A pair of unshared electrons can coordinate certain metallic ions to give complexes. In the case of ferrous ion, the orthophenanthroline complex is quite stable and is intensely red in color. The complex is sometimes called Ferroin.A pair of unshared electrons can coordinate certain metallic ions to give complexes. In the case of ferrous ion, the orthophenanthroline complex is quite stable and is intensely red in color. The complex is sometimes called Ferroin