Spin‐Controlled Binding of Carbon Dioxide by an Iron Center: Insights from Ultrafast Mid‐Infrared Spectroscopy

The influence of the spin on the mode of binding between carbon dioxide (CO 2 ) and a transition‐metal (TM) center is an entirely open question. Herein, we use an iron(III) oxalato complex with nearly vanishing doublet–sextet gap, and its ultrafast photolysis, to generate TM‐CO 2 bonding patterns and determine their structure in situ by femtosecond mid‐infrared spectroscopy. The formation of the nascent TM‐CO 2 species according to [L 4 Fe III (C 2 O 4 )] + + hν → [L 4 Fe(CO 2 )] + + CO 2 , with L 4 =cyclam, is evidenced by the coincident appearance of the characteristic asymmetric stretching absorption of the CO 2 ‐ligand between 1600 cm −1 and 1800 cm −1 and that of the free CO 2 ‐co‐fragment near 2337 cm −1 . On the high‐spin surface ( S =5/2), the product complex features a bent carbon dioxide radical anion ligand that is O‐“end‐on”‐bound to the metal. In contrast, on the intermediate‐spin and low‐spin surfaces, the product exhibits a “side‐on”‐bound, bent carbon dioxide ligand that has either a partial open‐shell (for S =3/2) or fully closed‐shell character (for S =1/2).