Dioxygen in the quintet O-2((5)Pi(g)) state is a weakly bound species near the entrance of the O(P-3) + O(P-3) recombination channel. It was predicted by ab initio calculations in 1977 and detected experimentally in 1999. Meantime, the O-2((5)Pi(g)) species was tentatively assumed as intermediate in transport properties calculations for the rarefied gases of the Earth's upper atmosphere, though its potential energy curve is still debated. Besides six other strongly bound low-lying states of dioxygen, the O-2((5)Pi(g)) state is an important potential candidate for modeling energy transfer and airglow of the upper atmosphere. A number of photochemical kinetic schemes designed to simulate energy flow upon atomic and molecular oxygen collisions in the rarefied mesosphere take into account a participation of the O-2((5)Pi(g)) state in energy relaxation processes responsible for terrestrial nightglow. All mechanisms of energy redistribution are based on the hard-sphere collision models. The possibility of chemical interactions between the quintet excited state of dioxygen and other atmospheric components has not been considered so far in photochemistry of the upper atmosphere. In the present paper, the chemical reactivity of the quintet O-2((5)Pi(g)) species is calculated for the first time in the framework of the density functional theory. Definitely, O-2((5)Pi(g)) is the most reactive species among all other metastable dioxygen states below 5.1 eV. Quintet products of the O-2((5)Pi(g)) state association with heavy inert gases, H2O, ND and CO2 are predicted to be chemically significant, while the complexes with abundant H-2 and He species are rather weak and not important even in the mesopause low-temperature region. The complex with N-2 molecule is unexpectedly stable with dissociation energy 4 kJ/mol, which can strongly influence the abundant termolecular association O + O + N-2 -> O-2 + N-2 process. Reaction with meteoritic ablated Mg atom produces metastable (5)A(1) excited state of MgO2 being more strongly bound than the ground (3)A(2) state of magnesium peroxide.