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.