Electron diffraction placed a cobalt hydride's hydrogen nearer the neutron reference than X-rays did

The reason to try electrons on this problem is physical. "The advantage of using electrons is that they interact more strongly with samples and diffract through a different mechanism than X-rays. This makes them more effective at determining the accurate location of the H atom," said Ryan Donnelly, the first author and a graduate student in Patrick Holland's lab at Yale ^[6]. In a standard X-ray map, the large metal atom obscures the view of the tiny hydrogen ^[3].

The comparison worked because it had an outside answer to check against. Neutron crystallography, the gold standard for precise measurements of this kind, needs a specialized facility, and there are only a handful in the world ^[4]. Oak Ridge National Laboratory in Tennessee, one of them, helped validate the study ^[5]. Donnelly then set his electron diffraction structure against several ways of analyzing X-ray data, and the hydride came out closer to the neutron position ^[9]. "We discovered that a microcrystal electron diffractometer (MicroED) can accurately locate the hydride better than X-ray methods," said Holland, the senior co-author and Whitehead Professor of Chemistry ^[2].

Two conditions have to hold for that. MicroED can reliably detect the hydride when the data are high quality and the refinement uses a dynamical scattering model ^[10].

Donnelly used one cobalt hydride crystal, and the paper's title names an open-shell cobalt complex ^[7][12][1]. The test case is a single compound, and the metal is cobalt. The phys.org account reports the electron placement as closer to the neutron result without stating the distances or the uncertainties ^[2]. Those figures are what another group would need before treating an electron structure as decisive in a dispute over where a hydride sits.

Access is what the authors press. The study describes MicroED with dynamical refinement as a practical, broadly deployable approach for detecting metal hydrides, potentially expanding access beyond specialized neutron facilities ^[11]. The instrument also works with crystals much smaller than traditional X-ray crystallography requires ^[8]. Yale's group has used it on a microcrystalline metal-organic framework, a rhenium catalyst and a natural product from total synthesis, three structures before this one ^[13][14]. "The MicroED system has been a boon to our research," said Holland ^[15]. The paper is in ACS Central Science ^[12].