Density functional theory studies on a designed photoactive {FeNO}6 nitrosyl and the corresponding photoinactive {FeNO}7 species: Insight into the origin of NO photolability

► DFT and TDDFT calculations on a photoactive {FeNO}6 (1) and its photoinactive {FeNO}7 (2) congener have been performed. ► The calculations support Fe(II)-NO+ and Fe(II)-NO assignment for 1 and 2, respectively. ► Low energy transitions from MOs with appreciable carboxamido character to Fe-NO antibonding MOs in 1 leads to NO photolability. ► The excess electron density on the NO moiety in 2 does not favor such transitions and makes it insensitive to light.