The CO substitution reactions in the complex, $Cp^*(C_5H_7)$CrCO with $PR_3(PR_3=PMePh_2,;P(OCH_3)_3,;PMe_2Ph)$ were investigated spectrophotometrically at various temperatures. For the reaction rates, it was suggested that the CO substitution reaction took place by first-order (dissociative) pathway. Activation parameters in decaline are ${Delta}H^{ eq}= 21.99{pm}2.4$ kcal/mol, ${Delta}S^{ eq}= 8.9{pm}7.1$ cal/mol·k. Unusually low value of ${Delta}S^{ eq}$, suggested an ${eta}^5-S{ o};{eta}^5$-U conversion of the pentadienyl ligand. At various temperature, the rates of reaction for the Cp(pdl)CrCO complexes increase in the order $Cp^*(C_5H_7)$-CrCO < Cp$(C_5H_7)$CrCO < Cp(2,4-$C_5H_{11}$)CrCO, which can be attributed to the usual steric acceration or electronic influence for the ligand substitution of metal complexes. This suggestion was confirmed by the extended-Huckel molecular orbital (EHMO) calculations, which revealed that the energy of $[Cp^*(U-C_5H_7)Cr]^{ eq}$ transition state is about 4.93 kcal/mol lower than that of [Cp(S-$C_5H_7)Cr]^{ eq}$ transition state, and the arrangement of the overlap populations between Cr and the carbon of CO is $Cp^*(C_5H_7)$CrCO > Cp($C_5H_7$)CrCO > Cp(2,4-$C_7H_{11}$)CrCO.