Transition-metal complexes of radical ligands can exhibit low-energy electronic transitions in the near-infrared (NIR) spectral region. NIR band energy and intensity sensitively depend on the degree of electronic coupling of the chromophore. Using the example of open-shell complexes derived from platinum and a 1,4-terphenyldithiophenol, we present a novel approach toward spectroscopically distinct NIR dyes for which the degree of electronic coupling correlates with the relative orientation of radical ligand and metal orbitals. Ligand/metal orbital alignment is modulated by auxiliary phosphine donors and selectively results in electron localized Class II-III or delocalized Class III structures that display distinct NIR transitions at 6500 and 4000 cm.

A heuristic four-parameter scheme captures and predicts the electronic properties of radical-ligand transition metal compounds, overcoming ligand specific descriptions.

The chemistry of radical-ligand complexes of the transition metals has developed into a vibrant field of research that spans from fundamental studies on the relationship between the chemical and electronic structures to applications in catalysis and functional materials chemistry. In general, fine-tuning of the relevant properties relies on an increasingly diversifying pool of radical-proligand structures. Surprisingly, the variability of the conformational freedom and the number of distinct bonding modes supported by many radical proligands is limited. This work reports on the angular constraints and relative geometric alignment of metal and ligand orbitals as key parameters that render a series of chemically similar thiyl/thiolate complexes of platinum(II) electronically and spectroscopically distinct. The use of conformational flexible thiophenols as primary ligand scaffolds is essential to establishing a defined radical-ligand [(S)Pt] core whose electronic structure is modulated by a series of auxiliary coligands at platinum.