IR Frequency of Manganese Carbonyl Complexes

[LogicX]

Homework Statement

This is for a lab report I'm trying to write. I need to figure out what the effect of different coordinations of a manganese carbonyl complex should be on the frequency of the carbonyl stretch in my IR spectrum.

Homework Equations

There are 4 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)]

The Attempt at a Solution

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?

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?

 

[nate808]

Hello,

Thank you for reaching out to the forum for help with your lab report. I can provide some guidance on how to approach this problem.

First, it is important to understand the concept of ligand field theory. This theory explains how the electronic structure of a metal ion is affected by the surrounding ligands. In general, ligands can be classified as either strong field or weak field based on their ability to split the d-orbitals of the metal ion. Strong field ligands, such as CO, have a large splitting effect and result in a low-spin complex. Weak field ligands, such as dppm, have a smaller splitting effect and result in a high-spin complex.

In your case, CO is a strong field ligand and dppm is a weak field ligand. This means that the coordination of dppm will not significantly affect the frequency of the carbonyl stretch in the IR spectrum. However, it is still important to consider the coordination of dppm, as it can affect the overall electronic structure of the complex and potentially impact other spectroscopic properties.

Next, the cis/trans arrangement of the remaining CO ligands can also affect the frequency of the carbonyl stretch. In cis arrangements, the CO ligands are closer to each other, resulting in a stronger interaction and a lower frequency. In trans arrangements, the CO ligands are farther apart, resulting in a weaker interaction and a higher frequency.

In summary, the coordination of dppm will not have a significant effect on the frequency of the carbonyl stretch, but it is still important to consider. The cis/trans arrangement of the remaining CO ligands will have a noticeable effect on the frequency, with cis arrangements resulting in a lower frequency and trans arrangements resulting in a higher frequency. I hope this helps guide your analysis. Good luck with your lab report!