Why Do Ligands Donate Electrons to Electropositive Transition Metals?

[hariharan venkatasu]

TL;DR Summary

Transition metal/ligands interaction

Transition metals, being metals, are electropositive.Ligands like ammonia,oxygen,chlorine etc ,on the other hand,are mostly electronegative(electron loving).How come the Ligands donate electrons to the electro positive(electron hating)transition metals when both have contradictory characteristics. Could you please clear my confusion?​

 

[Tom.G]

Try starting with just the introduction (before the Table of Contents) here:
https://en.wikipedia.org/wiki/Electronegativity

Then go on to:
https://www.khanacademy.org/science...onds-and-reactions/v/electronegativity-trends

and the one that immediately, and automatically, follows it:
https://www.khanacademy.org/science...ctions/v/electronegativity-and-chemical-bonds

And perhaps read more of the Wikipedia article if you are interested. It gives some background too.

If you need to go further, we can probably get some chemistry folks here to jump in.

Cheers,
Tom

 

[hariharan venkatasu]

Thank you for your prompt reply.My question was how can a ligands like ammonia, chlorine,oxygen etc donate electron to a transition metal?These ligands are elctronegative in character and are reluctant to part with their electrons.More over transition metals, being metals are electropositive,and will be unwilling to accept electrons.They prefer to part with their valence electrons.How this anomaly could be explained?

 

[PeterDonis]

how can a ligands like ammonia, chlorine,oxygen etc donate electron to a transition metal?

Under what circumstances does this happen? Can you give more specifics?

 

[jedishrfu]

I found this reference that talks about the complex bonding and may answer your question

https://www.theodysseyonline.com/transition-metals-crash

 

[Borek]

Put complexes aside for a moment. Let's try with something simpler. Imagine you have a water molecule. Oxygen is highly electronegative, so it attracts electrons - yet it has two free electron pairs, that it can - despite hydrogen being way more electropositive, even comparable with Mo - donate to a proton and create a hydrogen bond (or even just a bond like in hydronium). Why is is it so? Well, first - these electron pairs despite being attracted to the oxygen still have large density of excess electrons, second - electronegativity is a property of an element, when it comes to ions it has to be treated with caution, H⁺ - having a positive charge - attracts electrons much more than the hydrogen electronegativity would suggest.

When it comes to complexes things are being complicated by other factors (see ligand field theory or crystal field theory for example), but you are still having cation and electron pairs, so at some level similar factors are at work.

 

[hariharan venkatasu]

Thanks a lot for the enlightening answer.But why a reference to Mo is made?

 

[Borek]

Only because being transition metal it has particularly high electronegativity, comparable with that of hydrogen.

 

[hariharan venkatasu]

Thanks a lot for clarification

 

[HAYAO]

This is a good question. Let's review electronegativity.

There are several ways the value is defined, but the most common ones you see is the Pauling scale. The equation is given as the absolute difference of electronegativity between two atoms A and B:
$$|\chi_{\rm A} - \chi_{\rm B}| = ({\rm eV})^{-1/2} \sqrt{E_{\rm d}({\rm AB}) - \frac{E_{\rm d}({\rm AA}) + E_{\rm d}({\rm BB})} 2}$$$\chi_{\rm A}$ and $\chi_{\rm B}$ are the "absolute" electronegativity, $E_{\rm d}({\rm AB})$, $E_{\rm d}({\rm AA})$, and $E_{\rm d}({\rm BB})$ are the bond-dissociation energy of compound AB, AA, and BB. Thus, you can see from this that the electronegativity in the Pauling scale is a relative measure of electronegativity compared to hydrogen atom in a compound with hydrogen (HF, LiH, etc.). The reason why hydrogen was chosen is because they form covalent bonds with most elements. The electronegativity for hydrogen is defined as 2.20. So the values for other elements are relative to that value.

The other scale is the Mulliken Electronegativity. This is given by the following formula:
$$\chi = \frac{E_{\rm i} + E_{\rm ea}} 2 \$$where $E_{\rm i}$ and $E_{\rm ea}$ are the ionization energy (energy released when one electron is removed) and electron affinity (energy released when one electron is acquired) of an neutral atom. This value is a lot different from the Pauling electronegativity (it even has the unit of energy!), but it's not a relative measure.So, what does these mean?

It means that electronegativity is the property of an neutral atom in respect to how they are expected to behave once a covalent bond is formed with another atom. So the OP's question stems from the confusion that the electronegativity comes into play before a bond is formed, as opposed to the reality that they come into play after a bond is formed. When an electropositive neutral atom A and an electronegative neutral atom B forms a bond, the electrons are more dense around atom B and sparse around atom A. But this is about neutral atoms as reactants, not about ions or compounds as reactants.

So let's talk about metal ion and ligands.

For example, let's say that you have Cr³⁺ and three Cl^- atom. At this point, the "electronegativity" is irrelevant here because they are already an ion. At this point, Cr³⁺ is positively charged, and Cl^- is negatively charged. Positive and negative attract each other by Coulombic force, thereby forming CrCl₃.

Another example, let's say that you have Cu²⁺ and six H₂O molecules. Once again, the "electronegativity" is irrelevant here because one is an ion, and the other is a compound (not neutral atom). Once again, Cu²⁺ is positively charged, and the H₂O molecules is polar and the electron is more dense around the oxygen atom (the lone pair electrons), although the molecule itself is neutral. Positive and negative attract each other by Coulombic force, thereby forming [Cu(H₃O)₆]²⁺.

Most people make the similar mistake by understanding it like OP did, and believe that electronegativity is what forms a bond. Electronegativity explains polarity in compounds and the bond affinity (ionic vs covalent), but it's not what causes the bond (it is indirectly related though).