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Topic: Nature of ligands in coordination compounds  (Read 1374 times)

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Offline jestquim

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Nature of ligands in coordination compounds
« on: July 08, 2021, 08:12:15 AM »
Typically, the following types of ligands are distinguished.

- Sigma-donor ligand. One that yields electrons to the metal center from its sigma orbitals.
- Pi-donor ligand. One that donates electron density to the metal center of the complex through its pi orbitals.
- Pi-acceptor ligand. One that has empty pi orbitals and receives electrons from the metal.

If these definitions are incomplete, or at worst incorrect, let me know.

Based on these definitions, you are asked to classify the following ligands into each of the three groups: H2O, en, Cl, PPh3, NH3, CO, η5-C5H5, Me3P, Ph, H.

I think it would be like this, but I don't quite know how to reason it out:

sigma-donor: H2O, NH3, Me3P
pi-donor: Cl
pi-acceptor: PPh3, Ph; n5-C5H5

The rest I wouldn't know, because although I know the definition, I don't know what to base it on to make the classification and justify it correctly.

Offline Orcio_Dojek

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Re: Nature of ligands in coordination compounds
« Reply #1 on: July 08, 2021, 09:27:25 AM »
You need to look at the atom connected to the metal.

For example P(CH3)3 is pi-acceptor, as P atom has empty d orbitals.

C6H5 is sigma donor as it is bonded to the metal through C atom (M replaces H).

And so on...

Beware, as [C5H5]- is not a pi-acceptor.

Offline jestquim

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Re: Nature of ligands in coordination compounds
« Reply #2 on: July 08, 2021, 10:11:35 AM »
But, for example, the electronic configuration of nitrogen in its ground state is 1s2 2s2 2p3. In that case, the p orbitals are partially occupied, which would be pi-acceptor or pi-donor?

Offline jestquim

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Re: Nature of ligands in coordination compounds
« Reply #3 on: July 08, 2021, 10:11:58 AM »
You need to look at the atom connected to the metal.

For example P(CH3)3 is pi-acceptor, as P atom has empty d orbitals.

C6H5 is sigma donor as it is bonded to the metal through C atom (M replaces H).

And so on...

Beware, as [C5H5]- is not a pi-acceptor.

But, for example, the electronic configuration of nitrogen in its ground state is 1s2 2s2 2p3. In that case, the p orbitals are partially occupied, which would be pi-acceptor or pi-donor?

Offline Orcio_Dojek

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Re: Nature of ligands in coordination compounds
« Reply #4 on: July 08, 2021, 10:21:26 AM »
If bonded like:

M-N=N

then - pi-acceptor

If bonded like:

       N
M <  |
       N
     
then - pi-donor.

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