In terms of its coordination properties, phenanthroline is similar to 2,2'-bipyridine (bipy) with the advantage that the two nitrogen donors are preorganized for chelation. Phenanthroline is a stronger base than bipy.  According to one ligand ranking scale, phen is a weaker donor than bipy.
Several homoleptic complexes are known of the type  [M(phen)].  Particularly well studied is [Fe(phen)], called "ferroin." It can be 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.  The pink complex [Ni(phen)] has been resolved into its Δ and Λ isomers.  The complex [Ru(phen)] is bioactive.
Copper(I) forms [Cu(phen)], which is luminescent.