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00:00:26
CATHERINE DRENNAN: All right.
00:00:27
So moving to today's
handout, this
00:00:31
is one of my favorite
parts of the course.
00:00:33
Honestly, when I first
started teaching 5.111,
00:00:37
I said transition metals
are rarely covered
00:00:41
in the intro chemistry courses.
00:00:43
Is it really necessary
to cover it here?
00:00:45
And I was told it
absolutely was.
00:00:47
It's one of the reasons
that a 5 on the AP exam
00:00:52
is not good enough, that you
have to take the Advanced
00:00:54
Standing exam.
00:00:55
Because the people who
teach inorganic chemistry
00:00:57
found that people who
placed out of 5.111
00:01:00
didn't do as well
in their course
00:01:02
as people who took 5:111 here.
00:01:04
So this is one of the reasons.
00:01:06
And then I started
teaching it, and I
00:01:08
realized this is
one of the-- even
00:01:10
though people haven't
seen it before
00:01:12
and sometimes will get a
little scare-- it's actually
00:01:15
one of the most fun units.
00:01:16
So I absolutely love
it, and hopefully you
00:01:18
will love it by the end.
00:01:19
People are like, we
never covered it.
00:01:21
Why are you covering it?
00:01:22
We're in chapter 16.
00:01:24
It's fun.
00:01:24
OK.
00:01:25
So transition metals,
d-block metals,
00:01:31
because they have
those d orbitals.
00:01:33
Yes, we're going back to
talking about orbitals again.
00:01:36
And they're called
transition metals
00:01:38
because you transition from
this part of the periodic table
00:01:41
with your, what
kind of orbitals?
00:01:43
AUDIENCE: s.
00:01:43
CATHERINE DRENNAN: s.
00:01:44
To this part of your periodic
table with your, what kind of
00:01:46
orbitals?
00:01:47
AUDIENCE: p.
00:01:47
CATHERINE DRENNAN: p.
00:01:48
So they are the
transition metals,
00:01:51
and they're often really
reactive and very cool.
00:01:54
And many of them, since
we're on a biological theme,
00:01:57
many of them are super
important in biology.
00:01:59
And I have some of these
written down in your notes,
00:02:02
but here they are up here.
00:02:04
In the transition metals,
you have a lot of metals
00:02:07
that we could not live without.
00:02:10
Iron carries oxygen to
our blood, very important,
00:02:13
hemoglobin.
00:02:16
We talked about cobalt just now.
00:02:17
That is the metal
in vitamin B12.
00:02:20
So we know why that's important.
00:02:22
We have zinc everywhere.
00:02:24
Nickel's important for
bacteria, not so much for us.
00:02:27
But bacteria is
important for us,
00:02:28
so therefore nickel's
important to us.
00:02:30
So all of these are
really important.
00:02:33
Also, this part of the periodic
table is a part of the table
00:02:37
that people love that want to
make pharmaceuticals or want
00:02:41
to make new kinds of
electrodes or batteries
00:02:44
or all sorts of things.
00:02:46
There's a bunch that
are used as probes.
00:02:49
We talked about imaging agents,
detecting cancer, and all sorts
00:02:52
of different things like that.
00:02:53
Many of these transition
metals are used in those probes
00:02:58
and also in pharmaceuticals.
00:03:01
And so this is sort
of a very rich part
00:03:04
of the periodic table,
where those d orbitals allow
00:03:07
for properties
that are incredibly
00:03:09
useful for our health and
for doing all sorts of stuff.
00:03:13
So I love this part.
00:03:15
Again, some of the biological--
global cycling of nitrogen.
00:03:19
We talked about nitrogen
fixation, that triple bond.
00:03:22
It's really hard to
break nitrogen apart,
00:03:24
but bacteria can do it.
00:03:26
It does it using
transition metals.
00:03:29
Fixing carbon,
hydrogenase, if you
00:03:31
want to make hydrogen fuel
cells that are more biological.
00:03:34
Again, biology uses
transition metals in this.
00:03:37
Making vitamins, making
deoxynucleotides, respiration,
00:03:41
photosynthesis, it's all
due to transition metals.
