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).