I have written many posts about organometallic compounds, particularly those that are on interest in organic LEDs and photovoltaic cells. Almost all quantum chemical treatments are based on density functional theory (DFT). But given that both transition metals and the excited states of conjugated organic molecules are typically strongly correlated I wonder about how reliable this is. For a while I have been wondering about a valence bond theory description of these materials. A key effect that needs to be taken into account is that of "back-bonding" and the Dewar-Chatt-Duncanson model. Hence, I was quite delighted when yesterday I stumbled across a 2007 Inorganic Chemistry paper, Valence Bond Approach to Metal-Ligand Bonding in the Dewar-Chatt-Duncanson Model.
A few key findings are:
-the importance of including back-bonding
-a VB wavefunction including back-bonding captures a lot of the correlation energy calculated by the CCSD(T) method
[n.b. there is typo on p. 11394 at the beginning of the paragraph, entitled "Correlation energy". VB-II should be VB-I]
-the component 3 of the VB function [see Figure above] corresponding to Pd2-L2+ has small weight.
-one must use "breathing orbitals" to get good results, i.e. when there are two electrons on Pd or L one makes one of the electrons more diffuse than the other [see Figure above].