The Shrinking World of Innocent Ligands: Conventional and Non-Conventional Redox-Active Ligands

This essay for EurJIC's cluster issue on cooperative and redox non-innocent ligands introduces the reader to redox-active ligands, which range from the small archetypical NO+/center dot/- and O-2(0/center dot-/2-) systems via the classical 1,4-dihetero-1,3-diene chelates (e. g. alpha-diimine, dithiolene, or o-quinone redox series) to pi-conjugated macrocycles. The increased attention paid recently to the redox activity of ligands in coordination chemistry has now prompted wider successful searches, resulting in the establishing of less-conventional examples such as cyanide, carbon monoxide, thioethers, or acetylacetonate derivatives as non-innocently behaving ligands. By considering situations with significantly covalent metal-ligand bonding, the cases of metal-oxo, metal-hydrido, and organometallic compounds will also be addressed, with a perspective on how pervasive non-innocent ligand behavior is. The materials and reactivity potential of redox-active ligands will be pointed out.