What is the highest number of covalent bonds observed?

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.

#"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.