The title phosphinidene complex reacted at room temperature with CS2 and SCNPh to give the phosphanyl derivatives [Mo2Cp{mu-kappa(2)(P,S):kappa(1)(S'),eta(5)-P(CS2)C5H4}-(CO)(2) (eta(6)-HMes*)] and [Mo2Cp{mu-kappa(2)(P,S):kappa(1)(P),eta P5-(C(NPh)S)-C5H4}(CO)(2) (eta(6)-HMes*)] respectively (Mes* = 2,4,6-(C6H2Bu3)-Bu-t),which result from a [2 + 2] cycloaddition of a C=S bond in the organic reagent to the Mo=P bond of the phosphinidene complex, with further insertion of S into the remaining Mo-P bond, in the CS2 reaction. The title complex also reacted with diazoalkanes N2CRR' at room temperature to give the corresponding phosphaalkene derivatives [Mo2Cp{mu-eta(2):kappa(1)(P),eta(5)-P(CRR')C5H4}(CO)(2)(eta(6)-HMes*)] (CRR' = CH2, CPh2, CH(SiMe3)). These products follow from a formal [2 + 1] cycloaddition of the carbene CRR' fragment to the Mo=P bond of the parent compound but were shown to proceed through a [3 + 2] cycloaddition of the diazoalkane molecule, followed by N-2 elimination. The diazomethane reaction allowed the identification at low temperature of a stabilized form of the intermediate product, the phosphanyl complex [Mo2Cp{mu-kappa(2)(P,N):kappa(1)(P),eta(5)-P(CHN2H)C5H4}(CO)(2)(eta(6)-HMes*)], which follows from a reversible 1,3-shift of a methylenic H atom from C to N. It was concluded that all of the above cycloaddition reactions are initiated by heteroatom coordination of the unsaturated organic molecule to the MoCp(CO)(2) fragment in the parent phosphinidene complex, this triggering the P-C bond formation step which leads to the products eventually isolated. The structures of the new complexes were determined by spectroscopic, diffractometric, and (in some cases) density functional theory methods.