Donating electrons back to ....

What are the dominant orbitals and interactions in organometallic complexes?

Chapter 3 of the book, Molecular Orbitals of Transition Metal Complexes, by Yves Jean, [which has a forward by Roald Hoffmann] has a nice discussion of the pi orbitals in organic molecules with the d-electrons on the transition metal, pi back-bonding and the Dewar-Chatt-Duncanson model.


In the left figure above shows a pi orbital of ethylene with a sigma bond interaction with a d orbital. This leads to donation of electron charge from the ligand to the metal.

The right figure shows the overap of a pi* (anti-bonding) orbital with a metal d-orbital. This leads to back-bonding and charge transfer from the metal to the ligand. Experimental signatures of this interaction include:

• an increase in the length of the C-C bond in the ligand
• a softening of the ligand C-C stretch frequency
• substantial spectral weight associated with a metal-to-ligand-charge-transfer (MLCT) optical transition
• basicity of the ligand (i.e., its ability to receive protons) [remember acids like to give up protons] increases by orders of magnitude.
  

The second last point is nicely described in papers by Zwickel and Creutz and Reimers and Hush. They show that the transition dipole moment for the MLCT transtion is proportional to the hybridisation parameter (Huckel or hopping parameter) between the metal d and the ligand pi* orbital.