question on VO(acac)-piperidine

[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.

[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? Here's a pretty pic. (http://i38.tinypic.com/25z5pwg.gif))

[warenzeichen]

about 40 cm^-1

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.

[alxm]

about 40 cm^-1


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


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

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-.

[warenzeichen]

Thanks for your answer.
and finally could you tell me how to calculate it ? It seems that I can not calculate it based on the information I knew in the IR( I don't learn the IR on metal in details.... But it looks pretty similar to IR but in more complex way...)

[ChemHater]

about 40 cm^-1

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.
Hello there,

The stretching frequency of v(V=O) band is 957 cm^(-1), could you please tell me if it is your experimental value or literature value?

Thanks