Introduction to Organometallic & 18 Electron Rule - Organometallic Chemistry Video Lecture | Chemistry for GRE Paper II

hey everybody welcome back to organic
chemistry now we're going to do
organometallic chemistry so here it is
and so we're going to do the
organometallic chemistry now from your
textbook it starts on page 999 it's from
the 10th edition of Solomon but
regardless it's always the same special
topic G so if you have an early edition
of Solomon just look for a special topic
G in the table of contents and you'll
find it so basically this is less
realistic it started with the
introduction so number 1 let's talk
about an introduction now we've talked
about organometallic chemistry before
last semester we talked about grignard
Xand alkyl lithium's and there are other
things that we've discussed like for
example h2 and palladium 2 in order to
break a double bond or a triple bond
down to a single bond is an
organometallic reaction we just never
really talked about the details until
now like how these things happen so it
turns out that organometallic chemistry
is a very broad subject and there's a
lot of different rules that are special
for this particular topic so we're going
to touch on this but we don't really go
into every single detail okay it's quite
different than the organic chemistry or
even gen chem in some cases that we've
learned about in terms of how we look at
these molecules and how we figure out
their interactions and reactions ok so
the first thing I want to talk about is
a general idea that what we're dealing
with is always a metal and this is
typically a transition metal and somehow
the metal has a coordination of a ligand
L now ligands are things that are not
metal so M equals metal and again it's
usually the transition metals ok and L
equals a ligand and these are non metals
so these are could be organic but it
doesn't always have to be organic but
they're nonmetals so there are things
that are not metal like and now the way
that we refer to them even though I drew
a bond between them is actually that
they're there's no real bonding here
this is called a coordination ok so
metals and ligands are well I should say
ligands coordinate around metals ok so
what that means is that their electrons
come close electrons are close to the
metal but it's not a true bond like in
the it's not a covalent bond this is
more ionic character to it and so we say
it's a coordination of ligands around
the metal ok so we do draw the customary
bond between them like I show you here
this red bond but it's really not a bond
it's a coordination so just think about
ligands as having electrons that is the
requirement and that those electrons are
coming close to the metal in order to
satisfy the metals a requirement ok and
so that's what we're seeing here now
another thing that's important to
recognize in this review is the idea of
oxidation and reduction so we we all
know what this means but I want to
refresh our memories
now what oxidation means is that
something lost or a loss of electrons so
whenever something loses electrons it is
becoming oxidized and reduction is the
opposite right it's a gain of electrons
so when something is reduced it's
because it's gaining electrons now we've
sought we've seen this idea of oxidation
and reduction back in chapter 11
last semester 11 and 12 and what we
found was that with organic chemistry
sometimes it's not a true loss or gain
or electrons it could be like for
example a more electronegative atom and
it's drawing electrons away from carbon
and so that's considered to be an
oxidation even though the electrons are
still between the carbon and that other
atom whereas in the case of metals so
with metals it's truly a loss or gain a
full
loss or gain of electrons okay so again
let me just refresh your memory if you
have a carbon as fluorine we could say
that this carbon is oxidized because the
electrons are moving towards fluorine so
this carbon is oxidized right compared
to let's say if it had let's say a
carbon with an H there and we replace
the H with an F right so that carbon
became oxidized now again this idea of
oxidation is not that it truly lost its
electrons but it's losing its electron
density it's moving away from that atom
the carbon okay whereas with metals it's
truly an oxidation so if I have let's
say a metal like let's say I have I
guess um I'll do ruthenium so if I have
ruthenium and it's connected to a carbon
right or it's I should say coordinated
with a carbon and let's say ch3 well
this ruthenium also being less
electronegative than carbon is oxidized
right but unlike the top case where the
carbon is not truly oxidized in the
sense that it completely lost its
electrons in the case of ruthenium it is
truly oxidized so this has a loss of
electrons and it would so for a carbon
it would be negative and the ruthenium
would be positive okay so this ruthenium
is truly a positive ruthenium it has a
plus charge okay it lost its electron
alright so there's a slight difference
when you thinking out this is actually
it's kind of interesting because in gen
chem we consider things the way I'm
describing now right
it's truly oxidized something positive
something negative but because we
switched our focus to organic chem we
learned that oxidation or reduction
doesn't necessarily mean a full loss or
gain of electrons and so now I'm kind of
bringing us back to what we would have
known from gen chem okay so this
ruthenium is equal to being ru plus and
ch3 minus that's what we're really
looking at here whereas you can't say
that this is not ch
three plus and F - that's not true right
we know that that's not the case for
carbon-fluorine but it's not plus and
minus its they're still bonded to each
other okay so that's my point so
whenever you think of oxidation or
reduction in the case of metals we're
going to think of it as being truly
positive or negative okay
so this is the idea so far now let me
just for our introduction I want to
bring up another point and that is I
guess I'll do it here is this whole idea
of the 18 electron rule now this is
another unusual idea but it's it's
really makes sense see metals and in
particular transition metals have D
orbitals okay so they have D orbitals
available to them so they have their S
their P and then they have D and so for
that reason the outer shell can be
filled to 18 electrons okay so whereas
like for example a carbon has s and P
right so for carbon it has an S orbital
and a p orbital so you have this right
here remember for P there's three P's px
py PZ and so for carbon it wants eight
electrons to fill its outer shell right
because remember carbon has a second
it's a second role element so it has a
2s and the 2p right so if you want to
fill up carbon then you go like this
right so we have all these electrons
here I kind of did it backwards right in
terms of energy increasing not that it
matters but it's kind of unusual to see
the P below the s so I'm going to do
that anyway so here it is you've got
carbon that when it fills up its
complete outer shell it would have eight
electrons but now let's think about a
metal so metals have their their 2's for
example and actually I'm going to get
rid of the number two because that's not
really applying here it has an S it has
its P one two three but it also has D
okay all transition metals have
you orbitals and if you remember it's
one for the esses three for the peas and
five for the ds5 degenerate orbitals
remember degenerate means that these
orbitals are equal and energy so
whenever you have a D it comes in five
as opposed to P which comes with three
okay and then you have F which actually
has seven but we don't have to worry
about f at all so if we fill this up
we've got our eight that we would expect
right so we have right here is eight but
now we have an extra five which gives us
another 10 electrons that you can fill
in here and that gives us 18 electrons
so because the metal has a D orbital it
would like to have 18 electrons in order
to completely fill up its outer shell
okay so that's the idea that's where
this 18 electron rule comes from so you
know when you think about it we always
talk about the octet rule but there's
nothing special about octet there's just
something special about completely
filling your outer shell so all atoms
want to fill their outer shell hydrogen
would like to fill its outer shell so it
gets one extra up gets two electrons it
you
you