[LogicX]

I'm trying to figure out the difference in frequency for different manganese carbonyl complexes:

Mn(Br)(CO)5

fac-[MnBr(CO)3(dppm)]

cis,mer-[MnBr(CO)2{P(OPh)3}(dppm)] (mer referring to the three P atoms bound to the metal)

trans,mer-[MnBr(CO)2{P(OPh)3}(dppm)]

I know that I need to use the spectroscopic series. CO is strong field ligand. So there will be pi backbonding which means the bond will be weakened. This means that the frequency will be lower. But I don't know how dppm (1,1-Bis(diphenylphosphino)methane) will affect the frequency. How do I tell what type of ligand it is, i.e. weak field or strong field?  How do I visualize whether there will be pi back donation or not?

I also don't know how the frequency would change based on whether the two remaining CO ligands are cis or trans to each other. I really have no clue how this affects anything. It changes the point group, so the number of peaks will be different, but I don't know if it would change the frequency or not. Does it not matter?

Can anyone give me some hints towards an answer?

[Yakimikku]

Hi LogicX,

I'm a little confused about what you are trying to figure out. Are you just trying to compare the vibrational frequencies of different types of M-L bonds? When I first saw that list of compounds, I would expect the question one would ask would be "how could you use spectroscopy to distinguish between the four complexes?". The answer to that lies in using the symmetry of the molecules--I could help you with that if that is the case.

To answer a few of your other questions:

You can tell a ligand is a weak or strong field ligand based on it's place on the spectrochemical series. There are some trends you can use to help remember--halogens are on the weak side and amines are strong.

Pi back bonding occurs when the ligand has an available pi antibonding orbital for a metal d-orbital to donate electron density into. Thus, you can predict such to happen with ligands that have pi bonds between the metal bound atom and the next atom in the ligand. So CO and CN^- ligands would work, but other ligands wouldn't (halogens, alkoxides, amines, etc.).

I hope I was at least a bit helpful so far!

[LogicX]

Hi LogicX,

I'm a little confused about what you are trying to figure out. Are you just trying to compare the vibrational frequencies of different types of M-L bonds? When I first saw that list of compounds, I would expect the question one would ask would be "how could you use spectroscopy to distinguish between the four complexes?". The answer to that lies in using the symmetry of the molecules--I could help you with that if that is the case.

Yes, that is one of the questions I was asked to solve.  But I am familiar with the use of character tables and I think I figured it out. 

You can tell a ligand is a weak or strong field ligand based on it's place on the spectrochemical series. There are some trends you can use to help remember--halogens are on the weak side and amines are strong.

Pi back bonding occurs when the ligand has an available pi antibonding orbital for a metal d-orbital to donate electron density into. Thus, you can predict such to happen with ligands that have pi bonds between the metal bound atom and the next atom in the ligand. So CO and CN^- ligands would work, but other ligands wouldn't (halogens, alkoxides, amines, etc.).

I hope I was at least a bit helpful so far!

Ok this is where I am having some trouble.  In addition to using a character table to figure out the number of carbonyl stretching peaks in IR, we are supposed to comment on the average frequency of the peaks.  This is determined by the extent of pi backbonding on the carbonyl, which is turn is dependent on what other types of ligands you have.  I just wasn't sure how to predict whether my other ligands, like dppm which is not listed in the spectrchemical series, are stronger or weaker field than carbonyl.  I'm guessing that it is weaker than carbonyl, but I have no solid scientific basis for that, only the fact that I know carbonyl is at the far end of the spectrochemical series.

I also didn't know how a ligand with different pi backbonding capability would affect the frequency based on if it was cis or trans to carbonyl.  But I have done some research and I think the answer is that trans ligands share the same d orbital of the metal, so they are competing for the electrons of the metal for pi backbonding.

[Yakimikku]

Okay, cool! I'm not an expert, but I'll try my best to give you good food for thought on your questions!

As far as the spectrochemical series is concerned, pi-backbonding ligands are on the far right side (strong field). This makes sense because the pi-backbonding adds some stability to some of the d-orbitals, further increasing the orbital splitting. So, as long as you don't have any other pi-backbonding ligands, I think you can safety assume that CO is the strongest field ligand. I've seen Ph₃P on the high side of the series and so I would say that dppm should also be strong from high structural similarity.

So yes, the pi-backbonding is related to the carbonyl stretching frequency. Since pi-backbonding with CO is a metal d-orbital donating into a pi antibonding orbital of the C-O bond, pi-backbonding is a weakening of the C-O bond. Thus, more pi-backbonding means weaker C-O and hence stretching occurs at lower frequencies.

