Here we report the completely reversible spin state switch of the naturally diamagnetic tris(bipyridine)iron(ii) complex and the spin crossover complex bis(2,6-bis(1H-pyrazol-3-yl)pyridine)iron(ii) by the variation of the pH followed by 1H-NMR, UV-Vis spectroscopy, and magnetic and relaxivity measurements in solution and as composite materials encapsulated in a zeolite matrix.