| Summary: | Based on the data of the gas electron diffraction/mass spectrometry (GED/MS) experiment, the composition of the vapor over rhenium tetrafluoride at T = 471 K was established, and it was found that species of the Re<sub>2</sub>F<sub>8</sub> is present in the gas phase. The geometric structure of the Re<sub>2</sub>F<sub>8</sub> molecule corresponding to D<sub>4h</sub> symmetry was found, and the following geometric parameters of the r<sub>h1</sub> configuration were determined: r<sub>h1</sub>(Re-Re) = 2.264(5) Å, r<sub>h1</sub>(Re-F) = 1.846(4) Å, α(Re-Re-F) = 99.7(0.2)°, φ(F-Re-Re-F) = 2.4 (3.6)°. Calculations by the self-consistent field in full active space approximation showed that for Re<sub>2</sub>F<sub>8</sub>, the wave function of the <sup>1</sup>A<sub>1g</sub> ground electronic state can be described by the single closed-shell determinant. For that reason, the DFT method was used for a structural study of Re<sub>2</sub>X<sub>8</sub> molecules. The description of the nature of the Re-Re bond was performed in the framework of Atom in Molecules and Natural Bond Orbital analysis. The difference in the experimental values of r(Re-Re) in the free Re<sub>2</sub>F<sub>8</sub> molecule and the [Re<sub>2</sub>F<sub>8</sub>]<sup>2−</sup> dianion in the crystal corresponds to the concept of a triple σ<sup>2</sup>π<sup>4</sup> (Re<sup>IV</sup>-Re<sup>IV</sup>) bond and a quadruple σ<sup>2</sup>π<sup>4</sup>δ<sup>2</sup> (Re<sup>III</sup>-Re<sup>III</sup>) bond, respectively, which are formed between rhenium atoms due to the interaction of d-atomic orbitals. The enthalpy of dissociation of the Re<sub>2</sub>F<sub>8</sub> molecular form in two monomers ReF<sub>4</sub> (Δ<sub>diss</sub>H°(298) = 109.9 kcal/mol) and the bond energies E(Re-Re) and E(Re-X) in the series Re<sub>2</sub>F<sub>8</sub>→Re<sub>2</sub>Cl<sub>8</sub>→Re<sub>2</sub>Br<sub>8</sub> molecules were estimated. It is shown that the Re-Re bond energy weakly depends on the nature of the halogen, while the symmetry of the Re<sub>2</sub>Br<sub>8</sub> (D<sub>4d</sub>) geometric configuration differs from the symmetry of the Re<sub>2</sub>F<sub>8</sub> and Re<sub>2</sub>Cl<sub>8</sub> (D<sub>4h</sub>) molecules.
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