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Highly correlatedab initiowave functions within the UCCSD(T)-F12 approach have been used to map portions of the potential energy surfaces and to study the stability of the first excited triplet states of the NCS−/CNS−and NCO−/CNO−anions. Thesea3Π states for linear geometries, or their3A′ and3A″ bent components, correlate with the lowest dissociation asymptote of NCX−(X = S and O) along the NC-X coordinates. TheX1Σ+linear ground states of these anions are known to be stable with respect to dissociation to theX2Π ground state of the corresponding neutral molecule with a rather large electron affinity. Thea3Π state of the NCS−anion is positioned below theXstate of the neutral at long NC-S distances and its minimum of energy is found for bent geometries. The stability of its two components in bent geometries has been investigated, and it is found that some anionic forms are stable with respect to theXstate of the neutral. The linear CNS−and CNO−isomers present a minimum only at long CN−X distances, located below the minimum of their corresponding neutral CNX ground states. [ABSTRACT FROM PUBLISHER]