[Plumbum]

Hi !

Can anyone here explain to me what  pie-donor sigma-donor pie acceptor are ? But please in an easy way I tried to google them but unfortanely i didnt understand !!

thank u

{MOD Edit -- title edited to be less funny}

[kriggy]

One pie acceptor here

Anyway, jokes aside:
Those are classes of ligands in coordination chemistry.
Ligands can be classified as:
sigma donors - when coordinated to metal, it forms one single sigma bond,for example NH₃
sigma donor and pi donor - when coordinated it forms one sigma bond and one or more pi bonds. The important thing here is that he pi bond is formed by moving electrons from ligand to orbitals of metal
sigma donor and pi acceptor - the sigma bond is formed as in examples before, but there is donation of electrons from metal to empty pi-anibonding orbital
There are some other types - for example sigma dnnor and d-acceptor where the back donation is to empty atomic d-orbitals of atom coordinated to metal.

Hope it helps, I tried as simple as possible but to answer this question some knowledge from general and inorganic chemistry is required

[Plumbum]

thank u so much !

I still have problems in applying this knowledge on a task, though. Can you help me out?

The task is:

Classify the following ligands as sigma donor, pi donor and pi acceptor ligand CO, Br-, en, C2O42-

[kriggy]

Sure I can help you
But what do you think? Could for example ethylenediamine be signa and pi donor?

[Plumbum]

thank u so much

Wow thats kinda difficult for me haha. I don't really know what I should pay attention to when answering this question but I'll give it a try !

For a pi bond it should have an empty orbital, shouldnt it? I might need a hint.

[kriggy]

For pi bond you need either 2nd electron pain to donate or empty pi antibonding orbital to accept electrons.
So, which conditons are met when we are talking about en?

[Plumbum]

I have no idea how to tell....I'm so sorry I really tried

I mean a pi donor offers a free pair of electrons to donate for a pi binding? But I don't know how I can tell, whether i have an empty orbital ( which would be a pi acceptor then)

So after thinking it through i think en is a sigma donor. The N has a free pair of electrons.

[kriggy]

Yes, it is sigma donor (basicaly all ligands are sigma donors but some of them can be pi donors/acceptors in addition). It has only 1 electron pair so its only sigma donor.
How can you tell? Well from examples we were told at lectures (It most likely will not fit all possible ligands):
a) has only 1 electron pair at coordinating atom?  sigma donor
b) has more than 1 electron pair at coordinating atom?  sigma and pi donor
c) has double or tripple bond at coordinating atom?  sigma donor and pi acceptor
d) is the ligand coordinating by phosphorous?  sigma donor and d acceptor

[Plumbum]

hm why do we have to free pair of electrons in en then?

http://www.ddesignmedia.de/Komplex_Chemie/HTML/GMS/Chelat/en.jpg

[kriggy]

Oh I see.
At each nitrogen you have only 1 electron pair. It doesnt matter if its bidentate ligand (can coordinate with more than one atoms) or not. The case with en is that it is sigma donor which means that each nitrogen forms one sigma bond with metal, so if its coordinated, there are two sigma bonds.
Do you understand?

[Plumbum]

yes I do ! Now for the rest of the molecules/ ions in the task Im gonna give it a try :

C2O4 2- -> each oxygen has three free pairs of elecrtrons which makes it  sigma and pi donor ?
Br- -> same here actually, 4 free  pairs of electrons  sigma and pi donor
CO -> has a double bond between C and O and O has two free pair of elctrons wich makes it  sigma donor and pi acceptor?

thx

[kriggy]

Yeah that what I think
Youre wellcome

[Plumbum]

oh really? thats so cool I think I finally understood thank u

[AdiDex]

Hey ,
I have a question...
In case of carbonyl ( CO ) carbon has only one electron pair , so how it can be a sigma and pi donor ligand .??
Since it has double bond .??

[kriggy]

It is not pi donor, its pi acceptor.