The molecular crystal [Ni(12aneS(4))](BF(4))(2) (12aneS(4) = 1,4,7,10-tetrathiacyclododecane) displays the uncommon ability to retain its crystallinity on facile reversible hydration/dehydration without requiring polymeric bonding and despite the significant structural changes required. The flexibility of the 12aneS(4) ring facilitates rearrangement of the complex in the solid state upon absorption of two equivalents of water, with the crystal structures showing a clear topotactic relationship. The well defined chromatic, structural and magnetic changes that are both highly sensitive and easily reversed makes this complex of interest as an environmentally responsive material and as a contrasting example to the commonly reported coordination polymers.