Static and Dynamic Electron Correlation in the Ligand Noninnocent Oxidation of Nickel Dithiolates

Metal dithiolates have a wide range of applications from catalysis to molecular conductors with the ligands being the source of electrons during electrochemical oxidation in an effect known as ligand noninnocence. Recent large-scale variational two-electron reduced-density matrix (2-RDM) calculations of the vanadium oxo complex and manganese superoxide dismutase show that quantum entanglement stabilizes the addition of an electron to the ligands, providing a quantum mechanical explanation for ligand noninnocence. In this paper, we confirm and explore the ligand noninnocence in the electron oxidation series of bis(ethylene-1,2-dithiolato)nickel or [Ni(edt(2))]((-2,-1,0)) with variational 2-RDM calculations. While previous wave function calculations of this series have selected only the ligand pi orbitals as the critical (active) orbitals to be correlated, we find that both ligand pi and nickel d orbitals must be correlated to generate a realistic picture of the electron-transfer process. Using the computed 2-RDM to seed a solution of the anti-Hermitian contracted Schrodinger equation, we predict that the singlet state is lower in energy than the triplet state, which is consistent with experimental observations.