Infrared absorption spectrometry is often used to analyze metal carbonyl complexes. In carbon monoxide gas, the vibration of C-O bond (generally expressed by νCO) appears at the position of 2 143 cm in the spectrum. The position of νCO is negatively correlated with the bonding strength between metal and carbon. The material location is co 2143ti (co) 61748v (co) 61859cr (co) 6 2000mn (co) 6 2100fe (co) 6 204fe (co) 5. The spectra of weakly symmetric complexes will also be more complicated. For example, in the spectrum of Fe2(CO)9, the vibration frequency of CO bond appears at 2082,2019 and 1829 cm.

The COordination mode of co ligands can also be seen in νco clusters. The νCO of the bridged (μ2) carbonyl ligand will be lower than that of the general capped carbonyl ligand 100-200 cm. The νCO of μ3 carbonyl ligand will be lower. The following are the νCO of typical rhodium clusters: carbonyl ligand positions Rh2 (Co) 8 2060, 2084 1846,1862 rh4 (co)12 2044, 2070, 20741884.