Ligand-Field-Dependent Behavior of Meta-GGA Exchange in Transition-Metal Complex Spin-State Ordering
Prediction of spin-state ordering in transition metal complexes is essential for understanding catalytic activity and designing functional materials. Semilocal approximations in density functional theory, such as the generalized-gradient approximation (GGA), suffer from several errors including delo...
| Main Authors: | , |
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| Other Authors: | |
| Format: | Article |
| Language: | en_US |
| Published: |
American Chemical Society (ACS)
2018
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| Online Access: | http://hdl.handle.net/1721.1/113022 https://orcid.org/0000-0001-9342-0191 |
| Summary: | Prediction of spin-state ordering in transition metal complexes is essential for understanding catalytic activity and designing functional materials. Semilocal approximations in density functional theory, such as the generalized-gradient approximation (GGA), suffer from several errors including delocalization error that give rise to systematic bias for more covalently bound low-spin electronic states. Incorporation of exact exchange is known to counteract this bias, instead favoring high-spin states, in a manner that has recently been identified to be strongly ligand-field dependent. In this work, we introduce a tuning strategy to identify the effect of incorporating the Laplacian of the density (i.e., a meta-GGA) in exchange on spin-state ordering. We employ a diverse test set of M(II) and M(III) first-row transition metal ions from Ti to Cu as well as octahedral complexes of these ions with ligands of increasing field strength (i.e., H₂O, NH₃, and CO). We show that the sensitivity of spin-state ordering to meta-GGA exchange is highly ligand-field dependent, stabilizing high-spin states in strong-field (i.e., CO) cases and stabilizing low-spin states in weak-field (i.e., H₂O, NH₃, and isolated ions) cases. This diverging behavior leads to generally improved treatment of isolated ions and strong field complexes over a standard GGA but worsened treatment for the hexa-aqua or hexa-ammine complexes. These observations highlight the sensitivity of functional performance to subtle changes in chemical bonding. |
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