Secondary coordination sphere electrostatic effects tune the valence manifolds of copper centers, impacting molecular geometries, photophysical properties, and redox potentials.

This report describes an isostructural series of dinuclear iron, cobalt, and nickel complexes bound by a redox-active macrocyclic ligand. The series spans five redox levels (34–38 e–/cluster core), allowing for a detailed investigation into both the degree of metal–metal interaction and the extent of ligand-based redox-activity. Magnetometry, electrochemistry, UV–vis–NIR absorption spectroscopy, and crystallography were used in conjunction with DFT computational analyses to extract the electronic structures of the six homodinuclear complexes. The isoelectronic, 34 e– species [(3PDI2)­Fe2(PMe3)2(μ-Cl)]­(OTf) and [(3PDI2)­Co2(PMe3)2(μ-Cl)]­(OTf)3 exhibit metal–metal single bonds, with varying amounts of electron density delocalization into the ligand as a function of the effective nuclear charge of the metal ions. One- and two-electron reductions of [(3PDI2)­Co2(PMe3)2(μ-Cl)]­(OTf)3 lead to isolable products, which show successive increases in both the Co–Co distances and the extent of reduction of the ligand manifold. This trend results from reduction of a Co–Co σ* orbital, which was found to be heavily mixed with the redox-active manifold of the 3PDI2 ligand. A similar trend was observed in the 37 and 38 e– dinickel complexes [(3PDI2)­Ni2(PMe3)2(μ-Cl)]­(OTf)2 and [(3PDI2)­Ni2(PMe3)2(μ-Cl)]­(OTf); however, their higher electron counts lead to high-spin ground states that result from occupation of a high-lying δ/δ* manifold with significant Ni–NPDI σ* character. This change in ground state configuration reforms a M–M bonding interaction in the 37 e– complex, but formation of the 38 e– species again disrupts the M–M bond alongside the transfer of electron density to the ligand.