The effect of substitution, by tertiary phosphines, of the CO groups on the stability of the Mo–Mo bond in the compound [π-C5H5(CO)3Mo]2 has been studied. Three types of derivative are formed: (i) compounds in which one CO is replaced, viz., C5H5(CO)3Mo–Mo(CO)2LC5H5, e.g., where L = PPh3 or P(Cyclohexyl)3; (ii) those in which two CO groups are replaced symmetrically, viz., C5H5(CO)2L Mo–Mo L(CO)2C5H5, e.g., where L = P(OBu)3 and P(OPh)3; (iii) substances with salt-like structures, viz., [π-C5H5L2(CO)2Mo][π-C5H5(CO)3Mo], e.g., where L = excess of PPh3, PEt3, PBu3, and Ph2P·CH2·CH2·PPh2. The conditions for obtaining these different derivatives are discussed.