A low-spin, macrocyclic iron(II) complex in an aqueous solution responds to the addition of a chemical reactant (dithionite) by transformation into a high-spin complex, detectable by measurement of the longitudinal relaxation time (T-1) of surrounding water hydrogen nuclear spins. The initial compound does not modify T-1 of pure water at concentrations as high as 4 mM. The response is pH-dependent, and the complex is robust at a variety of conditions.