A reversible octahedral to trigonal-bipyramidal cobalt coordination change in an aquo-accessible coordination network

An aquo-accessible chromatic change in a crystal was induced by means of a reversible coordination geometry shift from octahedra in [Co(C12H8N2)(HO3P-C2H4-PO3H)(OH2)(2)] (1) to trigonal-bipyramid in [Co(C12H8N2)(HO3P-C2H4-PO3H)] (2) at the cobalt(II) center; the crystal-to-crystal phase transition was confirmed by powder X-ray diffraction and solid-state UV-vis diffuse reflectance spectra.