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 1 H 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.