Question

What is the highest number of covalent bonds observed?

Answer 1

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.

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`"N"_2` is a simple example of a triple bonded molecule. Each `"N"` can overlap:

• its `2p_z` orbital to make a sigma (`sigma`) bond
• its `2p_x` orbital to make one of the pi (`pi`) bonds
• its `2p_y` orbital to make one of the pi (`pi`) bonds

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

`:"N"-="N":`

However, in the `"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)` `bbsigma` overlap
• one `d_(xz)`-`d_(xz)` `bbpi` overlap
• one `d_(yz)`-`d_(yz)` `bbpi` overlap
• either a `d_(xy)`-`d_(xy)` or a `d_(x^2-y^2)`-`d_(x^2-y^2)` `bbdelta` overlap

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

Bond length: `~~` `"202 pm"`.

(Note: `"Ar"` probably just means aromatic ring, not argon.)

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