The coordination chemistry of 1,2,4,5-tetrazines

1,2,4,5-Tetrazine and its 3,6-disubstituted derivatives exhibit a particular coordination chemistry, characterized by electron and charge transfer phenomena and by the ability of these heteroatom-rich ligands to bridge metal centers in various ways. A very low-lying π* orbital localized at the four nitrogen atoms is responsible for intense low-energy charge transfer absorptions, electrical conductivity of coordination polymers, unusual stability of paramagnetic radical or mixed-valent intermediates and for often well-resolved EPR hyperfine structure in the radical complexes. Substituted 1,4-dihydro-1,2,4,5-tetrazines have also been used as bridging ligands. The structural consequences of electron transfer as well as the capability for efficient and variable metal–metal bridging render the tetrazines as valuable components of supramolecular materials.