The CO substitution reactions in the complex, $Cp(S-C_5H_7)CrCO$ with $PR_3(PR_3=PMePh_2,;P(OCH_3)_3,;PMe_2Ph)$ were investigated spectrophotometrically at various temperatures. From the reaction rates, it was suggested that the CO substitution reaction takes place by first-order (dissociative) pathway. Activation parameters in decaline are ${Delta}H^{ eq};=;24.58;kcal;{cdot};mol^{-1},;{Delta}S^{ eq};=;3.05 cal;{cdot};mol^{-1}{cdot}K^{-1}$. Unusually low value of ${Delta}S^{ eq}$ suggests an ${eta}^5-S;{ o};{eta}^5-U$ conversion of the pentadienyl ligand. This was confirmed by the extended-Huckel molecular orbital (EHMO) calculations, which revealed that the total energy Of $Cp(S-C_5H_7)CrCO$ is about 6.84 kcal/mol more stable than that of $Cp(U-C_5H_7)CrCO$ and the energy of $[Cp(U-C_5H_7)CrCO^{ eq}$ transition state is about 4.25 kcal/mol lower than that of $[Cp(S-C_5H_7)Cr]^{ eq}$ transition state.