What do you mean under "each metal complex" ?

each metal complex mean complex of APDC with Ag, Cd, Co, Cu, Fe, Zn, Ni and Pd

It may be the "synergic effect" (a mixture of extractants is more efficient than any of them) when both ligands enter the inner or outer coordination sphere and decrease hydrophobicity, see e.g. Zolotov's paper

http://www.sciencedirect.com/science/article/pii/0022190270806583