The elusive mono-hydride tri-anion [HRu₄(CO)₁₂]³⁻ (4) has been isolated and fully characterized for the first time. Cluster 4 can be obtained by the deprotonation of [H₃Ru₄(CO)₁₂]⁻ (2) with NaOH in DMSO. A more convenient synthesis is represented by the reaction of [HRu₃(CO)₁₁]⁻ (6) with an excess of NaOH in DMSO. The molecular structure of 4 has been determined by single-crystal X-ray diffraction (SC-XRD) as the [NEt₄]₃[4] salt. It displays a tetrahedral structure of pseudo C_3v symmetry with the unique hydride ligand capping a triangular Ru₃ face. Variable temperature (VT) ¹H and ¹³C{¹H} NMR experiments indicate that 4 is fluxional in solution and reveal an equilibrium between the C_3v isomer found in the solid state and a second isomer with C_s symmetry. Protonation–deprotonation reactions inter-converting H₄Ru₄(CO)₁₂ (1), [H₃Ru₄(CO)₁₂]⁻ (2), [H₂Ru₄(CO)₁₂]²⁻ (3), [HRu₄(CO)₁₂]³⁻ (4) and the purported [Ru₄(CO)₁₂]⁴⁻ (5) have been monitored by IR and ¹H NMR spectroscopy. Whilst attempting the optimization of the synthesis of 4, crystals of [NEt₄]₂[Ru₃(CO)₉(CO₃)] ([NEt₄]₂[7]) were obtained. Anion 7 contains an unprecedented CO₃²⁻ ion bonded to a zero-valent Ru₃(CO)₉ fragment. Finally, the reaction of 6 as the [N(PPh₃)₂]⁺ ([PPN]⁺) salt with NaOH in DMSO affords [Ru₃(CO)₉(NPPh₃)]⁻ (9) instead of 4. Computational DFT studies have been carried out in order to support experimental evidence and the location of the hydride ligands as well as to shed light on possible isomers.