 transition - metal based reactions have found wide use in organic synthesis and are used frequently to functionalize small molecules.1,2 however , there are very few reports of using transition - metal based reactions to modify complex biomolecules3,4 , which is due to the need for stringent reaction conditions ( for example , aqueous media , low temperature , and mild ph ) and the existence of multiple , reactive functional groups found in biopolymers . here we report that palladium(ii ) complexes can be used for efficient and highly selective cysteine conjugation reactions . the bioconjugation reaction is rapid and robust under a range of biocompatible reaction conditions . 
 the straightforward synthesis of the palladium reagents from diverse and easily accessible aryl halide and trifluoromethanesulfonate precursors makes the method highly practical , providing access to a large structural space for protein modification . 
 the resulting aryl bioconjugates are stable towards acids , bases , oxidants , and external thiol nucleophiles . 
 the broad utility of the new bioconjugation platform was further corroborated by the synthesis of new classes of stapled peptides and antibody - drug conjugates . 
 these palladium complexes show potential as a new set of benchtop reagents for diverse bioconjugation applications . 
Supplementary Material