The Energy Difference Between the Triply-Bridged and All-Terminal Structures of Co₄(CO)₁₂, Rh₄(CO)₁₂, and Ir₄(CO)₁₂: A Difficult Test for Conventional Density Functional Methods

The M4(CO)12 molecules Co4(CO)12, Rh4(CO)12, and Ir4(CO)12 have two low-lying structures, the all-terminal structure with Td symmetry and the triply bridged structure with C3v symmetry. A total of 45 density functional theory (DFT) methods have been used to predict structures and vibrational frequencies for Co4(CO)12, Rh4(CO)12, and Ir4(CO)12. The different DFT methods show a broad range of energy differences ΔE = E(Td) - E(C3v). For Rh4(CO)12, none of the 45 DFT predictions is within 11 kcal/mol of the 2005 experimental value of 5.1 ± 0.6 kcal/mol reported by Allian and Garland (Dalton Trans. 2005, 1957 - 1965). For the challenging Ir4(CO)12 molecule, 21 DFT methods predict the correct Td structure, while 24 DFT methods predict the C3v structure to lie lower in energy. This research reveals many peculiar problems in the computation of the vibrational frequencies for the all-terminal structure.