The ReOX2(met) compounds (X = Cl, Br) adopt a distorted octahedral structure in which a carboxylato oxygen lies trans to the Re=O bond, whereas the equatorial plane is occupied by two cis halides, an NH2, and an SCH3 group. Coordination of the SCH3 unit creates an asymmetric center, leading to two diastereoisomers. X-ray diffraction studies reveal that the crystals of ReOBr2(D,L-met)(.)1/2H(2)O and ReOBr2(D,L-met)(.)1/2CH(3)OH contain only the syn isomer (S-CH3 bond on the side of the Re=O bond), whereas ReOCl2(D-met) and ReOCl2(D,L-met) consist of the pure anti isomer. H-1 NMR spectroscopy shows that both isomers coexist in equilibrium in acetone (anti/syn ratio = 1:1 for X = Br, 3:1 for X = Cl). Exchange between these two isomers is fast above room temperature, but it slows down below 0degreesC, and the sharp second-order spectra of both isomers at -20degreesC were fully assigned. The coupling constants are consistent with the solid-state conformations being retained in solution. Complexes of the type [ReOX2(His-aa)]X (X = Cl, Br) are isolated with the dipeptides His-aa (aa = Gly, Ala, Leu, and Phe). X-ray diffraction work on [ReOBr2(His-Ala)]Br reveals the presence of distorted octahedral cations containing the Re= O3+ core and a dipeptide coordinated through the histidine residue via the imidazole nitrogen, the terminal amino group, and the amide oxygen, the site trans to the Re=O bond being occupied by the oxygen. The alanine residue is ended by a protonated carboxylic group that does not participate in the coordination. The constant pattern of the H-1 NMR signals for the protons in the histidine residue confirms that the various dipeptides adopt a similar binding mode, consistent with the solid-state structure being retained in CD3OD solution.