The linking reagent of claim 33, wherein Z has the formula: ##STR7## wherein X2 is a halogen or a substituted amino;R3 is selected from the group consisting of alkyl, alkoxy and phenoxy; X4 is selected from the group consisting of halogen, amino and O- ; and R5 is selected from the group consisting of alkyl, alkoxy, aryloxy and H, provided that R5 may be H only if X4 is O-. 35.