Understanding the self-assembly of cluster aggregates remains an important challenge in coordination chemistry. A chelating tetrazine-based ligand was employed to isolate an unprecedented tetranuclear Hg 4 complex exhibiting Hg I /Hg II mixed valence, which also contains metal–metal bonds. Single-crystal X-ray diffraction, X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance, and theoretical approaches (density functional theory) were employed to shed some light on the structure and self-assembly of this discrete molecule.

Reducible 3,6-bis(3,5-dimethyl-pyrazolyl)1,2,4,5-tetrazine was employed to isolate supramolecular air-stable [Co ] and [Zn ] squares, which were achieved via careful selection of counterions rather than the use of reducing agents. Magnetic susceptibility studies revealed a strong radical-Co exchange coupling (J /hc=-118 cm , -2J formalism) with a spin ground state of S =4, whereas the unreduced analogue revealed negligible coupling between the Co centers (J /hc=-0.64 cm ). Radical-radical coupling was also probed in the [Zn ] congener, which led to J /hc=-15.9(5) cm . These results highlight the versatile air-stable coordination chemistry of tetrazine and the importance of exploiting easily reducible delocalized radical to promote strong exchange coupling between spin carriers.