The effect of base on the solution behavior of oxo (D-penicillaminato) (L-penicillaminato)rhenate(V) (1) was investigated by H-1 NMR, resonance Raman, and UV-visible spectroscopy. The H-1 NMR spectrum of 1 consisted of two sets of sharp pen signals in DMSO-d(6) and D2O below pH 8 (penicillamine penH(4), the subscript on H indicating the number of dissociable protons present). The data were consistent with ReO(D-penH(3))(L-penH(2)) and its enantiomer ReO(L-penH(3)) (D-penH(2)), with penH(3) coordinated by N, S, and carboxyl O. These enantiomers have a cis-N-2,cis-S-2 coordination of the pen ligands, both CO2 groups anti to the oxo ligand and one CO2 coordinated trans to the oxo ligand (form I). In D2O, between pH 8.6 and 10.1, the signals broadened, collapsed, and reemerged as one set of signals, but no accompanying changes were observed in either the resonance Raman or the UV-visible spectra. The NMR spectral changes were attributed to base-catalyzed interconversion between two enantiomers; both CO2 groups are deprotonated in D2O, and they alternate between ligated and deligated states. Near pH 11, a new form (II) in slow exchange with form I was detected by NMR spectroscopy. The Re=O Raman band of I (963 cm(-1)) was replaced by a new midfrequency band (930 cm(-1)). The UV-visible bands (346 and 492 nm) decreased in intensity. With increasing pH and near pH 12, the H-1 NMR signals of II shifted. The midfrequency Re=O band was replaced by a low-frequency band (845 cm(-1)), and both the UV and visible bands continued to decline as new shorter wavelength bands emerged. These spectral changes were consistent with deprotonation of II to give II’. The H-1 NMR spectra of 1 and ReO(D-penH(3))(D-penH(2)) were nearly identical at pH 12. At this pH, the latter complex is a trans-dioxo species with a Re=O band at 846 cm(-1); the similar NMR spectrum and Re=O band of 1 suggest that II’ is a trans-dioxo species also. This conclusion was supported by studies in methanol which showed that [ReO(OCH3)(D-penH(2))(L-penH(2))](2-) did not deprotonate.