The Li+ salt of the unsaturated anion [Mo2Cp2(mu-PCy2)(mu-CO)(2)](-) reacted with P-4 in tetrahydrofuran at room temperature to give the title complex. This fluxional anion reacted with MeI and ClCH2Ph to give the diphosphenyl complexes [Mo2Cp2(mu-kappa(2):kappa(2)-P2CH2R)(mu-PCy2)(CO)(2)] (R = H, Ph), with the incoming electrophile being attached at the basal P atom of the Mo2P2 tetrahedron via the lone electron pair (P-P-CH3 = 122.8(1)degrees). In contrast, reactions with ClER3 (ER3 = GePh3, SnPh3, PbMe3, PbPh3) gave neutral complexes [Mo2Cp2(mu-kappa(2):kappa(2)-P2ER3)(mu-PCy2)(CO)(2)] having the incoming electrophile attached at the basal P atom but defining an acute P-P-E angle close to 90 with elongated P-P lengths of ca. 2.20 angstrom. These complexes undergo an easy fluxional process involving an exchange of the ER3 group between the P atoms that could be properly modeled through DFT calculations, and some of them displayed minor isomers in solution. Their structure could be rationalized as derived from the interaction of the electrophile with high-energy orbitals of the anion having both sigma(Mo-P) and pi(P-P) bonding character. Reaction with BrSiMe3 gave instead the agostic phosphenyl complex [Mo2Cp2(mu-kappa(2):kappa(1),eta(2)-HP2)(mu-PCy2)(CO)(2)], formally derived from the attachment of a proton to a basal Mo-P edge of the anion (computed length 2.810 angstrom) and displaying an unusually low P-H coupling of 4 Hz. A similar structure, with the agostic H atom replaced with SnH3, was found to be a satisfactory model for the minor isomer of the tin compound and represents a third and unprecedented coordination mode of the diphosphorus ligand. The agostic complex undergoes a fluxional process involving the intermediacy of the nonagostic isomer [Mo2Cp2(mu-kappa(2):kappa(2)-P2H)(mu-PCy2)(CO)(2)], which was computed to display a geometry comparable to the major isomers of the ER3 compounds (P-P = 2.183 angstrom; P-P-H = 81.7 degrees).