IR frequency question.

LogicX

1. The problem statement, all variables and given/known data

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.

2. Relevant 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)]

3. 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?