Abstract A low-temperature 1H-NMR study suggests the tetranuclear cluster dication [H 6Ru 4(C 6H 6) 4] 2+ ( 1) to contain an H 2 ligand that undergoes, upon warming of the solution, an intramolecular exchange with the four hydride ligands at the Ru 4 framework. Whereas two of the three NMR signals at −120°C in the hydride region show T 1 values in the range 200–300 ms, the least deshielded resonance at δ=−17.33 ppm exhibits a T 1 value of only 34 ms, characteristic of an H 2 ligand. A re-examination of the single-crystal X-ray structure analysis of the chloride salt of 1 supports this interpretation by a short distance of 1.14(0.15) Å between two hydrogen atoms coordinated as a HH ligand in a side-on fashion to one of the triangular faces of the Ru 4 tetrahedron. The distance between one of the two hydrogen atoms of the H 2 ligand and one of the four hydride ligands is also very short [1.33(0.15) Å], suggesting an additional H 2⋯H interaction. The presence of this H 3 unit over one of the three Ru 3 faces in 1 may explain the deformation of the Ru 4 skeleton from the expected tetrahedral symmetry. Density functional theory (DFT) calculations on 1 indicate a very soft potential energy surface associated with the respective displacement of the three interacting cofacial hydrogen atoms. In accordance with these results, the cluster dication 1 tends to loose molecular hydrogen to form the cluster dication [H 4Ru 4(C 6H 6) 4] 2+ ( 2). The equilibrium between 1 and 2 can be used for catalytic hydrogenation reactions.