Electron Counting in 18 Electron Rule - Organometallic Chemistry Video Lecture | Inorganic Chemistry

sup guys I want to talk about some
inorganic chemistry stuff and
specifically I want to talk about
electron counting and how you can use
electron counting to follow the 18
electron rule so what the 18 electron
rule does is it allows you to analyze in
organic compounds specifically just
metal compounds and you can come up with
oxidation states based on the assumption
that it will follow the 18 electron rule
or you can predict stability that kind
of stuff so let's just go ahead and
delve into it
there are two methods the way people
look at this and they are creatively
referred to here as method a and method
B method a is the donor pair method and
the way you look at this with method a
is you think of the ligands donating
electrons so the Chargers on the ligands
of course must be known if you're going
to have the ligands donate electrons and
also the oxidation state on the metal
must be known unless you assume it to
comply with the 18 electron rule and
then you can just figure out by some
little math really quick that what the
oxidation state is the other hand you
can use method B which is way easier to
use because you don't need to know the
oxidation state you don't need to know
the charge and the ligands and basically
you can do it quicker so with method B
it's a little bit different from method
a the way you tally up electrons and
specifically with charges the ligands
just count differently in method B so if
you have a positive charge and a ligand
usually it'll count as one more under B
and you have negative charge on a ligand
and B it'll count as one less than under
a so get to figure out what the charges
on an electron you kinda just have to
look at a table so for instance carbonyl
is always two chlorine it counts as two
under a but only counts as one under B
so that kind of stuff all the halogens
actually are two under a
and one under B but let's look at some
metals so remember you have to count the
the electrons on the metal and the
ligand so let's start out with iron iron
has a couple things going on first you
need to look at the electrons in the D
shell
so remember irons a transition metal in
the first row so we're looking at the 3d
shell and there are six and then so it
also means it's got an S shell appeared
the 4s shell and there's two electrons
in there so a grand total via accessible
electrons you've got eight and so for
instance manganese which is right next
to iron manganese the number is number
25 and iron is number 26
so for manganese we have 3 d 5 plus 4 s
2 equals 7 and despite them having
different numbers they both make
compounds that satisfy the 18 electron
rule so let's take a look at that so
we're going to make a chlorine manganese
compound with 5 carbonyls on it and this
is just going to be a typical octahedral
molecule there's a little abbreviation
for octahedral under this method unless
you know that it is going to be
following the 18 electron rule I have to
tell you what the oxidation state is so
in this case it's going to be 1 and that
means the manganese has one less
electron than it would under its neutral
state so in this case we're normally at
7 it's only going to count as 6
electrons here and then we have a
chlorine
so you're going to see that counts as
two
and finally we have five carbonyls each
counting is two so that will give us ten
and really quickly we have 18 electrons
so this compound does obey the 18
electron rule now let's check out method
B remember this was method a up here
so for manganese we don't have to take
into account the oxidation state we just
say what the neutral number of electrons
is with MN zero and we know it's seven
because we just did it up here seven and
we have a chlorine but we don't take
into account the charge so we just say
donates one electron and then of course
we have five carbonyls so then we get
once again ten electrons and you can see
that this also adds up to eighteen
electrons and it doesn't always work but
most the time does work so let's look at
a different compound let's look at
ferrocene now ferrocene is everybody's
favorite molecule because the symmetry
is interesting and it's pretty cool and
you've got this double cyclopentadienyl
thing going on which is pretty awesome
to say the least but you also need to
know something extra when you're working
with cyclopentadiene you need to know
about hapticity which is represented by
ADA just the Greek letter ADA and it's
called hapticity
and hapticity it's pretty standard to be
5 with cyclopentadienyl with benzene
it's fairly standard b6 and so on with
that trend but it can vary and it can
change to satisfy the 18 electron rule
so hapticity should also be given under
method a we are going to assume or we
are going to be told that fe is in the 2
oxidation state so remember what it was
under 8 0 it was 8 so under here it's
going to count as 6 right because we're
going to subtract 2 electrons from it ok
and then the way you'll see this written
we have two of these 2 a 2 v
cyclopentadienyl z-- c 5 h 5 because
remember it has to have a negative
charge in it for it to have aromaticity
those are huncle's rules believe so
under this we have to add that negative
charge on the method a because remember
we're taken to account the charges so
this counts as 6 electrons but there are
two of them so times 2 and that gives us
18 electrons total
now you can see pretty quickly bees just
going to be really simple because we
don't worry about the oxidation state we
just assume Fe to be neutral and there's
our 8 electrons and then we are given
the hapticity of the cyclopentadienyl
rings at 5 each and we ignore the charge
on that and so we're going to get 10
here 5 times 2 is 10 and we have 18 so
just as a general little aside most
hydrocarbon ligands are going to have a
negative 1 charge on them that kind of
thing is going on and it just especially
common that these are going to have a
negative 1 charge on them because they
need that negative charge to have
aromaticity benzene may or may not
because benzene actually doesn't want to
have a charge on the dev aromaticity and
then there is another compound called
cyclo hepta tri enol and this is usually
a 2 7
so it's a seven ring compound and that
has a positive charge on to maintain
aromaticity so that's a case where
you're gonna have a positive charge on
one of those hydrocarbon complexes or
hydrocarbon ligands and for instance
that counts us six under a and under B
it counts us seven because you just go
straight off what our apt isset e is and
an a you got to subtract one so that's
it for my little introduction to
electron counting and all the 18
electron rule