As for how the other ligands effect the pi-backbonding? This is where I get less confident about my thoughts. I agree though that the effect can be described with orbital theory. Since pi-donation results from metal-ligand pi-bonding, what matters most is likely the ability for other ligands to have pi-interactions with the metal. For your question, you would have to consider bromides, phosphines, and carbonyls.

Another idea I have is a structural trans effect. A strong trans-influencing ligand trans to a carbonyl would increase the M-CO bond length, decreasing orbital overlap, thus weakening pi-donation. So the effect would vary based on the trans ligand in the order bromide < phosphines < carbonyls. I'm not sure if this is an acceptable answer or if considered correct or not by the scientific community.

[ni0kin]

I have the same question.  I'm trying to figure out the point group for those 4 molecules and to be honest I have no clues on how to do so.  My best guess is an O point of symmetry but then all of them do not contain the C3 symmetry element.  Can someone give me insight on how to do this and apply it to character table to find  irreducible representation and the  number of CO band stretch expect.  Sorry if my grammar is bad. Thanks   

[crazygirl]

THE SAME WITH ME,HOW TO DO?

[cheese (MSW)]

The coordination chem. of TM can be divided into two classes: classic (Werner-type) cmplxs with M in M(II) and M(III) (and  M(IV) for 2nd & 3rd row TM); and nonclassical cmplxs  with π-acceptor ligands, (organometallic cmplxs) with M in a 0 low +ve or low -ve oxdn state.  Just as VSEPR is of little value in describing classical cmplxs, CFT is of little use in the description of nonclassical TM cmplxs.   Organometallic cmplxs are usually 18 e⁻ cmpds (especially metal carbonyl cmplxs such as the Mn(I) cmplxs under discussion here) so the d AOs are filled; there is  no d-d splitting  and cmplxs are diamagnetic  (and suitable for study by NMR techniques).
Likewise putting these cmplxs in the right point group and determining the # of ν(CO) vibs is not fruitful, except for Mn(CO)5Br (pt gr?). The other cmplxs have low symmetry (symmetry op(s) in fac-Mn(CO)3P2Br?) and exhibit the same # of ν(CO) vibs as there are CO ligands in the molecule. 
In the interpretation of the spectra of the Mn(CO)5-n(Pn)Br ( n= 2,3 ) cmplxs in the carbonyl stretching region (~2125-1775 cm^-1) consider the following:
For fac-M(CO)3L3  (C3v) cmpds exhibit an A1 and a lower energy E ν(CO) IR pattern;
The mer form has three strong ν(CO) bands.  In fac-M(CO)3X2Y cmplxs the E mode is split.
ν(CO) depends on the amount of backbonding  M→CO (in what way?)
Mn(CO)5Br: ν(CO) vibs higher than in Mn2(CO)10 : why?  Think in terms of the χ(Br).
Addition of a P ligand: compared to CO  P ligands are better σ-donors but poorer π-acceptors than CO (PF3 is a remarkable exception).  If a CO (or two or three) ligand is replaced by a P-donor ligand, what would you predict would be the effect on the backbonding  M→CO to the remaining CO ligands in the P-substituted molecule and hence the ν(CO)? (It is a dramatic shift.)
Tolman arranged P ligands in their order of σ-donor - π-acceptor properties according to the A1 vib in Ni(CO)3P cmplxs.  It is called the Tolman electronic parameter (Wikipedia).  Part of the series: 
(weak σ-donor- strong π acceptor) PF3 (2110 cm^-1)> P(OMe)3> PPh3>PMe3> P(Bu-t)3 (2045 cm^-1) ((strong σ-donor- weak π-acceptor).   [ν(CO)s from memory so check.] C-13 NMR shifts can also be used.  Also see my recent answer to Pt(PPh3)4.
Last point: would you expect the A1 symmetric ν(CO) vib for a  trans-OC-Mn-CO grouping to be weak or strong, especially compared to a cis-Mn(CO)2 grouping?   I will give it to you that the sym A1 ν(CO) vib for a  trans-OC-Mn-CO grouping is at a higher energy (frequency) compared to the corresponding asymmetric  ν(CO) vib, and the stretches in the cis cmpd.  This is because in the symmetric vib there is stretching of the π e⁻ density in opposite directions (←OC-Mn-CO→) which takes more energy than in the asymmetric stretch →OC-Mn-CO→ .