We've recently discussed the PMO theory in class. Today we went through the difference of carbenes and phoshines as ligands for metal complexes.
I just wanted to check if I fully understood the lecture...
The picture shows the σ and π interactions of a Fischer carbene and a phosphine ligand towards metal.
The carbene - metal sigma interaction is between the C(nσ) and an empty M(dz2). This leads to a stabilization of the orbital. The σ interaction can be increased by destabilizing the C(nσ) energy level. This is done by changing the substituents of the N to EDG.
The carbene - metal interaction is a π-backbonding interaction from the M. This can be stabilized by stabilizing the C(nπ) energy level. This is done by changing the substituents of the N to EWG.
The confusion here is, if you stabilize the π-interactions, the σ-interactions will automatically be decreased. Is there one kind of interaction that you "care" more for?
Are the metal energy levels affected the same way? Stabilized (decreased) by EWG ligands and destabilized (increased) by EDG ligands?
The same thoughts were applied for the phosphine except in this case, you can destabilize (increase) the energy level by changing the sterical bulk of the substituents. When you increase the size of the substituents you force the phosphine from a tetrahedral to a tetragonal pyramidal geometry. This would then destabilize the energy level since the orbital self-overlap is decreased.
For the phosphine the π interactions are between the M -> P-C(σ*)
Does this affect the phosphine in another way than for the carbene since the π interactions were between
M -> C(nπ)
Thank you for any feedback. Spell check didn't work when I tried it...