New carbide clusters in the cobalt sub-group. Part 7. Preparation and structural characterization of carbido-hexa-µ-carbonyl-heptacarbonyl-polyhedro-hexarhodate(2–) as its bis(tetraphenylphosphonium) salt

Abstract

The anion [Rh₆C(CO)₁₃]^2– has been prepared by refluxing a solution of K₂[Rh₆C(CO)₁₅]·6CH₃O(CH₂)₄OCH₃ in PrⁱOH at 100 °C for 2 h under nitrogen and pumping off the evolved CO. The new anion has been characterized by a crystallographic study on the salt [PPh₄]₂[Rh₆C(CO)₁₃], which crystallizes in the monoclinic space group P2₁/c with cell constants a= 24.232(8), b= 12.587(5), c= 19.754(7)Å, and β= 92.0(5)°. The structure has been solved by direct methods and refined by least-squares calculations to R= 0.055 for 4 350 significant diffraction intensities. The anion contains a distorted octahedron of rhodium atoms with the carbide ion in the centre [Rh–Rh 2.733–3.188(2), Rh–C(carbide) 1.99–2.15(1)Å]. Six of the carbonyl groups are edge bridging, spanning consecutive edges of the three octahedron equators, and seven are terminal. The idealized molecular symmetry is C_s-m. A strong dependence of the metal–metal distances on the ligand geometry has been noticed, and the driving force of the octahedron deformation is very likely the steric requirement of the interstitial carbon.