Why Quintuple and Sextuple covalent bonds are rare?

Answer 1

A couple of reasons. Firstly, a sextuple bond involves six electron pairs, twelve bonding electrons in total. On small atoms, there are simply not enough orbitals to be able to point in the correct orientation for constructive interference (i.e, for a bond to be formed). Secondly, these electrons have to be distributed over a larger surface area to be stable, otherwise the electron pairs repel too much for the bond to be energetically stable, so these bonds are only possible for larger atoms. Due to electron repulsion, these bonds are relatively unstable on their own, and are usually stabilised by large (usually bidentate) ligands.

The more complex reason refers to Molecular Orbital Theory. If only using p- and s-orbitals, or even hybridised orbitals, the maximum number of orbitals per atom is four, so only eight molecular orbitals can be formed on a diatomic molecule: four bonding and four anti-bonding orbitals. This means that the highest bond order possible would be four, but due to the placment of molecular orbital energy levels, the highest bond order is three. D-orbitals are required for quintuple/sextuple bonds, and d-shell chemistry is less well-studied than the s- or p-blocks. This d-orbital interaction is most energetically favourable for metals, which do not usually exhibit covalent bonding, since metallic bonding is energetically preferable. Dimolybdenum (one of the two compounds known with a sextuple bond) only exists at 7K (seven degrees above absolute zero) in the gaseous phase, at very low energy.

So that's why higher bond orders than three are almost unheard of. However more research is currently going into these electron-dense bonds due to their unusual chemistry.

Answer 2

Quintuple and sextuple covalent bonds are rare because they require the overlap of multiple atomic orbitals, which becomes increasingly difficult and energetically unfavorable as the number of bonds increases. The atoms involved must also have the appropriate electronic configurations and spatial orientations to allow for such extensive bonding. Additionally, the high electron density associated with quintuple and sextuple bonds can lead to steric repulsion and destabilization of the bond.