On Fe–Fe Dumbbells in the Ideal and Real Structures of FeGa<sub>3</sub>

The intermetallic phase FeGa<sub>3</sub> belongs to the rare examples of substances with transition metals where semiconducting behavior is found. The necessary electron count of 17 ve/fu can be formally derived from eight Fe–Ga and one Fe–Fe two-center–two-electron bond. The situation is reminiscent of the well-known Fe<sub>2</sub>(CO)<sub>9</sub> scenario, where a direct Fe–Fe two-center–two-electron bond was shown to not be present. Fe–Fe interaction in FeGa<sub>3</sub> and its substitution variants represents the crucial point for explanation of electronic, thermal transport, and optical properties of this material. Chemical bonding analysis in position space of FeGa<sub>3</sub> and Fe<sub>2</sub>(CO)<sub>9</sub> on the basis of the topology of the electron localizability indicator distribution, QTAIM atoms, two- and three-center delocalization indices, domain natural orbitals, IQA analysis, and an evaluation of the Fe–Fe dissociation energy yields a complete picture of the partially compensated Fe–Fe bond, which is nevertheless strong enough to be of decisive importance. Structural reinvestigation of differently synthesized single crystals leads to the composition Fe<sub>1+<i>x</i></sub>Ga<sub>3</sub> (0 ≤ <i>x</i> ≤ 0.018), where the additional Fe atoms are predicted from DFT/PBE calculations to yield a magnetic moment of about 2 μ<sub>B</sub>/Fe2 atom and metallic in-gap states. Accompanying magnetization and ESR measurements are consistent with this picture.