I was wondering about Hydrogen and wether or not it can form Pi bonds.
I dont think Hydrogen can form Pi bonds.
Since a double bond is made up out of one Sigma and one Pi bond, a Hydrogen atom would need to form a double bond for this to happen.
I dont see any reason why a Hydrogen atom would need to make a double bond since it is nice and stable with just a single bond. A Hydrogen atom only needs 1 additional electron to obtain a Noble gas configuration, so there would be no need for a double bond.
No, 1s atomic orbitals and pi atomic orbitals don't have the same symmetry, thus they don't interact to form molecular orbitals.
The only time I have ever seen hydrogens with forming more than 1 bond, if you can call it that, is in things like carborane cages. I'm not sure of the theory behind it, but carboranes are stable structures with hydrogens that have more than 1 bond.
If you take inorganic chemistry, you'll learn that, basically, any orbital (atomic or molecular) that has the same symmetry as the 1s atomic orbital of the hydrogen can interact (in relevance to group symmetry applications, such as LCAO and the theories). Two pz atomic orbitals, can approach the s orbital from opposite sides and interact with the proper symmetry, more or less covalent. Another example pertaining this subject that gravenewworld has mentioned is the cobaloximes, such as Co[R](dmgH)2(B), where the two dmgH molecules are coordinately bonded to the cobalt in a square planar fashion and are linked together by hydrogen bonds.
its not always neccessary that there be a sigma bond for the formation of a pi bond. the C2 molecule(yes, such a molecule exists), consists of only two pi bonds, making a double bond. however, in most cases a double bond is made up of one sigma and one pi bond, but not in this case.
If I am not mistaken, the formation of a pi bond requires side on side overlap of the px and py atomic orbitals. Seeing as hydrogen only has a single 1s orbital, it is energetically unfavourable and/or impossible to form a pi bond to hydrogen.
It is theoretically possible to have a pi-bond to hydrogen, the necessary p-orbitals are there, but they are relatively high in energy. So, I would agree with jer83 that it is energetically unfavorable, although not impossible.
And there are examples of pi bonds without sigma bonds. I saw someone talk about a 4-membered ring system where there was evidence of an interaction of two p-orbitals on atoms at opposite corners of the ring. They referred to it as "banana bonding."
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