Revisiting [Fe(bipy)2Et2]: electronic structure and new considerations on the thermal decomposition mechanism

• The previously proposed decomposition mechanism of [(bipy) 2 FeEt 2 ] is discredited. • Complex [(bipy) 2 FeEt 2 ] contains Fe III and a non-innocent (bipy) 2 •- ligand system. • Bipy/THF ligand exchange takes place with low energy cost and a spin state change. The present computational investigation discredits a previously proposed mechanism for the thermal decomposition of the title compound in THF, according to which β-H elimination from one Et ligand would occur after an initial rearrangement of one bipy ligand from κ 2 to κ 1 . It is alternatively proposed that the initial step is full replacement of one bipy ligand with one solvent molecule, to yield 5-coordinate [(bipy)(THF)FeEt 2 ]. The title compound, the 5-coordinate [(κ 2 -bipy)(κ 1 -bipy)FeEt 2 ], the 5-coordinate [(bipy)(THF)FeEt 2 ] and the 6-coordinate [(bipy)(THF) 2 FeEt 2 ] complexes have been analyzed in all possible spin states. While [(bipy) 2 FeEt 2 ] is diamagnetic, but suggested by the calculation to rather adopt an open-shell (broken-symmetry) singlet with a low spin ( S = ½) Fe III and non-innocent bipy ligands, [(bipy) 2 ] •- , the other three systems have a broken-symmetry triplet ground state ( S = 1 overall) with an intermediate spin ( S = 3/2) Fe III . The low computed Gibbs energy cost for the bipy/THF substitution (5.6 kcal mol -1 ) makes [(bipy)(THF)FeEt 2 ] a kinetically competent intermediate. Full bipy dissociation prior to the β-H elimination step is also consistent with the previously observed dependence of the decomposition rate on the solvent donor properties. A DFT investigation of the thermal decomposition of [(bipy) 2 FeEt 2 ] discredits a previously proposed mechanism and illuminates on the non-innocent behavior of the bipy ligand in the starting material and ligand exchange product.