X-ray absorption study of the geometry structure of Co2+/Co3+ in ammonia solution

Abstract The investigation of the geometrical structure of transition metal ions in solution is mandatory to determine the role of transition metal ions in many chemical reactions. In this contribution, the local structure of Co 2+ /Co 3+ in ammonia solution was investigated by X-ray absorption spectroscopy (XAS). We found that Co 2+ ions are coordinated by six NH 3 ligands arranged in a Jahn-Teller (JT) distorted octahedral (DT Oc) geometrical configuration with four long Co-N eq bonds (2.17 A) and two short Co-N ax bonds (2.03 A). At variance, Co 3+ ions in [Co(NH 3 ) 6 ] 3+ or [Co(NH 3 ) 5 Cl] 2+ clusters occur depending on the presence of activated carbons during the oxidation reaction process from Co 2+ to Co 3+ . By means of full multiple-scattering theoretical (MXAN) analysis of the Co K-edge XANES (X-ray absorption near edge structure), we show that [Co(NH 3 ) 6 ] 3+ is characterized by a regular octahedral (Re Oc) geometrical structure with a Co–N distance 1.91 A long while [Co(NH 3 ) 5 Cl] 2+ by a distorted octahedral geometrical structure with four Co-N eq bonds (1.94 A), a shorter Co-N ax bond (1.91 A) and a Co–Cl bond (2.12 A).