New carbide clusters in the cobalt subgroup. Part II. Crystallographic characterization of the di-µ3-carbonyl-hexa-µ-carbonyl-carbidoundecacarbonyl-polyhedro-octarhodium

Abstract

The title compound crystallizes in the triclinic space group P with cell constants a= 9·18(1), b= 17·76(2), c= 10·46(1)Å, α= 75° 57′(6′), β= 69° 4′(6′), γ= 92° 22′(6′), Z= 2. The structure was determined by Patterson and Fourier methods from X-ray single-crystal counter data and refined by block-diagonal least-squares calculations to R 0·0234 for 3423 significant reflections.

The molecule has C₁ symmetry. The Rh₈ metal atom cluster can be described as a monocapped prism plus one edge-bridging atom; Rh–Rh distances range from 2·699 to 2·913(3), mean 2·81 Å. The carbide atom occupies the centre of the prism with mean Rh–C 2·127 Å. The carbonyl groups are distributed on the cluster surface: two triple bridging, six-edge-bridging, and eleven linearly; their bonding parameters are as expected. The cluster geometry and the ligand stereochemistry suggest the most probable mechanism of formation of the molecule to consist in the insertion of two [Rh(CO)₂]⁺ groups on the [Rh₆(CO)₁₅C]^2– anion.