[Mark S 2014]

How should the orbital overlaps look in triangular triatomic hydrogen ? I know that when all three AO's are in phase its pretty much just a triangular sort of shape with rounded edges. The other two I can't get my head round how the 3 orbitals will interact. Any links to pictures or a push in the right direction would be much appreciated !

[Corribus]

Have you done any group theory, using character tables?

[Irlanur]

as a rule of thumb you can remember that you will always have degenerate orbitals if you have at least a 3n axis.

[Mark S 2014]

No, I've never heard of group theory sorry. I have only really considered simple diatomics so far. By 3n axis do you mean three nuclei ? Would this mean two degenerate anti bonding orbitals then ?

[Corribus]

Ok, never mind then; I won't try to show you how to generate them. Triatomic hydrogen isn't stable, but the triatomic cation is. It's also shaped like a triangle. The wikipedia article has a picture of what the MOs look like. There is one bonding orbital and two degenerate antibonding orbitals (one node each). In one of these antibonding orbitals, the node runs through a hydrogen atom. In the other, the node bisects them. (Just think of drawing a cross on top of an equaliateral triangle. The arms of the cross are the two possible places for nodes to be.) Both of these orbitals have the same energy. The reason that the triatomic hydrogen molecule is unstable compared to triatomic hydrogen cation is because in the former, the extra electron goes into a destabilizing antibonding orbital. Oxidation removes this electron, leaving two electrons in a single bonding orbital. Note that it's a single two electron bond spread over three atomic centers, a rather unusual situation.

[Mark S 2014]

Thanks corribus, I really appreciate that you consistently replied with great information. Much appreciated, really did help a lot.

[Irlanur]

In one of these antibonding orbitals, the node runs through a hydrogen atom. In the other, the node bisects them.

Please note that this choice is more or less arbitrary and you could find pictures where it is done differently.

[Corribus]

Well of course the choice of molecular axes is almost always artificial. The choice is not arbitrary, though. By convention we deliberately choose them this way because it makes the math easier and it's easier to see the spatial orientation of the nodes. If you want to choose a different axis system, then by all means - but doing so usually accomplishes little beyond confusing beginners.