The CO substitution reactions of the complex, $Cp(S-2,4-Me_2C_5H_5)CrCo$ 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 took place by first-order (dissociative) pathway. Activation parameters in decaline were ${Delta}H^{ eq};=;22.0;kcal{cdot}mol^{-1}$, ${Delta}S^{ eq}=;-3.8cal{cdot}mol^{-1}{cdot}K^{-1}$. Unusually low value of ${Delta}S{ eq}$ suggests an ${eta}^5-S{ o}{eta}^5-U$ conversion of the pentadienyl ligand. This suggestion was confirmed by the Extended-Huckel molecular orbital (EHMO) calculations, which revealed that the total energy of $Cp(S-2,4-Me_2C_5H_5$)CrCO is about 0.42 kcal/mol more lower than that of $Cp(U-2,4-Me_2C_5H_5)CrCO$ and the energy of $[Cp(U-2,4-Me_2C_5H_5)Cr{cdots}CO]^{ eq} $ transition state is about 2.43 kcal/mol lower than that of $[Cp(S-2,4-Me_2C_5H_5)Cr{cdots}CO]^{ eq}$ transition state.