A compound of formula (I): wherein: X is an electron withdrawing group, and each X is independently selected; Y is ligand; Z is selected from the group consisting of cyano, C 1-10 perfluoroalkyl and –C(=O)OR wherein R is a hydrocarbyl; each R1 is independently a substituent; and p in each occurrence is independently 0, 1, 2, 3 or 4.