00:03:45
All right.
00:03:45
So we'll start with
just one example,
00:03:48
or one of In Our
Own Words segment.
00:03:50
And this focuses on
nickel, which is something
00:03:53
very important in bacteria.
00:03:55
And this is actually an example
from a collaborative project
00:03:58
between my lab and course 6.
00:04:00
And I know a lot
of you are thinking
00:04:02
about being course
6 majors, so I
00:04:04
thought I would tell you
about some research of Collin
00:04:07
Stultz, a course 6 professor.
00:04:09
So he was doing some
computational analysis
00:04:11
on these proteins
that we're studying.
00:04:15
So many of you at this
point in the semester
00:04:17
probably feel like you
might be getting an ulcer.
00:04:19
But unless you have
H. pylori in your gut,
00:04:21
you probably are not
actually getting an ulcer.
00:04:24
And you just take a little
B12, you'll feel a lot better.
00:04:26
OK.
00:04:27
So here's the video.
00:04:27
[VIDEO PLAYBACK]
00:04:28
- My name is Sarah Bowman, and
I am a post-doctoral fellow
00:04:31
at MIT.
00:04:33
I am working on
studying a protein
00:04:36
from Helicobacter pylori,
which is pathogenic bacteria.
00:04:41
Its kind of ecological niche
is in mammalian stomachs.
00:04:46
It's actually very difficult
to treat using antibiotics,
00:04:50
because a lot of times
when you're given
00:04:52
antibiotics they're going
to actually be broken apart
00:04:55
by the acidity of the stomach
before they actually ever
00:04:59
get to killing the
Helicobacter pylori.
00:05:02
Transition metals in
biological systems
00:05:05
are actually really important.
00:05:07
They increase the
range of reactivity
00:05:09
that proteins and enzymes
are able to access.
00:05:14
Nickel is a transition metal.
00:05:16
I mean, it's a transition
metal that's actually fairly
00:05:19
rare in biological systems.
00:05:22
So one of the big things that
H. pylori uses nickel for
00:05:26
is an enzyme called urease.
00:05:29
Urease requires something like
24 nickel ions, which is a lot.
00:05:36
Urease is one of
the proteins that
00:05:38
allows for a lot of buffering
capacity of the organism,
00:05:42
of the H. pylori organism.
00:05:45
The stomach pH is very
low, so pH 2-something.
00:05:50
And this bacteria has to
swim through the stomach
00:05:53
and then colonize it.
00:05:54
And you'd think that the stomach
would just break it apart
00:05:57
like it breaks apart your food.
00:06:00
But in fact, the bacteria
itself has mechanisms in place
00:06:05
that allow it to
create buffers that
00:06:07
allow it to move
through the stomach
00:06:09
and live in the stomach.
00:06:11
And one of those enzymes, and
it's really important for that,
00:06:14
is urease.
00:06:15
00:06:18
In humans, nickel, as
far as we can tell,
00:06:21
is not essential
for any enzymes,
00:06:24
whereas in Helicobacter
pylori, for instance, nickel
00:06:29
is an essential
transition metal.
00:06:31
And so a really intriguing
thing to kind of think about
00:06:35
is just whether we could somehow
target the nickel requirement
00:06:42
in this organism and
in other bacteria that
00:06:46
would allow us to kill these
pathogenic bacteria while not
00:06:50
doing anything that would
be harmful to humans.
00:06:55
[END PLAYBACK]
00:06:55
CATHERINE DRENNAN:
So I like that video
00:06:57
partly because it brings back
acid-base and buffers, as well
00:07:02
as talking about
transition metals.
00:07:04
And I love the bacteria
being attacked by the acid
00:07:09
and then making a buffer
and saving itself.
00:07:12
It's awesome.
00:07:12
OK.
00:07:14
So one of the reasons why
these transition metals are
00:07:18
so powerful, they can
do so many things,
00:07:21
is that they like
to form complexes,
00:07:24
and they like to form complexes
with small molecules or ions.
00:07:32
And those ions often will
have a lone pair of electrons,
00:07:36
and the metal wants
that electron density.
00:07:38
It wants the benefit
of that lone pair.
