Ligand exchange reactions between a Hoveyda-Grubbs-type complex and 2-alkoxybenzylidene in a biomolecule assist in the delivery of a ruthenium complex unit to the biomolecule. Complexes having an electron-withdrawing group in their ligands efficiently transfer a ruthenium complex unit onto peptides. This method is also applicable for adenylate kinase, an experimental model protein. The ligand exchange reaction originates from ruthenium-olefin specific interactions and potentially provides a bioorthogonal method for chemical modification of biomacromolecules with transition metal complexes.