I

n terms of its coordination properties, phenanthroline is similar to 2,2'-bipyridine (bipy) but binds metals more tightly since the chelating nitrogen donors are preorganized. Phenanthroline is however a weaker donor than bipy.[5]

Many homoleptic complexes are known. Particularly well studied is [Fe(phen)3]2+, called "ferroin." It was used for the photometric determination of Fe(II).[6] 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.[7] The pink complex [Ni(phen)3]2+ has been resolved into its Δ and Λ isomers.[8] Copper(I) forms [Cu(phen)2]+, which is luminescent.[9][10]