pi backbonding

dinoflagellates

Hi there,
hate asking people this but what is pi backbonding? is it where two electrons move go back into an empty pi orbital or is it where two elctrons in a pi bond go back into an empty orbital or are both right
Thanks very much i have searched the web to no avail.
Also does anyone know any good science search engines

Capt'n Midnight

Going by memory - can lookup the book later.

In carbon you have 1s , 2s and three P shells
When all four valancies are used with Single bonds the outer four shells hybridise together to form four equal energy electron pairs. Because it's 4 the structure is tetrahydral.

With other hybridisations of shells you get different structures. With a Double bond (and two single bonds) there are three shells involved and so you get three bonds about 120 degrees apart. - You can imagine the double bond as a single bond with electron clouds above and below it forming the other one.

With a tripple bond again you have a single bond and electron clouds above and below, but this time with others to each side - and the bond angle is 180 degrees.

Not clearly explained
the images on these sites aren't that clear either
http://dl.clackamas.cc.or.us/ch106-02/pibonds.htm
http://dl.clackamas.cc.or.us/ch106-02/structur2.htm

http://www.wellesley.edu/Chemistry/chem120/hybrids.html - images are better

Zaphod

Pi backbonding is a bonding interaction between the d orbitals of a metal and Pi* orbitals of a ligand.

So in the case of CO as the ligand, you generally get a sigma bond where the ligand donates the electron pair of the carbon into the empty orbital of the metal, while the metal back donates electrons from filled d orbitals into the Pi* orbital of CO.

More info:
http://www.ilpi.com/organomet/carbonyl.html

dinoflagellates

thanks very much, that is what i was looking for!!!!!