[Rh12Sb(CO)27]3-. an example of encapsulation of antimony by a transition metal carbonyl cluster

The reaction of Rh(CO)₂acac with triphenylantimony in the presence of cesium benzoate in tetraethylene glycol/dimethyl ether solution resulted in the selective formation of Rh₁₂Sb(CO)₂₇]^3- (66% yield) after 3 h of contact time under ≈400 atm of carbon monixide and hydrogen (CO/H₂  1) at 140–160°C. The cluster has been isolated as the Cs(18-Crown-6)₂]⁺, (CH₃)₄]⁺, (C₂H₅)₄N]⁺, (Ph₃P)₂N]⁺ and PhCH₂N(C₂H₅)₃]⁺ salts. The (C₂H₅)₄N]₃ Rh₁₂Sb(CO)₂₇] complex has been characterized via a complete three-dimensional X-ray diffraction study. The complex crystallizes in the space group R$\text{3}$c with a  23.258(13) Å, c  22.811(4) Å, V  10 686 ų and p(calcd.)  2.334 g cm^-3 for mol.wt. 2503.66 and Z  6. Diffraction data were collected with an Enraf-Nonius CAD 4 automated diffractometer using graphite-monochromatized Mo-Kα radiation. The structure was solved by direct methods and refined by difference-Fourier and least-squares techniques. All non-hydrogen atoms have been located and refined: final discrepancy indices are R_f  3.5% and R_wf  4.6% for 3011 reflections. The anion's structure consists of twelve rhodium atoms situated at the corners of a distorted icosahedron with contacts of 2.807(1), 2.861(1), 2.874(1), 2.999(1), 3.017(1) and 3.334(1) Å and rhodium—antimony contacts of 2.712(0) Å. Rhodium—rhodium bond distances of 2.807 and 3.017 Å are in the range usually found for these complexes although a distance of 3.334 Å may be longer than expected from bonding interactions. The sum of the covalent radii of antimony and rhodium, 2.80 Å, is intermediate between the two observed RhSb contacts. The anion cluster structure is that of distorted icosahedron. This polyhedron has previously been found in B₁₂H₁₂]^2- but not with transition metal clusters. A comparison between the structures of rhodium carbonyl clusters and boranes shows the occurrence of similar structural features. Applications of bonding theories based on the boranes, such as Wade's rules, to rhodium carbonyl clusters shows the extent in which these rules are obeyed.