A　novel　pure　inorganic　cobalt-containing　hetero-polyoxoniobate　H9KNa2(H2O)(10)[Co(H2O)(2)(SiNb18O54)]center　dot　15H(2)O　(1-purple)　has　been　constructed　from　crescent-shaped　[SiNb18O54](14-)　units.　In　compound　1-purple,　the　[SiNb18O54](14-)　units　are　connected　by　[Co(H2O)(2)](2+)　ions　to　form　a　chiral　helical　chain.　The　adjacent　helical　chains　with　left/right-handedness　are　bridged　by　K+　ions　to　generate　a　2D　inorganic　layered　structure,　where　the　water　molecules　were　trapped　by　coordination　and　hydrogen　bonds.　Compound　1-purple　shows　reversible　chromotropic　behaviour　in　the　solid　state,　changing　from　purple　to　dark　blue　(1-blue)　upon　heating　or　desorption.　Interestingly,　fast　reversible　blue　to　purple　color　transition　can　only　be　achieved　in　the　presence　of　water　or　vapour.　The　chromism　is　demonstrated　to　stem　from　the　quantitative　desorption/adsorption　of　water　by　UV-vis-NIR　spectroscopy　and　2D　IR　correlation　spectroscopy.　In　addition,　the　proton　conductivity,　water　vapor　adsorption　and　magnetic　properties　are　also　investigated.　The　dependence　of　coordinated　water　molecules　with　Co(ii)　centres　on　the　proton　conductivity　is　discussed.