The ligand polyhedral model: its application to carbonyl clusters containing thirteen carbonyl groups

Abstract

The Ligand Polyhedral Model (LPM) has been applied to the series of carbonyl species [Fe₄(CO)₁₃]²⁻, [Fe₃Co(CO)₁₃]⁻, [Ru₃Co(CO)₁₃]⁻, [Ru₃Rh(CO)₁₃]⁻, [Ru₃Ir(CO)₁₃]⁻, [Os₃Ir(CO)₁₃]⁻ and [Co₂Ru₂(CO)₁₃] containing thirteen carbonyl ligands. The model predicts that the 13-vertex polyhedra with the minimum energies are the face-capped icosahedron and edge-bridged icosahedron and that they are very close in energy. Polyhedra with next higher energies are the variously capped cubeoctahedra and capped anti-cubeoctahedra. In this sense these 13-vertex arrangements are clearly related to the polyhedra adopted by the 12-CO species [Co₄(CO)₁₂] and [Ir₄(CO)₁₂]. Significantly, the structures of the carbonyl cluster anions [Fe₄(CO)₁₃]²⁻, [Fe₃Co(CO)₁₃]⁻, [Ru₃Co(CO)₁₃]⁻, [Ru₃Rh(CO)₁₃]⁻, [Ru₃Ir(CO)₁₃]⁻ (one isomer), and [Co₂Ru₂(CO)₁₃] all contain these capped icosahedra in line with prediction. These structures are discussed for clarity in terms of their close 12-CO relatives with an icosahedral CO shell with T or C_3v symmetry. Of special interest is the variety of orientations that the central, tetrahedral metal unit adopts within the capped icosahedron leading to the range of different structural forms observed experimentally. As anticipated, the anion [Os₃Ir(CO)₁₃]⁻ and a second isomer of the anion [Ru₃Ir(CO)₁₃]⁻ with a larger central metal core have a capped cubeoctahedral arrangement of thirteen CO ligands. The diverse range of structural types found for species containing thirteen carbonyl groups are readily understood in terms of the LPM. There is no other approach which so easily explains the observed structural types and isomeric forms seen.