Tools for Predicting the Nature and Magnitude of Magnetic Anisotropy in Transition Metal Complexes: Application to Co(II) Complexes

Abstract

This work addresses the question of the identification of the excited states that are mainly responsible for the magnitude and nature of the magnetic anisotropy in high-spin mononuclear transition metal complexes. Only few states are actually responsible for the single ion magnetic anisotropy, and these states can be anticipated from rather simple rules. We show that in high-spin complexes atomic selection rules still prevail and that molecular selection rules from the symmetry point group are more selective than those of the double group. The predictive power of these rules is exemplified on a penta-coordinate Co(II) complex investigated with correlated ab initio calculations, including relativistic contributions. The electronic structure of excited states coupled to the ground state through spin-orbit coupling informs us about the nature (either axial or planar) of their contribution to the anisotropy. From this information, it is possible to anticipate the nature and strength of the ligand field and predict the magnetic anisotropy, which may guide the synthesis of improved anisotropic complexes. Such results can also be used to improve the quality of ab initio calculations of the spin Hamiltonian parameters and to reduce the computational cost.

Magnetic Anisotropy in Pentacoordinate NiII and CoII Complexes: Unraveling Electronic and Geometrical Contributions

“…The calculated E / D ratio corresponds to the value that gives the best calculated EPR spectrum, which allows us to be confident in analyzing the anisotropy behavior of 2 . Calculation with ten quadruplets and 40 doublets in the SO‐SI space shows that only the first, third, and fourth excited quadruplets and the ninth doublet are strongly coupled to the ground state by SOC . Here again two‐state calculations were performed.…”

Quantitative Estimation of Ising‐Type Magnetic Anisotropy in a Family of C3‐Symmetric CoII Complexes

“…This fact can be explained by using complex 1 as an example. The first four excited states for this complex, denoted as 4 Φ n , are close enough in energy to the ground state to have a significant impact on the magnitude of D (Table ) –. As stated above, the ground state and the low‐lying excited states are highly multideterminant (Table S23 in the Supporting Information); Figure shows the wavefunction compositions for the ground state ( 4 Φ 0 ) and the first excited state ( 4 Φ 1 ) of complex 1 .…”

“…As may be observed, the negative contribution to D arises from the coupling between the b and e determinants of 4 Φ 1 with the c and d determinants of 4 Φ 0 , because they involve excitations between orbitals having the same mL values. The interaction between the determinants b ( 4 Φ 1 ) and e ( 4 Φ 0 ), which should imply an excitation with “double” negative contribution, may play a role as well –…”

A Bis‐Binuclear NiII Complex with Easy and Hard Axes of Magnetization: Complementary Experimental and Theoretical Insights

“…Calculations at the CASPT2 level were also performed to introduce dynamic correlation as a second‐order perturbation , . The procedure of extraction of ZFS parameters from the effective Hamiltonian theory, and the computed energies and wave functions has already been successfully used , . Broken‐symmetry DFT (BS‐DFT) calculations were performed by using the GAUSSIAN package to evaluate the exchange coupling J for the binuclear complex .…”