Does the metal–metal sextuple bond exist in the bimetallic sandwich compounds Cr₂(C₆H₆)₂, Mo₂(C₆H₆)₂, and W₂(C₆H₆)₂?^†

Abstract

Although evanescent at best, the bare dimers of the elements Cr, Mo, and W have been identified as possible candidates for the sextuple metal–metal bond. The corresponding dibenzene sandwich compounds Cr₂(C₆H₆)₂, Mo₂(C₆H₆)₂, and W₂(C₆H₆)₂, satisfy the ‘18-Electron Rule’, and might achieve high-order metal–metal bonds and longer lifetimes at the same time. Twenty-two different DFT methods have been used to evaluate this possibility. Based on the present Wiberg bond index and molecular orbital analyses, however, only quadruple metal–metal bonds are predicted for the electronic ground states of Cr₂(C₆H₆)₂, Mo₂(C₆H₆)₂, and W₂(C₆H₆)₂, rather than the sextuple or even quintuple bonds, for both singlet and triplet electronic states. It is possible to force the hypothesised D _6h sextuple bond electron configuration, but the resulting energy is 39 kcal/mol above the D _2h quadruple bond structure for Cr₂(C₆H₆)₂. However, the constrained sextuple bond structure for Mo₂(C₆H₆)₂ lies 19 kcal/mol above the MoMo singlet. For W₂(C₆H₆)₂ the sextuple bond structure is predicted to lie only 3 kcal/mol above the WW structure. Thus the answer to the question raised in our title is ‘almost’ for the tungsten–sextuple bond.

Acknowledgements

This research was supported by the U. S. National Science Foundation (grant number CHE-1054286), National Natural Science Foundation of China (No. 20903006), and the Beijing Natural Science Foundation (No. 2092008).

^†Special Issue of Molecular Physics in Honor of Professor Werner Kutzelnigg.

Notes

^aRE are energies relative to the singlet D _2h conformer results in Table 1.

^aRE are energies relative to the singlet D _2h conformer results in Table 1.

^aRelative energies with respect to the singlet D _2h conformer in Table 4.

^aEnergies are relative to the energies of the singlet D _2h conformer (Table 6).