Coupled Potential Energy Surfaces Strongly Impact the Lowest-Energy Spin-Flip Transition in Six-Coordinate Nickel(II) Complexes

Pseudoctahedral 3d⁸ nickel(II) complexes with strong ligands are promising candidates for spin-flip luminescence. This study illustrates however that spin-flip luminescence from six-coordinate d⁸ nickel(II) complexes is not easily observed as for molecular d³ chromium(III) compounds. A series of five nickel(II) complexes are explored and their lowest-energy singlet and triplet excited states characterized by DFT, CASSCF, experimental spectroscopy and theoretical models, revealing a large degree of detrimental mixing of states via spin-orbit coupling, leading to non-radiative decay. These models provide insight into key differences between the nickel(II) complexes and their strongly luminescent chromium(III) analogues.