Mapping the electronic state crossing of a tetra-coordinated d8 complex using multi-reference quantum chemical methods

Abstract

The metalloproteins rubredoxin and [NiFe]-hydrogenase perform important biological functions, participating in electron transfer pathways and hydrogen oxidation/reduction respectively. The active sites of both metalloproteins have several low lying electronic spin states, providing the potential to transition from one spin state to another, which is believed to be important for these proteins’ functionality. To better understand discrepancies in the active site properties as it transitions between the tetrahedral and square planar coordinations, a simplified crystal field model consisting of a d8 Ni(II) ion and four negative point charges simulating the ligands around the Ni(II) ion is studied. For comparison, a Ni-substituted rubredoxin active site model is also characterized by quantum chemical computational methods. Pseudo-Tanabe-Sugano diagrams for the square planar and tetrahedral coordinations are created. Energies of the triplet and singlet states arising from the lowest energy 3F and 1D atomic terms are calculated and analyzed as the coordination of the point charges changes from tetrahedral to square planar.