00:07:42
So when you have this
lone pair, the metal
00:07:46
will come in contact
with that lone pair,
00:07:48
and it'll make a very
happy, very happy metal.
00:07:51
And we can think
about this interaction
00:07:54
here as the donor atoms
are called ligands.
00:07:59
And now let's review something
we learned before about
00:08:02
whether this is a Lewis
acid or a Lewis base then.
00:08:05
00:08:18
OK, 10 more seconds.
00:08:19
00:08:35
That's right.
00:08:36
So it's a Lewis base.
00:08:39
So if we put this
up here, donor atoms
00:08:42
are called ligands,
which are Lewis bases,
00:08:45
and the Lewis bases donate
the lone pair of electrons.
00:08:48
And again, we can think about
the definition that we've been
00:08:51
more used to, where
a base is taking H .
00:08:55
It's accepting the
proton from the acid.
00:08:58
But there, when it's taking
H , it's taking H without its
00:09:01
electrons.
00:09:02
So it's actually donating its
loan pairs to form a bond.
00:09:06
And then we can think
about Lewis acids.
00:09:09
So the acceptor atoms, which
are our transition metals,
00:09:13
are Lewis acids.
00:09:14
They accept the lone pair.
00:09:16
And when an acid that has
a proton on it loses H ,
00:09:21
it is taking the
electrons with it,
00:09:24
because H is leaving
without its electrons.
00:09:26
So these definitions
work, but these are
00:09:28
sort of more broad definitions.
00:09:31
So here, our metals, any
of our transition metals,
00:09:33
are going to be our
acceptors, our Lewis acids.
00:09:36
And here are a bunch of ligands.
00:09:37
We have water.
00:09:38
We have NH3.
00:09:39
We have CO.
00:09:40
They have lone pairs.
00:09:41
They can be donor atoms.
00:09:43
And the ligands form
complexes with the metals.
00:09:47
And the kind of
complexes-- they're
00:09:48
often called
coordination complexes,
00:09:51
and that's a metal that's
surrounded by ligands.
00:09:55
And here's a little example,
a metal in the middle,
00:09:58
and it has the
ligands around it.
00:10:00
So let's consider this
coordination complex now
00:10:03
and think about what this
picture is telling us.
00:10:08
So we have our
coordination complex.
00:10:10
We have cobalt in the middle,
and we have NH3 groups
00:10:14
as our donor ligands.
00:10:16
And here this bracket indicates
the overall charge is plus 3.
00:10:21
Again, the transition metal
is going to be the Lewis acid.
00:10:25
It's going to be accepting
the lone pairs from the Lewis
00:10:29
bases, which are the
ligands, or the donor atoms.
00:10:34
Now, we can think
about a new term
00:10:36
called "coordination number."
00:10:39
And that's simply
the number of ligands
00:10:41
that are bound to the metal.
00:10:44
So a CN number of 6 would
indicate six ligands make up
00:10:51
what's called the
primary coordination
00:10:54
sphere, which is the things
that are bound directly
00:10:58
to the metal.
00:11:00
So CN numbers for transition
metals range from 2 to 12,
00:11:05
but 6 is probably
the most common.
00:11:08
So before we think about the
shapes of these molecules,
00:11:12
let's just look at
the notation for this,
00:11:17
so coordination
complex notation.
00:11:20
So I would write this
structure up here
00:11:24
within brackets--
cobalt bracket NH3.
00:11:29
You have parentheses around NH3.
00:11:31
There's six of those.
00:11:33
Another bracket here
with a plus 3 charge,
00:11:36
indicating the
charge on everything,
00:11:38
this whole structure.
00:11:40
But often, coordination
complexes with a plus charge
00:11:44
will have counterions around.
00:11:46
So there might be, say, three
chlorine minus ions around,
00:11:50
and so that could be
written like this,
00:11:52
or it could be
written like this.
00:11:54
If you see Cl3 outside
of those brackets,
00:11:57
it means that
they're counterions.
00:12:00
So if I looked at this, I'd
say NH3 is within the brackets.
00:12:03
That means it's bound to the
cobalt. So that would tell you
00:12:06
there are six things bound to
the cobalt. The Cl is outside.
