Role of Dispersion in the Molecular Geometries of Mn(III) Complexes*

Transition metal complexes are well known for their applications in diverse research areas such as molecular magnetism, molecular electronics, and catalysis. Theoretical modelling of such phenomena requires accurate determination of electronic structure and that cannot be achieved without reliable molecular geometries. Here we investigate the molecular geometries of manganese(III) complexes. For the high-spin state’s geometry, the density functionals significantly overestimate the Mn-N_amine bond distances, although the geometry for the intermediate-spin state is well-described. Comparisons with several wavefunction-based methods demonstrates that this error is due to the limited ability of density functional theory (DFT) to recover dispersion beyond a certain extent. Among the methods employed for geometry optimization, Møller-Plesset perturbation theory (MP2) appropriately describes the high-spin geometry, but results in an elongated Mn-O distance in the intermediate spin-state. On the other hand, complete active space second-order perturbation theory (CASPT2) results in a good description of the geometry for the intermediate spin state, but also sufficiently recovers dispersion performing well for the high-spin state. Despite the fact that the electronic structure of both spin states is dominated by one electron configuration, CASPT2 offers a balanced approach leading to molecular geometries with much better accuracy than MP2 and DFT. A scan along the Mn-N_amine bond demonstrates that coupled cluster methods (i.e., DLPNO-CCSD(T)) also yield bond distances in agreement with experiment, while multiconfiguration pair density functional theory (MC-PDFT) is unable to recover dispersion well enough, analogous to single reference DFT.