cis and trans, infrared

i have to say what the geometric isomers are formed by inspection of IR! [Mo(CO)4(PPh3)2] < is my compound

Well that's quite important for the question!

If it's trans (PPh3 - M - PPh3) then all the C=O shifts would be the same and thus you would only see one C=O peak. If its cis (PPh3 - M - CO) then you should have two C=O peaks

thanks for that information
What is your theory behind this and why is this so?
I need to be convinced

Well that's quite important for the question!

If it's trans (PPh3 - M - PPh3) then all the C=O shifts would be the same and thus you would only see one C=O peak. If its cis (PPh3 - M - CO) then you should have two C=O peaks

For trans, could you not see two peaks? One for a symmetric and one for an asymmetric stretch?

Four year old thread

Whilst there will be asymmetric and symmetric stretching for the linear OC-M-CO arrangements, only the asymmetric stretch will be detected since the symmetric stretch leaves the dipole moment unchanged.

Edit: Though actually looking it up, the cis should have four peaks apparently possibly due to two types of stretching being detected for each type of CO there(?) I haven't done any of this in years

You can, and really ought to, solve this more rigorously by using group theory. That will tell you exactly how many bands you should expect to be active for IR for each combination.
This works particularly well for carbonyl groups.
You can't always tell simply by looking at the molecule.

Thanks for that.

Also, as said above, the wavenumber of the streches will be important. There's a thing called the trans effect which is the effect a ligand has on the bonding between the metal and the ligand trans to it, due to the fact that ligands trans to each other interact with the same metal d orbitals. As CO is a better pi acceptor than phosphines, when a CO is trans to a phosphine, it will have higher degree of back bonding, and hence a weaker CO bond (but stronger M-C bond) than when a CO is trans to a CO.

Explained pretty badly but you might remember it :P

The actual wavenumber is not a reliable indicator at all. The only reason IR is useful in this case is because CO peaks are so characteristic that IR is ideal. It's the number of peaks which helps you out here and distinguishes the two isomers. You can even calibrate it and get the ratio of each isomer present in a mixed sample.

By the way, you've described trans influence not trans effect.
Trans influence is the weakening of the bond trans to the ligand in question. It's a thermodynamic effect and affects the stability of the complex. You've described it the wrong way round as well. A CO trans to a CO will have a weaker M-C bond than a CO trans to a phosphine precisely because CO has a stronger trans influence than a phosphine.

You've described it the wrong way round as well. A CO trans to a CO will have a weaker M-C bond than a CO trans to a phosphine precisely because CO has a stronger trans influence than a phosphine.

Trans effect on the other hand is a kinetic effect and affects the rate of substitution trans to the ligand in question. The two aren't the same. Mind you a ligand with a strong trans influence will usually also have a strong trans effect. Not always though. There are some striking exceptions.

Well we've covered a lot of content where the wavenumbers of the peaks can be diagnostic. Of course, not as diagnostic as the number, but still something you'd talk about if asked to compare the two spectra. You would get different wavenumbers for the ones trans to a CO and the ones trans to a PPh3.



Trans effect can be used to describe both of them. You can talk about the kinetic and thermodynamic parts. It is also common to use trans influence but trans effect is not wrong.


That is what I meant to say, just wrong the wrong thing, damnit. It's a friday afternoon and I've spend the day inhaling chloroform in the lab Of course, one orbital donating into two forms a weaker bond than donating into one - a case of typing the wrong thing



I know that the two parts are effected by different things - though this is usually studied when looking at ligand substitution and the effects on either the ground state and how it is destabilised, or the stabilisation of the transition state.


I appreciate the comments and thanks for pointing out my mistake, but I do (I like to think), know what I'm talking about :P

Perhaps I misunderstood you. I understood you were claiming trans effect and trans influence to be essentially the same thing. If that's truly what you think then I'm afraid this is quite simply wrong. I may however have misunderstood. It is late at night.

The trans effect consists of two phenomena:
• Ground state contribution (also known as trans influence): the weakening of a trans - positioned ligand in its ground state (thermodynamic control)
• Transition state contribution (also known as trans effect): the stabilisation of the transition state by a trans -
ligand (kinetic control)

You can have this debate with my lecturer. I'm reading it straight out of the notes. I'm saying that the term trans effect can be used to cover what you are calling the trans influence as well as your trans effect.

You are either misunderstanding the notes or he's made an error bud.
You should talk to him. I've worked in this area professionally as a research scientist. Lecturers make mistakes and students misread notes. I worked in research last year which required me to know the difference.
If trans effect and trans influence are the same you might want to question why they each have a different series associated with them.
You'll find the different series in primary literature.

You will find many examples in science where more than one term is used to describe the same thing. This is one of those cases. I am perfectly capable of understanding some pretty simple english in my lecture notes. The trans effect is simply the effect a ligand has on the one trans to it, there are kinetic and thermodynamic components of this effect, but they come under the same umbrella and it is perfectly correct to use an umbrella term. Sometimes the kinetic and thermodynamic components are given more specific names, but not always.

For what it's worth, wikipedia agrees with me.

If you are using Wikipedia as a source of information at the age of 22 (??)then you seriously have a lot to learn.

This is a common interview question BTW. Good luck trying to insist those two terms are interchangeable.
It would be a real shame if someone on here genuinely needing help read your post and went down the wrong path because of it but that's life.