The reactivity of the "masked" terminal nickel sulfide complex, [K(18-crown-6)][(L-tBu)Ni-II(S)] (L-tBu = {(2,6-(Pr2C6H3)-Pr-i)NC(Bu-t)}(2)CH), with the biologically important small molecules CO and NO, was surveyed. [K(18-crown-6)][(L-tBu)Ni-II(S)] reacts with carbon monoxide (CO) via addition across the Ni-S bond to give a carbonyl sulfide complex, [K(18-crown-6)][(L-tBu)Ni-II(S,C:eta(2)-COS)] (1). Additionally, [K(18-crown-6)][(LtBu)NiII(S)] reacts with nitric oxide (NO) to yield a nickel nitrosyl, [(L-tBu)Ni(NO)] (2), and a perthionitrite anion, [K(18-crown-6)][SSNO] (3). The isolation of 3 from this reaction confirms, for the first time, that transition metal sulfides can react with NO to form the biologically important [SSNO](-) anion.