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Phenanthroline is a heterocyclic organic compound. As a bidentate ligand in coordination chemistry, commonly abbreviated "phen," it forms strong complexes with most metal ions. In terms of its coordination properties, phen is similar to 2,2'-bipyridine.


Phenanthroline may be obtained by refluxing o-phenylenediamine, sulfuric acid, glycerol, a solution of arsenic acid, and water.

Ferroin and analogues

The complex [Fe(phen)3]2+, , called "ferroin," is used for the photometric determination of Fe(II). It is used as a redox indicator with standard potential +1.06 V. The reduced ferrous form has a deep red colour and the oxidised form is light-blue. Ferroin is used as a cell permeable inhibitor for metalloproteases in cell biology.

The pink complex [Ni(phen)3]2+ has been resolved into its Δ and Λ isomers. The analogous [Ru(phen)3]2+ has long been known to be bioactive.

Related ligands

In the related compound "bathophenanthroline," the 4 and 7 positions are substituted by phenyl groups.

As an indicator for alkyl lithium reagents

Alkyl lithium reagents form deeply colored derivatives with phen. The alkyl lithium content of solutions can be determined by treatment of such reagents with small amounts of phen (ca. 1 mg) followed by titration with alcohols to a colourless endpoint.


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