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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: Plumbum on September 09, 2014, 11:37:11 AM

Title: pi-donor,sigma-donor, pi acceptor
Post by: Plumbum on September 09, 2014, 11:37:11 AM
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}
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: kriggy on September 09, 2014, 03:56:56 PM
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
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: Plumbum on September 11, 2014, 08:44:32 AM
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-


Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: kriggy on September 11, 2014, 10:12:37 AM
Sure I can help you :)
But what do you think? Could for example ethylenediamine be signa and pi donor?
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: Plumbum on September 11, 2014, 02:29:32 PM
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.
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: kriggy on September 13, 2014, 07:52:16 AM
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?
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: Plumbum on September 13, 2014, 08:39:47 AM
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.
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: kriggy on September 13, 2014, 03:39:12 PM
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?  :rarrow: sigma donor
b) has more than 1 electron pair at coordinating atom?  :rarrow: sigma and pi donor
c) has double or tripple bond at coordinating atom?  :rarrow: sigma donor and pi acceptor
d) is the ligand coordinating by phosphorous?  :rarrow: sigma donor and d acceptor
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: Plumbum on September 14, 2014, 06:36:05 AM
hm why do we have to free pair of electrons in en then?

http://www.ddesignmedia.de/Komplex_Chemie/HTML/GMS/Chelat/en.jpg
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: kriggy on September 14, 2014, 12:54:24 PM
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?
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: Plumbum on September 14, 2014, 06:16:13 PM
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
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: kriggy on September 15, 2014, 03:08:23 AM
Yeah that what I think :)
Youre wellcome
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: Plumbum on September 15, 2014, 05:59:01 AM
oh really? thats so cool I think I finally understood :D thank u
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: AdiDex on September 16, 2014, 12:02:07 AM
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 .??
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: kriggy on September 16, 2014, 04:20:16 AM
It is not pi donor, its pi acceptor.
Title: Re: pi-donor,sigma-donor, pi acceptor
Post by: AdiDex on September 16, 2014, 01:28:56 PM
Okk.....i read it wrong....
Title: Re: pi-donor,sigma-donor, pi acceptor
Post by: kriggy on September 16, 2014, 02:42:09 PM
Haha, happens to me like.. all the time :D
If you have more questions regarding this topic or similar feel free to ask
Title: Re: pie-donor,sigma-donor, pie acceptor
Post by: AdiDex on October 16, 2014, 02:51:29 AM
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?  :rarrow: sigma donor
b) has more than 1 electron pair at coordinating atom?  :rarrow: sigma and pi donor
c) has double or tripple bond at coordinating atom?  :rarrow: sigma donor and pi acceptor
d) is the ligand coordinating by phosphorous?  :rarrow: sigma donor and d acceptor

Why did you said that ligand should have double or triple bond for sigma donor and pi acceptor..?
Title: Re: pi-donor,sigma-donor, pi acceptor
Post by: kriggy on October 19, 2014, 02:32:16 PM
Because the back donation goes into pi-antibonding orbitals. And for them to exist, you need pi bond.
Or at least that is my reasoning (we didnt cover it that much at class)
Title: Re: pi-donor,sigma-donor, pi acceptor
Post by: AdiDex on October 19, 2014, 11:57:00 PM
Hmm.. Seems logical . Thank you... :)
Title: Re: pi-donor,sigma-donor, pi acceptor
Post by: Irlanur on October 20, 2014, 10:20:32 AM
Quote
Because the back donation goes into pi-antibonding orbitals. And for them to exist, you need pi bond.

That's not true. the symmetry of an orbital concerning ligand-symmetry has nothing to do with the symmetry of this orbital in a complex. (of course there are cases which exist more often than others...)

consider di-hydrogen. the sigma-bonding orbital of H2 can be used as a sigma-donating orbital in a complex. the sigma-antibonding orbital of H2 has pi-symmetry in a complex and can serve as a pi-acceptor. (provided that the binding is side-on and not end-on)