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. The oxidation state of hydride ions in oxide hosts is a matter of debate. Here, the authors address this question with a range of techniques and suggest that the electron density near an incorporated hydride ion is less than that at the hydrogen in a hydroxide ion, contrary to formal valence arguments.