The aqueous solution chemistry of niobium is rather unexplored, and well characterized aqua complexes are scarce. In this work, a new niobium aqua ion is obtained upon treatment of Zn-reduced ethanolic solutions of NbCl5 with HCl in the presence of a sulfide source. The red aqua ion, obtained upon cation-exchange chromatography, forms readily the thiocyanate complex which has been crystallized as Cs4.26Na1.74[Nb4SO5(NCS)(10)].0.33H(2)O. X-ray crystallography revealed an unprecedented metal-metal bonded tetranuclear Nb-4(mu(4)-S)(mu(2)-O)(5)(4+) core with a capping mu(4)-S ligand.