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#Post#: 1824--------------------------------------------------
18 Electron rule - Urgent
DIR By: raynechem
Date: January 2, 2013, 12:06 pm
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sir plz help me with this questio,
How is 18-electrone rule helpful in determining the number of
metal-metal bonds in Fe3(co)12 and Co4(Co)12 ? explain with
example that 18-electrone rule is not useful in distinguishing
between bridging carbonyl group and terminal carbonyl group??
#Post#: 1828--------------------------------------------------
Re: 18 Electron rule - Urgent
DIR By: Chemist@
Date: January 2, 2013, 2:10 pm
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Iron has 8 valence electrons and it need 10 more for 18. In the
first compound, there are 3 iron atoms=24 electrons, they need
+30 electrons. As these aren't too complex compounds you can
assume that each carbonyl group donates 2 electrons .No matter
if it donates both electrons to one iron atom or each electron
to two iron atoms, you don't need to know the structure.
12*2=24. 6 more needed, so here the Fe-Fe bond has to interfere.
When a Fe-Fe bond is made, each iron gets an electrons from the
other iron atom, without losing its electron (its not a dative
bond), so when two iron atoms bond, both gain one electron
therefore 2*1=2. The three iron atoms needed 6 electrons and
6/2=3 meaning that three metal-metal bonds are made. You do for
the cobalt compound.
For the second question, I think that it is because you don't
know whether the carbonyl group is attached to one iron atom or
two or even to more. Here is the structure of the iron compound
HTML http://en.wikipedia.org/wiki/File:Fe3%28CO%2912.png.
You see
that some CO are terminal and some are bridging, but luckily
they donate the same number of electrons (2). For tougher
examples where this is not the case, see:
HTML http://en.wikipedia.org/wiki/Metal_carbonyl
(bridging
carbonyls).
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