The transition metal carbonylate, PPN+(${eta}^5-MeCp)Mn(CO)_2Cl^-$ undergoes a novel ligand substitution reaction with PR3 (R=Me, Et, OEt, $C_6H_5$ in THF at elevated temperatures (40 $^{circ}C$ up to 60 $^{circ}C)$ under the pseudo-first-order reaction conditions (usually 20-fold excess of PR3 with respect to metal carbonylate concentrations) where chloride is displaced by PR3. The reaction follows overall first order dependence on [(${eta}^5-MeCp)Mn(CO)_2Cl^-$]; however, the negative entropy changes of activation (${Delta}S^{ eq}$=-19.3 e.u. for $P(OEt)_3$; ${Delta}S^{ eq}$=-16.4 e.u. for $PPh_3$) suggest the existence of the intermediate, ((η3-MeCp)Mn(CO)2(THF)Cl-, which eventually transforms to the product (${eta}^5-MeCp)Mn(CO)_2(PR_3)$.