Forums
Dear Colleagues,
I hope all is well. Today, in my course I talked about the 18 electron-rule and gave the students a few problems to practice. I asked the students to determine if the complex Fe2(CO)9 followed the 18 electron rule. So, I go through the example and explain the the M-M bond contributes one electron. A student asked me "Where does the electron come from in the bond? Are you counting an electron twice from Fe?" I explained that I am not counting electron twice from Fe. What is the best way to explain this? I never had a student ask me where does the electron come from before. (Ha) The formation of an M-M single bond provides an extra electron for each iron. Any thoughts on helping me explain this to an undergraduate?
I would say that the electron comes from the other iron atom. It is shared just like electrons in molecular hydrogen. Sometimes you may also encounter a dative bond in a metal-metal bonded system that is a classic Lewis acid-base adduct.
If you use the CBC method of counting
http://www.columbia.edu/cu/chemistry/groups/parkin/cbc.htm
you treat the other Fe atom as an X type ligand. So, even thought it is an iron atom with lots of electrons, it is like every other X type ligand providing one electron to form a bond.
Chip,
That is certainly an interesting website!
SNC
I would say each iron atom provides one electron to form a metal-metal covalent bond, and like in counting number of electrons in a nitrongen atmn in a nitrogen molecule to see if N satisfies the octet, we count all the bonding electrons to belong to each of the bonded atoms.
Dr. Dishakjian
Hi Sossi,
That's a good explaination as well.
SNC