[Winga]

About constructing a new MO diagram from HOMO-LUMO interactions.





For this kind of interactions, both HOMO & LUMO of butadiene and ethene are shown to construct a new MO diagram.

How about Mg²⁺ + H₂O?
The HOMO of H₂O interacts with Mg²⁺ LUMO, should I also indicate the interaction between the LUMO of H₂O and HOMO of Mg²⁺?

[alkemist]

It is not really appropriate to compare the two, since butadiene and ehene form a new compound, i.e. new chemical bonds.  Mg2+ and H2O interaction is ionic, and no chemical bond is really formed.  HOMO-LUMO diagrams are usually used to represent the new chemical bonds and how their energies compare to the energies of individual levels of each component.

[maxyoung]

HOMO of Mg2+ is the 2p orbitals, and LUMO of H2O is the antibonding sigma bond. The energy gap between these two orbitals is quite large. According to MO theory, there would be little interaction between the two orbitals. You really don't need to show this interaction at all--really we don't hear somedody saying Mg2+ donates electrons to H2O.

[Winga]

HOMO of Mg2+ is the 2p orbitals, and LUMO of H2O is the antibonding sigma bond. The energy gap between these two orbitals is quite large. According to MO theory, there would be little interaction between the two orbitals. You really don't need to show this interaction at all--really we don't hear somedody saying Mg2+ donates electrons to H2O.

But my lecturer drew both HOMO-LUMO interactions to explain the adduct formed from Mg²⁺ and H₂O.
I think it is not necessary to draw it in this case, but necessary for the butadiene-ethene reaction, right?

[maxyoung]

Don't know why your lecturer shows both. Does he try to show you something about symmetry of the orbitals(e.g. they have the correct symmetry to overlap)?

[Winga]

Don't know why your lecturer shows both. Does he try to show you something about symmetry of the orbitals(e.g. they have the correct symmetry to overlap)?

Nothing to do about the symmetry, I think she wanted to tell us that similar energy of 2 orbitals to interact would give larger stabilization and larger destabilization, while 2 orbitals are very different in energy (e.g. Mg2+ HOMO and H2O LUMO) would give smaller stabilization and smaller destabilization.