A new water soluble Re(I) complex with a zwitterionic structure, Bu₄N[(bpy)Re(CO)₃(dcbpy)] (where Bu = butyl; bpy = 4,4′-bipyridine; dcbpy = 2,2′-bipyridine-5,5′-dicarboxylate), was successfully synthesized and characterized by elemental analysis, ¹H NMR, FTIR and ESI. Protonation studies in aqueous solutions of the Re(I) complex showed three acid–base equilibriums with pKₐ₁ = 5.0, pKₐ₂ = 3.0 and pKₐ₃ = 2.0. pKₐ₁ was assigned to the protonation equilibrium at bpy while pKₐ₂ and pKₐ₃ could be ascribed to protonation/deprotonation of the two carboxylates groups in the dcbpy ligand. With the aid of TD-DFT calculations the nature of the electronic transitions responsible for the pH-dependent UV–vis spectroscopy of the Re(I) complex was identified. At pH = 7 the lower energy band of the complex has MLLCTRe(CO)3→dcbpy character while at pH < 2 it switches to MLLCTRe(CO)3→bpy. This change in the nature of the lower energy band is responsible for the overall spectral changes in the 350–500 nm range after protonation of the Re(I) complex.Centro de Química InorgánicaInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas