1. A ligand having a structural formula (I): wherein R 1 , R 2 , R 3 and R 4 are independently hydrogen, nitro, amino or thiocyano; P 1 , P 2 , P 3 , P 4 and P 5 are independently oxo, halo, hydroxyl or C 1-7 alkyl; X is methylenyl, 1,1-cyclobutylenyl, 1,1-cyclopentylenyl or 1,1-cyclohexanylenyl; A 1 , A 2 and A 3 are independently hydrogen, C 1-3 alkyl, C 1-3 benzylalkyl, C 1-3 methoxybenzylalkyl, diphenylmethyl or C 1-3 isothiocyanobenzylalkyl; Z is a group binding to Y; and Y has a structural formula (III): wherein n is an integer in a range from 2 to 8.