why oxygen, cannot act as a pi-donor while NO can?

queries

Hi,

I have learnt that pi-donor are those ligands which donate their pi electrons to metal centre while pi-acceptor accepts electron from filled d(pi) orbital to their pi* orbital.
Now, I'm confused, why oxygen, having two pi* electrons cannot act as a pi-donor while NO, having one pi* electron only can?

Thank you!

sjb-2812

Have you actually been told that oxygen cannot act in this way, or is it more that you haven't seen any examples yet?

queries

Actually my professor said so, but I just wonder why

chemisttree

What is a pi bond? Is it present in an oxygen atom?

What is a pi electron? How does it relate to a pi bond?

edit: It just occured to me that you might be referring to molecular oxygen. If that is the case, you should know that oxygen is paramagnetic... that is, it has two unpaired electrons. Try to resolve that bit of information with the Lewis structure!

alxm

Oxygen can act as a pi-donor. It seldom does because oxygen is electronegative and simply doesn't like donating electrons in general, pi or otherwise.

NO can act as a pi donor because you can form a nitrosyl species, e.g. when binding to a metal atom:
M⁺..NO* <->M-NO⁺

The corresponding dioxygenyl ion for oxygen is very rare and very high in energy. OTOH oxygen can act as a d-acceptor, forming a superoxo-species. (pretty much all the oxygen-consuming reactions in biology are performed by metal centers in enzymes, e.g. cytochrome c oxidase.)

chemistree: You don't know what a pi bond is, or you're not using your head? Of course he's talking about molecular oxygen. And why bring up Lewis structures when the discussion is already phrased in terms of more sophisticated MO theory?

chemisttree

Please name one. I'd be very interested to learn about it.