Development of a contemporary ligand-field program for 3d transition-metal complexes

Ligand-field theory (LFT) is a quantum-mechanical model which calculates the physical properties associated with the presence of an incomplete d shell. In this work, the development of a contemporary ligand-field program for 3dn transition-metal complexes is reported. The program, Kestrel is designed to be easy-to-use and performs real time calculations of the d-d transition energies, EPR g-factors, paramagnetic susceptibilities, and single-ion molecular magnetic (SiMM) behaviour of a complex using its real molecular geometry. Kestrel also features the prediction of UV-Vis, circular dichroism (CD), and magnetic circular dichroism (MCD) spectra in non-centrosymmetric systems. The ligand-field model uses metal-ligand bonding parameters to parameterise the ligand field and Racah parameters are used to simulate the effects of interelectronic repulsion. The multiconfigurational effects arising from spin-orbit coupling are also treated. Spectroscopic intensities arising from the electronic dipole mechanism are simulated using transition dipole moment parameters calculated for each ligand using metal-ligand polarisation parameters. Herein, the application of Kestrel to three contemporaneous case studies demonstrates the practical use of the software. First, the program is used to analyse how the SiMM behaviour of three homoleptic cobalt(II) complexes changes with variation in the molecular geometry and metal-ligand bonding. The study outlines suggestions for future synthetic work to enhance the SiMM behaviour of these systems. The second case study focuses on the reproduction of the variable-temperature MCD spectrum of a characterised intermediate-spin (S=1) iron(IV) oxo complex. The analysis shows that Kestrel’s assignment of the d-d bands, which differs from that in the literature, can reproduce the reported experimental data. Lastly, the program is used to analyse the full reported experimental characterisation of the resting state and substrate bound lytic polysaccharide monooxygenase (LPMO) enzyme LsAA9. The analysis was able to characterise the electronic structure of the copper(II) ion in this enzyme.