Hydride ions in oxide hosts hidden by hydroxide ions

The true oxidation state of formally 'H^-' ions incorporated in an oxide host is frequently discussed in connection with chemical shifts of ¹H nuclear magnetic resonance spectroscopy, as they can exhibit values typically attributed to H⁺. Here we systematically investigate the link between geometrical structure and chemical shift of H^- ions in an oxide host, mayenite, with a combination of experimental and ab initio approaches, in an attempt to resolve this issue. We demonstrate that the electron density near the hydrogen nucleus in an OH^- ion (formally H⁺ state) exceeds that in an H^- ion. This behaviour is the opposite to that expected from formal valences. We deduce a relationship between the chemical shift of H^- and the distance from the H^- ion to the coordinating electropositive cation. This relationship is pivotal for resolving H^- species that are masked by various states of H⁺ ions.