Dimer Rhenium Tetrafluoride with a Triple Bond Re-Re: Structure, Bond Strength

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(2)F(8) is present in the gas phase. The geometric structure of the Re(2)F(8) molecule corresponding to D(4h) symmetry was found, and the following geometric parameters of the r(h1) configuration were determined: r(h1)(Re-Re) = 2.264(5) Å, r(h1)(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(2)F(8), the wave function of the (1)A(1g) 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(2)X(8) 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(2)F(8) molecule and the [Re(2)F(8)](2−) dianion in the crystal corresponds to the concept of a triple σ(2)π(4) (Re(IV)-Re(IV)) bond and a quadruple σ(2)π(4)δ(2) (Re(III)-Re(III)) bond, respectively, which are formed between rhenium atoms due to the interaction of d-atomic orbitals. The enthalpy of dissociation of the Re(2)F(8) molecular form in two monomers ReF(4) (Δ(diss)H°(298) = 109.9 kcal/mol) and the bond energies E(Re-Re) and E(Re-X) in the series Re(2)F(8)→Re(2)Cl(8)→Re(2)Br(8) 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(2)Br(8) (D(4d)) geometric configuration differs from the symmetry of the Re(2)F(8) and Re(2)Cl(8) (D(4h)) molecules.