We describe the synthesis and spectral characterization of a rhenium metal−ligand complex. Thiscomplex reacts with sulfhydryl groups via an iodoacetamide side chain on the phenanthroline ligandand displays a high limiting anisotropy near 0.35 when excited at 442 nm. When covalently linked tohuman serum albumin, this complex displayed a mean decay time of about 1 μs. This decay time isappropriate for measuring rotational correlation times on the microsecond time scale as may occurfor large macromolecular complexes.