Quad Bond

Yes, but not for carbon.

The reason carbon can't four-bond is because of its tetrahedral geometry. Even with only half of the orbitals hybridized, one orbital on each carbon atom will still face "away" from the bond.

As it turns out, quadruple bonds, pentuple bonds, and even a sextuple bond have all been observed. Where? Metal-metal bonds in complexes, especially the heavier metals with loose electrons. I believe Tungsten and Rhenium both exhibit this odd behavior.

My theory as to why this happens? d-orbitals.

 

Correct. Overlap between d-orbitals in metal-metal covalent bonds can produce a delta bond which is required to explain quadruple bonding species.

 

Haha! I knew it!

Thanks for proving me right. Honestly, sometimes physics is so screwed up I think the intuitive answer, though right in this case, could ever be correct.

 

Specifically I remember about some Rhenium and Technetium compounds with a formal Re-Re or Tc-Tc double bond.

 

Doesn't mean it can't be higher as well.

 

I don't know about the names, but I can tell you that the highest order bond observed is a sextuple bond, sharing 12 electrons.