A new chelate, mercaptoacetyl-L-histidinyl-S-benzyl-L-cysteine methyl ester was synthesized by standard peptide coupling techniques and reacted with ReOCl3(PPh3)(2) to give two diastereomers, 7a and 7b. The two isomers were separated by reversed-phase HPLC and characterized by NMR spectroscopy and electrospray mass spectrometry. An X-ray structure of one isomer, 7a, confirmed that the chelated complex was analogous to other Re-N2S2 compounds in that it formed a square pyramidal complex where the four donor atoms were the base of the pyramid and the oxygen attached to the rhenium was at the apex. The S-benzyl group, as expected, was cleaved during the formation of 7, and the resulting complex was a zwitterion where the rhenium was formally -1 and the counterion was the protonated imidazole ring.