AbstractAn empirical linear relationship between the ratio of cis interaction constants and the ratio of stretching force constants was derived and a method for determining the ratio of cis interaction constants for ligand substituted metal carbonyls was proposed. Results of the present work would enable us to assess the validity of the Cotton‐Kraihanzel approximation of k=kc and to evaluate the correctness and the reliability of Some anomalous carbonyl force constants evaluated with other methods. The reliable correlation of the ratio of stretching force constants with the ratio of interaction constants would be helpful to clarify some ambiguous vibrational assignments made previously for the carbonyl bands of metal carbonyls, or to analyze isotope spectra of carbonyl bands for other related molecules in the same series.