Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Markovnikov on October 20, 2009, 01:31:46 PM
-
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.
Carbene
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?
Phosphines
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...
-
Hey good question and good job showing all of your work. This question is way too advanced for me but if Arctic-Nation or nj_bartel comes through or one of the mods comes here they will be able to help. I just wanted you to know that at least someone read your thread.
-
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?
Excellent questions and analysis. It's a complicated subject for sure.
With respect to the back-bonding of the metal to the carbene, think about the atomic orbitals involved in the sigma bond and the back-bond. Sigma bond arises from an sp2-hybridized orbital that is filled, while the back bond involves a vacant p-orbital. As a result of the orthogonality of these orbitals, the effect of adding electron density to the empty p-orbital will be only a minimal rise in the energy of the sp2[/sub] orbital. Think of the carbon equivalent (an alkene), does the pi-bond significantly affect the sigma bond energy?
As for the metal orbitals you need to know something about the number of electrons you are looking at to really comment of how EDG vs. EWG ligands will operate. Generally EWG-type ligands will bind more strongly with low-valent metals. Think of the EWG as providing other places for the relatively high-lying electrons to go without actually oxidizing the metal.* EDG ligands will bind strongly to higher oxidation state metals, which you can think of in electrostatic terms (to a first approximation). Of course certain donor atoms are more suited to particular metals for a number of reasons.
For phosphines the type of back-bonding again depends on the nature of the phosphine. It will probably not be through the M-P sigma* however since that would involve an empty metal atomic orbital and an occupied P a.o. (the reverse of how you get the M-P bond in the first place) so the orbitals aren't really occupied in the right way for that kind of bonding. More likely is bonding to a sigma* orbital from the P and one of the substituents on the phosphine. It's kind of like hyperconjugation in reverse.
Finally, just another general comment on your description - It is not rigorously correct to say that metal orbitals (atomic orbitals) are pushed up or down in energy by the presence of a ligand. The metal orbitals are static for the metal itself. Once you start adding ligands you get orbital mixing that leads to molecular orbitals composed of the atomic orbitals. The atomic orbitals themselves don't change, just the way and degree that they are populated.
* Sometimes ligands will actually be reduced by low-valent metals even while still bound to the metal. This is called "non-innocent" behavior.
-
Thank you very much for your answer movies! It was certainly most helpful!
I'm sorry for the late reply, I'm having time management problems in real life. So much to do - so little time!