Chelate complexes of Ru(II) (12a,b) featuring an η 5 -C 5 ,η 1 -N-C 5 H 4 (CH 2 ) n NH 2 ligand (n=2, 3) have been synthesized as arene-selective protein modification reagents. The new chelates produce η 6 -arene complexes from a wide variety of arenes, from 1,4-di-t-butylbenzene to phenylalanine-containing peptides and proteins. In methanol or nitromethane at room temperature (r.t.), kinetic products are formed by coordination of the Ru center to N- and S-donors (primary amines, histidine, methionine, cystine), but warming to 60 o C leads to migration of the Ru center to a phenylalanine ring, with the formation of η 6 -arene complexes as robust thermodynamic products. Notably, in water the complex 12a reacts at r.t. with the 27-residue protein secretin, which contains a phenylalanine residue, to give an η 6 -arene complex, which can be purified by HPLC and analyzed by electrospray and MALDI-MS.