The phosphinidene complex [Mo2Cp(mu-kappa(1):kappa(1),eta(5)-PC5H4)(CO)(2)(eta(6)-R*H)] (2; Cp = eta(5)-C5H5; R* = 2,4,6-(C6H2Bu3)-Bu-t) has substantially different Mo-P bonds and displays a high reactivity located at the short Mo-P bond. Sideways cycloaddition or addition processes are observed toward RCCR, HCl, and [Fe-2(CO)(9)], to give respectively metallacyclobutene and arylphosphide-bridged and heterometallic phosphinidene-bridged derivatives, a behavior reminiscent of the nucleophilic mononuclear phosphinidene complexes (carbene-like behavior), which is in good agreement with the ground-state electronic structure of 2 derived from density functional theory calculations. However, the reaction of 2 with [Co-2(CO)(8)] implies the addition of two cobalt fragments to its short Mo-P bond and thus reveals a carbyne-like behavior of compound 2. In most of the new products, the P atom displays an unprecedented trigonal-pyramidal-like environment, instead of the expected tetrahedral distribution of bonds.