Synthesis and crystallographic characterization of the carbidopentadecacarbonylhexarhodate dianion in its bis(benzyltrimethylammonium) salt, the first example of a trigonal prismatic cluster of metal atoms

Abstract

Carbidopentadecacarbonylhexarhodate dianion has been obtained by reaction of dodecacarbonyltetrarhodium with methanolic sodium hydroxide under carbon monoxide with successive addition of chloroform. The structural characterization has been carried out on the salt [NMe₃(CH₂Ph)]₂[Rh₆(CO)₁₅C], which crystallizes in the monoclinic space group C2/c with a= 22·17(2), b= 11·46(1), c= 18·51 (2)Å, β= 112° 43′(6′), Z= 4. Diffractometer data have been refined by least-squares methods to a final R of 0·033 for 1654 statistically significant reflections. The anion [Rh₆(CO)₁₅C]^2– has precise C₂ and idealized D_3h symmetry. The six rhodium atoms define a trigonal prism whose edges are associated with symmetrical bridging CO groups, the remaining six carbonyl ligands are linearly bonded one per each rhodium atom. The carbide atom occupies the centre of the prism. The mean basal and inter-basal Rh–Rh distances are 2·776(3) and 2·817(2)Å respectively; mean Rh–C(carbide) 2·134(6)Å. The mean Rh–C and C–O distances for linear, basal bridging, and inter-basal bridging CO groups are 1·89(1), 1·13(1); 2·12(1), 1·14(2); and 2·04(1), 1·17(2)Å.