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Sibrina Collins, The Charles H. Wright of Museum of African American History
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18-Electron Rule

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?

James G. Goll, Edgewood College
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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.

Sibrina Collins, The Charles H. Wright of Museum of African American History
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Thanks James! Sometimes we are asked the simplest questions, and we get stumped on how to respond!
Chip Nataro, Lafayette College
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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.

Sibrina Collins, The Charles H. Wright of Museum of African American History
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Chip,

That is certainly an interesting website!

SNC

Sossi Dichakjian, Lebanese University
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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

Sibrina Collins, The Charles H. Wright of Museum of African American History
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Hi Sossi,

That's a good explaination as well.

SNC