# What is the highest number of covalent bonds observed?

May 28, 2017

A common maximum number of covalent bonds is $3$, though bonds of higher order have been observed. Three is a common maximum when $d$ orbitals are not available for use.

$s$ orbitals tend to make poorer overlap than the ${p}_{z}$ orbital of the same $n$ (less outwardly polarized), so in cases without hybridization, ${p}_{z}$ orbitals are favored for the $\sigma$ bond, and the other two $p$ orbitals can be used for the remaining two $\pi$ bonds.

${\text{N}}_{2}$ is a simple example of a triple bonded molecule. Each $\text{N}$ can overlap:

• its $2 {p}_{z}$ orbital to make a sigma ($\sigma$) bond
• its $2 {p}_{x}$ orbital to make one of the pi ($\pi$) bonds
• its $2 {p}_{y}$ orbital to make one of the pi ($\pi$) bonds

Thus, three bonds are made in total: one $\sigma$ and two $\pi$ bonds.

$: \text{N"-="N} :$

However, in the ${\text{Re"_2"Cl}}_{8}^{2 -}$ anion let's say, rhenium makes a quadruple bond:

As usual, the first three bonds are one $\sigma$ and two $\pi$, but the fourth bond is a $\delta$ bond. When it comes to rhenium in this scenario, it is probably:

• one ${d}_{{z}^{2}}$-${d}_{{z}^{2}}$ $\boldsymbol{\sigma}$ overlap
• one ${d}_{x z}$-${d}_{x z}$ $\boldsymbol{\pi}$ overlap
• one ${d}_{y z}$-${d}_{y z}$ $\boldsymbol{\pi}$ overlap
• either a ${d}_{x y}$-${d}_{x y}$ or a ${d}_{{x}^{2} - {y}^{2}}$-${d}_{{x}^{2} - {y}^{2}}$ $\boldsymbol{\delta}$ overlap

Quintuple bonds are rare but one of these can be seen below.

Bond length: $\approx$ $\text{202 pm}$.

(Note: $\text{Ar}$ probably just means aromatic ring, not argon.)

It would have one $\sigma$, two $\pi$, and two $\delta$ bonds.