[xshadow]

Hi!!

I don't understand why in some complex like M-CO (where M= K+,Na+,Cs+.Rh+)  the stretching frequency of the CO bond  INCREASES (blue shift)


In this complex there is a sigma donation,from the HUMO of CO to the metal  (no backdonation,no "d" electrons).
How can sigma donation increase the freqeuncy  (i.e. the strenght  of this bond)  ??

CO """loses""" some electrons when acts like a sigma donator ,so the CO bond shouldn't be less strong?)


Thanks!!

[xshadow]

No  one??

I don't understand this blue shift of CO

[xshadow]

Someone can explain me why there is a blue shift when CO interacts with M (where M=Na+,Li+,K+,Cs+)

I know that there is sigma donation from the HOMO of CO to "M" center.


Now if there is a blue shift it means  that the CO bond becomes stronger than before...BUT how this can be possible?
(a sigma donation from CO towards another species...how can this increase the CO bond strenght)

Please help me!!
Thanks.

[Corribus]

Perhaps not exactly what you are looking for, but similar enough.

https://en.wikipedia.org/wiki/Pi_backbonding

[xshadow]

Perhaps not exactly what you are looking for, but similar enough.

https://en.wikipedia.org/wiki/Pi_backbonding

In that I case I understand why...The metal send some electron density towards an antibond orbital thanks the backdonation
So it is clear that the CO bond becomes weaker...


But here is different...the CO   makes a sigma donation from its HOMO( a non-bonding orbital with a very little bonding character)  to the empty "metal" orbital.

How can this creates the opposite situation? I.e. a CO bond weaker??

[Corribus]

Maybe it would be helpful if you would provide some actual data for me to look at, then we can work through it.
There are plenty of places that discuss CO ligand interactions with metals:

e.g. (first hit through google)

https://organometallicchem.wordpress.com/2012/01/17/epic-ligand-survey-carbon-monoxide/

But you seem to have a very specific question that's framed in a very vague way.

[chrugel]

Hi, I see that this post is older, yet if you still need an answer, here it is:
The sigma donation has (more or less) no effect on the CO bond strength. As there is no pi-backdonation into the CO pi* orbital, the bond is not weakened. The effect that is responsible for the shortening (blue shift) of the CO bond is electrostatic in nature: the electron density in CO is polarized to the oxygen in the free CO. When the carbon binds to the cationic metal, this polarization is (partially) compensated (coulombic attraction between positive charge on metal and electrons in binding orbitals). This results in a more covalent CO bond and hence a shortening/strengthening of it. These effects are not considered in MO-LCAO models, which is why it is hard to account for them beforehand.
I hope this is the answer you were looking for.

[Flatbutterfly]

There are thousands of stable carbonyl cmplxs of TM.  The TM are typically in a zero or low oxdn state (e.g., hexacarbonylchromium(0),Cr(CO)6, is an air-stable, white crystalline solid;  ν(CO) = 1984 cm^-1).  To rationalize the bonding in these cmplxs we have to look at the HOMO-LUMO interactions.
For the HOMOs and LUMOs on CO we must use the sp mixing MO scheme:
CO: 10 valence e⁻ =  σ1(2e⁻)σ2 (2e⁻)π1(4e⁻) σ3(2 e⁻)………..π2*(0e⁻) σ4*(0e⁻)
The HOMO is σ3 and is located mainly on C and is donated to the σ MO on the Cr:  O≡C:→Cr.
(hybridization such as sp^3d^2 is not used). The π* MOs have the correct symmetry and energy to overlap with the filled tg Cr AOs and there is π back-donation that gives the M-CO partial double-bond character.  It is also called synergic bonding: the buildup of e⁻ density on the metal by the σ donation is offset by the π back-donation. Up until ~20 years ago this was fine; the
ν(CO) of typical metal carbonyl cmplxs come in the 2125 - 1850 cm^-1 range of the IR. Borane carbonyl O≡C:→BH3 exhibits the CO stretch at 2165 cm^-1 some 22 cm^-1 above that of uncoordinated carbon monoxide. [1]
There were then reports of stable binary metal carbonyls [(MCO)6]^n+ where π bonding must be small due to the contraction of the d AOs.  These cations have high CO stretching frequencies (e.g., [Ir(CO)6]^3+ ; ν(CO) = 2295 cm^-1) [2].
The most remarkable cmplx of this type is Hg(CO)2]2+ (2(Sb2F11)^-1) where ν(CO) = 2280cm^-1) [3].  In the nonTM Hg (especially Hg^2+) the 5d e⁻ are considered as core electrons that do not participate in bonding.  These observations are rationalized that the bonding is mainly  σ bonding and the  HOMO σ3 on CO is slightly antibonding in character (C 2p AO at higher energy); and donation of electron density therefore increases the CO bond order resulting in the shift to higher frequencies in this and similar cmplxs.
I think there is more to this argument but let me leave it at that.
[1] https://en.wikipedia.org/wiki/Borane_carbonyl
[2] Organometallics 1997, 16, 4807.
[3] (Aubke and co-workers) Inorganic Chem. 1996, 35, 82.