I have been reading about introductory Molecular Orbital Theory lately. Till now, I have used Valence Bond Theory to evaluate bonding. I have a few questions about it:-

  1. In VBT, we used the idea that bonds form when orbitals overlap. In Oxygen molecule, two bonds forms in VBT as the other orbitals were far enough to not overlap . But in MOT we say that all the orbitals overlap to form new MOs. Aren't the two statements contradictory?

  2. In Coordination Compounds, when we study about non-classical $\pi$-acceptor ligands,we say that Bond Order of the ligand decreases when it receives electron pair from transition metal in its Anti-Bonding orbital. However, if I try to use VBT for the same, I feel that Bond Order should increase or at least remain the same as any kind of overlapping leads to stability in VBT. Where am I wrong?

Please try to explain the matter without using complex mathematics.

  • $\begingroup$ Roughly speaking, the two theories are all but unrelated. You don't have orbitals in VBT. Also, you don't really have bond orders in MOT. There is simply no area where they could possibly contradict each other. $\endgroup$ – Ivan Neretin Oct 12 '20 at 8:43
  • $\begingroup$ The two theories necessarily describe the same sum total of electron density at any point in space, but break that sum into different components. Just as 5 and 7 are different numbers from 9 and 3, but 5+7 and 9+3 are both equal to 12. $\endgroup$ – Andrew Oct 12 '20 at 11:57
  • $\begingroup$ The statement that orbitals do not overlap is wrong. They might not overlap much.Also, the idea of sharing electron density using anti-bonding orbitals is bound to confuse, I agree. It's best to think of involvement of anti-bonding orbitals resulting in high-energy MOs which, as the name suggests, reduce sharing of electron density. $\endgroup$ – Buck Thorn Oct 13 '20 at 4:13

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