# Question on VO(acac)-piperidine

1. Oct 5, 2009

### warenzeichen

I am curious of the reason why the V=O stretching here has a lower frequency than that in VO(acac)2. Is it related to resonance? I am not sure.

2. Oct 5, 2009

### alxm

Yeah, you can explain it in terms of resonance.

Basically (ignoring the two existing bonds to vanadium) you have V=O <--> -V-O- where in the latter case, some ligand forms an additional bond to the vanadium atom, creating a cation somewhere else.

In the case of VO(acac)2, an oxygen forms an additional bond, leaving a cation on the carbon it is bonded to. With piperedine, the nitrogen can form a second bond to vanadium, with its lone pair, making the nitrogen a cation.

The nitrogen cation is more stable than the carbocation, so the V-O- resonance form is more stable with piperidine, so the V=O double-bond is weaker, so the V=O stretch has a lower frequency.

(Updated: I did a calculation, it's a pretty small shift, about 20 cm-1? http://i38.tinypic.com/25z5pwg.gif")

Last edited by a moderator: Apr 24, 2017
3. Oct 5, 2009

### warenzeichen

the V=O stretching vibration for VO(acac)2 at 995cm^-1 and for VO(acac)-piperidine it is at 957cm^-1

I think there should be four bonds except the V=O ? also, one acac is substituted by piperidine, isn't it?

Thanks.

4. Oct 5, 2009

### alxm

Well, close enough - well within the margin of error. :)

Well, with VO(acac)2 you'll have four oxygens liganding, but with two bonds which you could view as resonating between the two acetylacetonate oxygen atoms. So, 'half' a bond to each oxygen would really be a better way of thinking of it. (and in the zwitterion-resonance form, you'd have two full bonds to one acac, and a carbocation on the same acac) With VO(acac) you'd replace one acetylacetonate with the piperidine (CH2)5N-.

5. Oct 6, 2009