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(edt₂)]^(-2,-1,0) with variational 2-RDM calculations. While previous wave function calculations of this series have selected only the ligand π orbitals as the critical (active) orbitals to be correlated, we find that both ligand π 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 Schrödinger equation, we predict that the singlet state is lower in energy than the triplet state, which is consistent with experimental observations.