X-Ray Diffraction Studies on the Structures of Hydrated Oxonium Ion, and the Chlorocobalt(II) and Tetrachlorocobaltate(II) Complexes in Aqueous Solutions

The structure of hydrated oxonium ion has been determined in a concentrated hydrochloric acid solution by the X-ray diffraction method. The oxonium ion H3O+ combines with four water molecules to form a trigonal pyramid structure in which three water molecules have an H3O+–H2O distance of 2.44 Å, while the water molecule at the apex of the pyramid is separated by 2.90 Å from the central oxygen atom of the oxonium ion. The structure of the CoCl+ determined in a relatively dilute lithium chloride solution (the Cl/Co mole ratio is 3.38) shows that the cobalt(II) ion is coordinated with five water molecules, the Co–OH2 distance being 2.14 Å. The Co–Cl distance within the complex is 2.35 Å. The CoCl42− complex has a regular tetrahedral structure both in a concentrated lithium chloride (Cl/Co=22.1) and a hydrochloric acid (Cl/Co=21.1) solutions with the Co–Cl bond length of 2.29 Å.