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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: Vital Y on February 09, 2016, 12:12:41 AM

Title: Stretching vibrational frequency in coordination compounds
Post by: Vital Y on February 09, 2016, 12:12:41 AM

The question was:

Arrange the following in increasing order of the stretching vibrational frequency of the CO ligand in the following complexes:

(i) [(PCl₃)₃Mo(CO)₃]
(ii) [(PPhCl₂)₃Mo(CO)₃]
(iii)[(PPh₂Cl)₃Mo(CO)₃]
(iv)[(PPh₃)₃Mo(CO)₃]

I think the answer would be (iv) < (iii) < (ii) < (i), because PPh₃ ligand donates electron density into the metal atom. This additional electron density is used in forming a back bond between the metal and the CO ligand. The back bond decreases the frequency. Chlorine however hinders the formation of the back bond by withdrawing electron density from phosphorus so that electron density of the metal will not be sufficient to form a back bond.

Is this the correct approach?

Title: Re: Stretching vibrational frequency in coordination compounds
Post by: clinz63 on February 09, 2016, 09:46:10 AM

Yup.

Title: Re: Stretching vibrational frequency in coordination compounds
Post by: Enthalpy on February 09, 2016, 03:41:04 PM

And masses play no role here?

Title: Re: Stretching vibrational frequency in coordination compounds
Post by: clinz63 on February 09, 2016, 08:53:55 PM

No because it's only the strength of the CO bond that matters here.

Title: Re: Stretching vibrational frequency in coordination compounds
Post by: Enthalpy on February 10, 2016, 12:10:28 PM

Why? Don't the PPh_xCl_3-x move here?

Title: Re: Stretching vibrational frequency in coordination compounds
Post by: clinz63 on February 10, 2016, 10:30:30 PM

Yeah, it does. However, the question is to arrange the complex in increasing vibrational frequency of CO. So yeah... We only consider what will affect the vibrational frequency of CO.

Title: Re: Stretching vibrational frequency in coordination compounds
Post by: Vital Y on February 11, 2016, 09:32:48 AM

Will having a low or high spin complex affect the stretching vibrational frequency?

Title: Re: Stretching vibrational frequency in coordination compounds
Post by: clinz63 on February 11, 2016, 11:40:23 PM

Hmm... Since we're talking about a complex with CO already in it, doesn't that make the complex high spin? So it's always high spin. But if it were low spin, I don't think the metal can backdonate unpaired electrons so C=O bond frequency should be higher.

Title: Re: Stretching vibrational frequency in coordination compounds
Post by: Vital Y on February 12, 2016, 01:50:51 AM

CO is a strong field ligand so the complex is a low spin complex. PPh₃ is strong field ligand as well, I'm not very sure about Chlorine substituted PPh₃.

Title: Re: Stretching vibrational frequency in coordination compounds
Post by: clinz63 on February 12, 2016, 06:15:52 AM

Yeah. Sorry. Yes. I meant that. Hahaha. It will always be low spin (I thought about the high energy difference between t_2g and e_g).

Title: Re: Stretching vibrational frequency in coordination compounds
Post by: Vital Y on February 12, 2016, 08:06:31 AM

If we swap the CO ligand with NO₃^- and see the vibrational frequency of the N-O bond, will the nature of the complex matter? Or is the back bonding explanation sufficient to determine which complex will have the most vibrational frequency. (NO₃^- is a weak field ligand)