In the present work, we investigated the proton-transfer reactions of Re(II)-nitrosyl complexes bearing halides and nicotinic acid (Hnic) or 6,6́-dinicotinic acid (H2dinic) trans-coordinated to the core {ReNO}3+ by potentiometry and UV − Vis absorption spectroscopy in a mixture DMSO:H2O (1:1; v:v). The presence of Hnic and H2dinic in title complexes promotes a marked decrease in the acidity (higher pK values) compared to the non-coordinated pyridine-derived ligands. The UV − Vis measurements indicate that the absorption spectra of complexes do not show any dependence on the acidity of the medium and the number of non-coordinated carboxylic units. Time-dependent Density Functional Theory (TD-DFT) calculations are performed to understand the optical properties of complexes and their response upon deprotonation. TD-DFT studies are conducted with PBE1PBE in combination with LANL2TZ on rhenium and 6–311++G(d,p) on non-metal atoms within the C-PCM approach. This methodology shows a reasonable match between the measured and simulated spectra keeping the computational cost low. The nature of all absorption bands is ascribed to a ligand-metal-to-ligand-metal charge transfer (LMLMCT), in which negligible contribution from carboxylic units is detected.