![]() For 3d transition-metal chemistry, DE can lead to pronounced errors in calculated bond dissociation energies, 16,23–27 barrier heights, 28,29 and properties of the density 30–33 within a given spin state as well as the relative energetic ordering of spin states. The high earth abundance and prevalence of 3d transition metals in enzymatic systems has motivated the widespread study 16,23–41 of electronic structure method accuracy in first-row transition-metal complexes. For DFT in particular, presently available exchange–correlation (xc) approximations suffer from one- and many-electron self-interaction errors, 15–19 commonly referred to as delocalization error 20–22 (DE). 14 The well-localized d or f electrons of open-shell transition-metal centers impart unique properties but also can lead to a significant number of low-energy spin and oxidation states that are challenging to describe on equal footing using approximate electronic structure methods. ![]() ![]() Introduction Approximate density functional theory (DFT) is widely used in studying the catalytic 1–6 and materials 7–13 properties of open-shell transition-metal complexes. Instead, evaluation of potential energy curves in 3d and 4d TMCs reveals that higher exchange sensitivities in 3d TMCs are likely due to the opposing effect of exchange on the low-spin and high-spin states, whereas the effect on both spin states is more comparable in 4d TMCs. Differences in delocalization, as judged through changes in the metal–ligand bond lengths between spin states, do not explain the distinct behavior of 4d TMCs. 0.0–0.3) in exchange fraction, almost no 4d TMCs do. The combined effect of reduced exchange sensitivities and the greater low-spin bias of most 4d TMCs means that while over one-third of 3d TMCs change ground states over a modest variation ( ca. We observe consistently lower but proportional sensitivity to exchange fraction among 4d TMCs with respect to their isovalent 3d TMC counterparts, leading to the largest difference in sensitivities for the strongest field ligands. ![]() We analyze hundreds of complexes assembled from four metals in two oxidation states with ten small monodentate ligands that span a wide range of field strengths expected to favor a variety of ground states. We compare the exchange sensitivity of adiabatic spin-splitting energies in pairs of mononuclear 3d and 4d mid-row octahedral transition-metal complexes. Increasing the amount of exchange in a functional typically shifts the preferred ground state in first-row TMCs from low-spin to high-spin by penalizing delocalization error, but the effect on properties of second-row complexes is less well known. Density functional theory (DFT) is widely used in transition-metal chemistry, yet essential properties such as spin-state energetics in transition-metal complexes (TMCs) are well known to be sensitive to the choice of the exchange–correlation functional. ![]()
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