Real-time Crystallographic Capture of Fe–NO Bond Photodissociation in a Nonheme Iron Nitrosyl Complex

The interplay between nitric oxide (NO ^• ) and iron (Fe) centers in metalloenzymes is central to many biological functions, with light-induced Fe − NO bond dissociation emerging as a key regulatory mechanism. Here, we present direct structural evidence for the stepwise dissociation of the Fe − NO bond in a crystalline nonheme iron nitrosyl complex, captured through in-situ photocrystallography under visible light. Marked Fe − NO bond elongation reveals real-time snapshots of excited-state fission at atomic resolution. Solution-phase studies confirm the generality of the observed photoreactivity. Multiconfigurational CASSCF calculations show the ground state as a resonance among Fe(I), Fe(II), and Fe(III) configurations, with a photoactive Fe ^I –NO ⁺ contribution enabling NO ^• release via metal-to-ligand charge transfer. Strategic placement of electron-withdrawing chlorides at the para-position of the pyridyl moiety further amplifies this character, promoting efficient NO ^• dissociation. These findings delineate a detailed mechanism of Fe–NO bond cleavage and provide rare structural insights into transient photoinduced processes central to NO signaling and metalloenzyme function.