Equivalent amounts of ReOX3(OPPh3)(Me2S) (where X = Cl, Br) and L-histidine (L-hisH) in acetonitrile yield ReOX2-(L-his), in which the amino acid monoanion is N,N,O-tridentate. X-ray diffraction work on both compounds shows that the three donors occupy a face in a distorted octahedron and the carboxylate oxygen is coordinated trans to the Re=O bond. The 2:1 complex [ReO(L-his)(2)]\ is obtained by reacting 2 equiv of L-histidine with ReO2\(PPh3)(2) in methanol in the presence of NaOCH3. H-1 NMR spectroscopy indicates that these complexes contain one N,N,O-tridentate histidine anion coordinated as above and one N,N-bidentate histidine anion, whose carboxylate group is free. By refluxing ReOX2(L-his) in methanol, the carboxylic groups esterify and two octahedral units condense into an oxo-bridged dinuclear complex {ReOX2(L-hisMe)}(2)O containing N,N-bidentate histidine methyl ester. The O=Re-O-Re=O backbone is approximately linear, and the two ReOX2(L-hisMe) units are related by a 2-fold axis through the central oxygen. Crystals of {ReOBr2(L-hisMe)}(2)O consist of an ordered phase containing two of the possible diastereoisomers in a 1:1 ratio. H-1 NMR spectra of these crystals include two sets of signals, consistent with the presence of two isomers with C-2 symmetry, and the spectra of the non recrystallized material confirm that these are the only two isomers formed.