Quantitative IR spectroscopic determination of the concentrations of cobalt carbonyl complexes in a three-component system

The three-component system consisting of Co₄(CO)₁₂, Co₂(CO)₈ and HCo(CO)₄ was analyzed by means of IR spectroscopy. A quantitative method was developed in order to enable the precise calculation of the concentrations of all three compounds simultaneously. The quantitative analysis was based upon the intensity of the bridging CO stretching bands at 1858 cm⁻¹ (A₂) and 1867 cm⁻¹ (A₁) of the polynuclear carbonyls, and the terminal CO symmetric stretching band of HCo(CO)₄ at 2116 cm⁻¹ (A₃). The mathematical expression for the concentrations of the three compounds required the precise knowledge of at least one of the four extinction coefficients of either Co₄(CO)₁₂, ϵ_J1 and ϵ_J2 or Co₂(CO)₈, ϵ_K1 and ϵ_K2. The reference extinction coefficient was ϵ_K2, because Co₂(CO)₈ was employed as the starting compound in all experiments performed in this study. In order to determine the extinction coefficient of HCo(CO)₄ at 2116 cm⁻¹, ϵ₃, intensities of this band were plotted as function of the corresponding concentrations of HCo(CO)₄, which were calculated by means of the three- component system method; from the slope of the straight line ϵ₃ could be directly calculated.