Light triggers the formation of HNO from a metal-nitrosyl species, facilitated by an intramolecular pendant thiol proton. Two {FeNO}(6) complexes (the Enemark-Felthan notation), [Fe(NO)((TMS)PS2)((TMS) PS2H)] (1, (TMS)PS2H(2). = 2,2'-dimercapto-3,3'-bis(trimethylsilyl)diphenyl)phenylphosphine; H is a dissociable proton) with a pendant thiol and [Fe(NO)-(TMS)PS2)((TMS)PS2CH(3))] (2) bearing a pendant thioether, are spectroscopically and structurally characterized. Both complexes are highly sensitive to visible light. Upon photolysis, complex 2 undergoes NO dissociation to yield a mononuclear Fe(III) complex, [Fe((TMS)PS2)((TMS)PS2CH(3))] (3). In contrast, the pendant SH of 1 can act as a trap for the departing NO radical upon irradiation, resulting in the formation of an intermediate A with an intramolecular [SH center dot center dot center dot ON-Fe] interaction. As suggested by computational results (density functional theory), the NO stretching frequency (nu(NO)) is sensitive to the intramolecular interaction between the pendant ligand and the iron-bound NO, and a shift of nu(NO) from 1833 (1) to 1823 cm(-1) (A) is observed experimentally. Subsequent photolysis of the intermediate A results in HNO production and a thiyl group that then coordinates to the Fe center for the formation of [Fe((TMS) PS2)(2)] (4). In contrast with the common acid-base coupling pathway, the HNO is not voluntarily yielded from 1 but rather is generated by the photopromoted pathway. The photogenerated HNO can further react with [Mn-III((TMS)PS3)(DABCO)] ((TMS)PS3H(3) (2,2'2 ''-trimercapto-3,3',3 ''-tris(trimethylsilyl)triphenylphosphine; DABCO = 1,4-diazabicyclo[2.2.2]octane) in organic media to yield anionic [Mn(NO)((TMS)PS3)](-) (5(-)) with a {MnNO}(6) electronic configuration, whereas [Mn-III((TMS)PS3)(DABCO)] reacts with NO gas for the formation of a {MnNO}(5) species, [Mn(NO)((TMS)PS3)] (6). Effective differentiation of the formation of HNO from complex 1 with the pendant SH versus NO from 2 with the pendant SMe is achieved by the employment of [Mn-III((TMS)PS3)(DABCO)].