Singlet–Triplet Transitions in the Dianionic Complex of Fullerene C 60 with Iron Carbonyl: (TBA +) 2 Fe 2 (CO) 6 -η 2, η 2 -C 60 2–

Interaction of fullerene C60 with Cs2Fe (CO) 4 in the presence of two equivalents of tetrabutylammonium iodide (TBAI) yields the first dianionic coordination complex of fullerene: (TBA+) 2Fe2 (CO) 6-η2, η2-C602- (1). According to magnetic data and DFT calculations, complex 1 has a diamagnetic ground (S = 0) and a triplet excited state (S = 1) populated above 80 K. As a result, a new broad EPR signal appears, whose integral intensity grows with temperature; the estimated singlet-triplet energy gap is 508 ± 8 K. The broad signal attributed to the triplet has g = 2. 0046 with a line width of 5. 6 mT at 140 K, and this signal is shifted to smaller g-factors (2. 0000) and narrows as the temperature increases. Calculations also show that the singlet-triplet gap is defined by the positions of the cations close to the Fe2 (CO) 6 fragment or the fullerene cage. According to DFT calculations, the negative electron density is mainly localized on the fullerene cage (-1. 051) in the singlet state, and this charge even increases to -1. 204 in the triplet state of the complex. Spin density localized on Fe2 (CO) 6 is concentrated on only one of the two iron atoms.