The equilibrium structure of Ru(CO)(5) in benzene has been investigated using FTIR spectroscopy and DFT calculations. It was determined that 68% of the Ru(CO)(5) molecules form a weak complex with a single benzene molecule under ambient conditions. At ambient temperature a Gibbs free energy of -1.92 kJ/mol and an equilibrium constant of 2.16 was determined for the complexation reaction. The DFT calculations predict that the complex formation occurs via electron donation from the complexing benzene to the carbonyl ligands in Ru(CO)(5) at an optimum distance of similar to 500 pm from the metal center to the benzene plane. (C) 2011 Elsevier B. V. All rights reserved.