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-reducedethanolic solutions of NbCl
5 with HCl in the presence of a sulfidesource. The red aqua ion, obtained upon cation-exchangechromatography, forms readily the thiocyanate complex which hasbeen crystallized as Cs
4.26Na
1.74[Nb
4SO
5(NCS)
10]·0.33H
2O. X-raycrystallography revealed an unprecedented metal-metal bondedtetranuclear Nb
4(
4-S)(
2-O)
54+ core with a capping
4-S ligand.