The four-coordinate scandium phosphinidene complex, [LSc(mu-PAr)](2) (L = (MeC(NDIPP)CHC(Me)-(NCH2CH2N(Pr-i)(2))), DIPP = 2,6-(Pr-i)(2)C6H3; Ar = 2,6-Me2C6H3) (1), has been synthesized in good yield, and its reactivity has been investigated. Although 1 has a bis(mu-phosphinidene)discandium structural unit, this coordinatively unsaturated complex. shows high and versatile reactivity toward a variety of substrates. First, two-electron reduction occurs when substrates as 2,2'-bipyridine, elemental selenium, elemental tellurium, Me3P=S, or Ph3P=E (E = S, Se) is used, resulting in the oxidative coupling of two phosphinidene ligands 2[PAr](2-) into a diphosphene ligand [ArP-PAr](2-). Complex 1 easily undergoes nucleophilic addition reactions with unsaturated substrates, such as benzylallene, benzonitrile, tert-butyl isocyanide, and CS2. This complex also shows a peculiar reactivity to CO and Mo(CO)(6), that includes C-P bond formation, C-C coupling and C-O bond cleavage of CO, to afford novel phosphorus-containing products. In the last two types of reactivity, reaction profiles have been computed (for the insertion of (BuNC)-Bu-t and the CO activation by 1) at the DFT level. The unexpected/surprising sequence of steps in the latter case is also revealed.