A variety of substituted derivatives of phen have been examined as ligands.  Substituents at the 2,9 positions confer protection for the attached metal, inhibiting the binding of multiple equivalents of the phenanthroline. Such bulky ligands also favor trigonal or tetrahedral coordination at the metal.  Phen itself form complexes of the type [M(phen)]Cl when treated with metal dihalides (M = Fe, Co, Ni).  By contrast, neocuproine and bathocuproine form 1:1 complexes such as [Ni(neocuproine)Cl].