structure of [Co₆C(CO)₁₅]^2– has been determined by single-crystal X-ray crystallography. It is monoclinic, space group C2/c, with a= 21.752(2), b= 11.350(1), c= 18.099(2)Å, β= 112.28(1)°, and Z= 4. The structure has been solved from 2 184 reflections, collected by counter methods, and refined by least-squares calculations to R= 0.074. The dianion has idealized D_3d symmetry and contains a trigonal prism of cobalt atoms, whose cavity accommodates an interstitial carbon atom. The structural parameters are carefully compared with those of the isostructural anions [Rh₆C(CO)₁₅]^2– and [M₆N(CO)₁₅]^–(M = Co or Rh) to show how the metal atoms, the interstitial species, and the anionic charge affect the bond parameters.