Quadruple bonds

Quadruple bonds

A quadruple bond is a type of chemical bond in which four pairs of electrons are shared between two atoms, resulting in a very strong bond. This type of bond is often formed between transition metals, such as molybdenum, and can be characterized by a bonding orbital occupying the space at opposite ends of the molecule. In a quadruple bond configuration, all eight electrons are in bonding orbitals, resulting in a formal quadruple bond.From: Remediation of Heavy Metals in the Environment [2019], New experimental and theoretical assessment of the dissociation energy of C₂ [2019], Wave function analysis with a maximum flow algorithm [2021]

Chemical Bond I: Lewis Scheme

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Published in Franco Battaglia, Thomas F. George, Understanding Molecules, 2018

Franco Battaglia, Thomas F. George

Also, the boron and carbon diatomic molecules raise problems. The boron diatomic molecule exists with a dissociation energy of about 3 eV, but its Lewis structure does not follow the octet rule, neither Boron lacks a sufficient number of electrons to fulfill the rule. Even accepting the single-bond structure (the triple-bond structure is not justified by the dissociation energy), the magnetic properties of the molecule are compatible with two unpaired electrons with parallel spin, a circumstance not contemplated by Lewis scheme. As for the carbon diatomic molecule, Lewis scheme predicts a quadruple bond, little justified by a dissociation energy of 6.2 eV and better suited to a double bond. Finally, we mention the oxygen molecule, O2, whose magnetic properties imply the existence of unpaired, parallel-spin electrons, unpredicted by the formula (6.9) from Lewis scheme.

Behind the success of modified coupled-cluster methods: addition by subtraction

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Published in Molecular Physics, 2019

Varun Rishi, Ajith Perera, Rodney J. Bartlett

Carbon dimer is another difficult problem where the effects of quasi-degeneracy can be seen even at the equilibrium geometry and obtaining the correct ordering of low-lying states is challenging for low-level ab initio methods. The ground state () of C has multi-reference character with contributions from two important configurations: and . Conventionally, it has been argued that carbon dimer has a double bond but some have argued in favour of a quadruple bond [66,67]. To correctly dissociate this bond of an unconfirmed bond order into two open-shell atomic fragments () is considered to be difficult [68–70].

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Chemical Bond I: Lewis Scheme

Behind the success of modified coupled-cluster methods: addition by subtraction