[killacam2]

Hi,

I was wondering if anybody could help with the following:

Explain for the differences in CO stretching frequencies shown below

Complex
Frequency

[Mn(CO)6]+
2090

Cr(CO)6
2000

[V(CO)6]-
1860

[Ti(CO)6]2-
1750

Evidently, they decrease as the central metal ion goes down period 4.

I know the following is true:

• For CO, the stretching frequency is lowered as other e- rich donators are added to the metal ion. however, all of the ligands in each of the complexes are CO ligands, so this can't be the reason.
• "Bridging" of M ions by the CO group lowers in stretching frequency. As the CO is bonded to more and more Metal ions, the frequency lowers. However, I dont think there is any bridging in any of the complexes above?

I know it has something to do with M ion and the bonding in the complex. Is it because the M-L bond is stronger progressively? Because I know this weakens the C-O bond. Please can somebody give me the reason behind the drop in CO stretch frequency as we go along the period.

THANK YOU!

[BluePill]

Just a quick look at it, I could see that the nuclear charge decreases from Mn to Ti. Therefore, Ti does not attract electrons appreciably and it has the greatest tendecy to donate electrons to the pi* orbital of CO. This reduces the strength of CO bonds (possibly due to increased electron repulsion).

It's just a just a hypothesis though. I'm still don't have a good grasp with MLCT or LMCT bonding.

[nox]

Figure out the formal charge on the metal center (bearing in mind that CO are considered neutral ligands). The answer should come to you right away.

[cheese (MSW)]

Note all the cmpds obey the 18 e⁻ Rule and the oxidation state range from Mn(I) in [Mn(CO)6]^+ to
Ti(-II) in [Ti(CO)6]^2-.  As you should know, the M-CO bond consists of e⁻ pair donation from σ3 on CO to a suitable vacant σ AO on the metal.  There is π-backbonding from M 3d→ π* CO.  Because the π* becomes occupied the CO bond order decreases (and what happens to ν(CO)?).  With this in mine can you now explain why the ν(CO) decreases as the e⁻ on the metal increases.  More advanced: from PMO theory there is better overlap when the AOs forming the MO are closer in energy.  What happens to the energy of the 3d AOs as M goes from M(+) to M(2-)?  It is given that the π* CO MOs are higher in energy than the 3d AOs.