New bis (imino)-N-heterocyclic carbene ligands have been synthesized, and the corresponding iron complexes have been isolated and characterized. Whereas imidazole- derived complexes exhibited exclusively bidentate binding modes, 4, 5, 6- trihydropyrimidylidene-based ligands adopted a tridentate pincer conformation analogous to complexes of 2, 6-bis (imino) pyridines. Bonding in the five-coordinate bis (imino)-N- ...