New carbide clusters in the cobalt sub-group. Part 10. Preparation and crystallographic characterization of dicarbido-octa-µ-carbonyl-hexadecacarbonyl-polyhedro-dodecarhodate(2–) as its bis(triphenyl-phosphine)iminium salt, [N(PPh3)2]2[Rh12C2(CO)24]

Abstract

The title compound has been prepared by heating at 70 °C the dianion [Rh₆C(CO)₁₅]^2– and H₂SO₄ in propan-2-ol. Its molecular structure has been determined by single-crystal X-ray crystallography: triclinic, space group P, with a= 17.190(4), b= 13.049(4), c= 11.581(3)Å, α= 103.24(1), β= 88.69(1), γ= 102.24(1)°, and Z= 1. The structure has been solved from 3 421 diffraction intensities, collected by counter methods, and refined by least-squares calculations to R= 0.050. The dianion has idealized D_2h-mmm symmetry and contains a Rh₁₂ polyhedron based on a square–rhomb–square sequence of layers arranged in such a way as to form two prismatic cavities sharing one edge. Two carbon atoms are encapsulated in these cavities. Relevant average bond distances are Rh–C(carbide) 2.12 and Rh–Rh 2.81 Å. Eight CO ligands bridge the edges of the square layers and 16 are terminal, one per metal atom in the outer squares and two for each atom of the central rhomb. The Rh–C and C–O average distances are 1.89, 1.10, and 2.04, 1.17 Å for terminal and bridging groups, respectively.