Abstract Cluster models are constructed for mononuclear Cr(II) and Cr(III) sites as well as dinuclear Cr(II) sites of the reduced Cr/SiO2-based Phillips catalyst. The binding energies and structures of oligocarbonyl complexes formed at these cluster models have been computed using density functional theory. Furthermore, harmonic frequencies and infrared intensities are calculated for the carbonyl stretching modes of these complexes. The resulting parameters are compared to literature spectra of CO-exposed Phillips catalysts and related model systems. A reassignment is proposed for the infrared spectrum of carbon monoxide on the reduced Cr/silica system.