Tunable Charge Delocalization in Dinickel Quinonoid Complexes

When a 2,5‐diamino‐1,4‐benzoquinonediimine C 6 H 2 (NR) 2 (NHR) 2 ( 2 ) is used as a bridging ligand, new dinickel( II ) complexes [(acac)Ni{μ‐C 6 H 2 ( ${^{\underline{}}}$ NPh) 4 }Ni(acac)] ( 3 a : R=Ph) and [(acac)Ni{μ‐C 6 H 2 ( ${^{\underline{}}}$ NCH 2 t Bu) 4 }Ni(acac)] ( 3 b : R=CH 2 t Bu) are obtained; upon one‐electron oxidation of these complexes delocalized mixed‐valence compounds are formed. An X‐ray diffraction study on 3 b reveals equalization of the bond lengths within each of the ligand 6π systems and a lack of conjugation between them. The oxidized state in 3 b + involves both the bridging quinonoid ligand and the metal centers, with a major contribution coming from the bridging ligand. Electrochemical and spectroscopic methods were used to study the influence of the N‐substituents of the tetranitrogen donor ligands 2 . In this combined experimental and theoretical (DFT) study, it is also shown that the electronic structure within the dinickel system can be altered by addition of a coordinating ligand such as pyridine. The latter favors the high‐spin configuration with semi‐occupied metal‐centered orbitals, leading to a metal–metal interaction in the mixed‐valence Ni II –Ni III 3 b + system.