The infrared spectra of a series of complexes containing the M(CO)2 group have been investigated, and absorption bands due to δ(MCO) and ν(MC) vibrations identified and reported. An attempt has been made to correlate the symmetry co-ordinates and observed band intensities for the δ(MCO) vibrations, and thereby to assign the spectra. The infrared active δ(MCO) vibrations of [Co(CO)4]2Hg, [Et3PCo(CO)3]2Hg, and [Et3PCo(CO)3]2 have also been assigned using the same principles.