Sextuple bond

The only known molecules with true sextuple bonds are the diatomic dimolybdenum (Mo2) and ditungsten (W2), which exist in the gaseous phase and have boiling points of 4,639 °C (8,382 °F) and 5,930 °C (10,710 °F) respectively.

A full quantum treatment requires a more nuanced picture, in which electrons may exist in a superposition, contributing fractionally to both bonding and antibonding orbitals.

In Roos et al's calculations, the effective bond order (EBO) could be determined by the formula

where ηb is the proportion of formal bonding orbital occupation for an electron pair p, ηab is the proportion of the formal antibonding orbital occupation, and c is a correction factor account­ing for deviations from equilibrium geometry.

Dichromium, while formally described as having a sextuple bond, is best described as a pair of chromium atoms with all electron spins exchange-coupled to each other.

[6] Laser evaporation of a molybdenum sheet at low temperatures (7 K) produces gaseous dimolybdenum (Mo2).

[7] Both ditungsten and dimolybdenum have very short bond lengths compared to neighboring metal dimers.

This equi­librium internuclear distance is signi­ficantly lower than in the dimer of any neighboring 4d transition metal, and sug­gestive of higher bond orders.

[8][9][10] However, the bond dissociation energies of ditungsten and dimolybdenum are rather low, because the short internuclear distance introduces geometric strain.

Like dichromium, dimolybdenum and ditungsten are expected to exhibit a 1Σg+ singlet ground state.

[8] Although sextuple bonding in homodimers is rare, it remains a possibility in larger molecules.

Theoretical computations suggest that bent dimetallocenes have a higher bond order than their linear counterparts.

Quantum chemistry calculations reveal, however, that the corre­sponding D2h singlet geometry is stabler than the D6h triplet state by 3–39 kcal/mol, depending on the central metal.