00:12:09
That indicates
it's a counterion.
00:12:11
There are three of them.
00:12:12
So there are three counterions,
which then tells you
00:12:15
the charge must be plus 3.
00:12:18
All right.
00:12:18
So there is our notation.
00:12:20
00:12:23
All right.
00:12:23
So now we're back to
thinking about geometries.
00:12:28
So this is one of the things
I love about this part.
00:12:30
I feel like some
people in the course
00:12:32
are just like, new
topic, new topic.
00:12:34
Oh, man, when is the new
material going to end?
00:12:37
Well, you find you get
enough into chemistry,
00:12:40
and you start revisiting topics
you've already seen before.
00:12:43
So this is great.
00:12:45
All right.
00:12:45
So coordination number 6.
00:12:49
We haven't maybe heard
coordination number 6,
00:12:51
but that's pretty
easy to remember.
00:12:53
It's the number of atoms bound.
00:12:54
What type of geometry is this?
00:12:57
00:13:00
You can just yell it out.
00:13:04
Right.
00:13:04
So that's octahedral geometry.
00:13:07
Again, the solid
triangles coming out
00:13:11
indicate they're
coming out at you.
00:13:13
Back dashes are going back,
and we have our axial.
00:13:17
All right.
00:13:17
So let's see how well you
remember CN 5 structures.
00:13:23
And you can keep
this up here, and you
00:13:25
can tell me what the name
of those two geometries are.
00:13:28
00:13:45
All right.
00:13:45
Why don't you take
10 more seconds.
00:13:47
And here are our structures
in real life down here.
00:13:50
00:14:02
People are just like,
I want to put see-saw.
00:14:04
No, no.
00:14:05
00:14:08
That is the parent
geometry of see-saw,
00:14:10
but not see-saw itself.
00:14:11
OK.
00:14:12
So we have the
trigonal bipyramidal
00:14:16
and the square pyramidal.
00:14:17
So I'm holding up the
square pyramidal right now.
00:14:21
And then we have the trigonal,
because it's trigonal
00:14:26
along here, bipyramidal.
00:14:28
So it's sort of like
one pyramid here,
00:14:30
one pyramid there,
so bipyramidal.
00:14:33
And if I took off one
and we had a lone pair,
00:14:36
then we would get our
friend the see-saw.
00:14:41
OK.
00:14:43
Next we have this.
00:14:46
What's that one called?
00:14:49
AUDIENCE: Square.
00:14:49
CATHERINE DRENNAN: Square--
00:14:51
AUDIENCE: Planar.
00:14:52
CATHERINE DRENNAN: Planar, yep.
00:14:55
And this one?
00:14:58
Tetrahedral.
00:14:58
00:15:02
And now CN number of 3.
00:15:09
What is this one?
00:15:10
AUDIENCE: Trigonal--
00:15:12
CATHERINE DRENNAN:
Trigonal planar.
00:15:13
It's in a plane kind
of, if I hold the bonds
00:15:16
and they don't fall off,
and it's kind of trigonal.
00:15:20
And then what
about the last one?
00:15:23
AUDIENCE: Linear.
00:15:23
CATHERINE DRENNAN: Linear.
00:15:25
OK.
00:15:25
And let's just run through and
think about the angles as well.
00:15:29
With octahedral,
what are our angles?
00:15:32
AUDIENCE: 90.
00:15:33
CATHERINE DRENNAN: 90.
00:15:34
Trigonal bipyramidal?
00:15:36
AUDIENCE: 90 and 120.
00:15:38
CATHERINE DRENNAN: 90
and 120, that's right.
00:15:41
So we have one 120 around here,
and then the top parts were 90.
00:15:50
OK.
00:15:52
We have the square pyramidal.
00:15:55
90.
00:15:56
Square planar?
00:15:57
AUDIENCE: 90.
00:15:58
CATHERINE DRENNAN: 90.
00:15:59
Tetrahedral?
00:16:00
AUDIENCE: 109.7?
00:16:02
CATHERINE DRENNAN: 109.5.
00:16:04
Give credit for 0.7 too.
00:16:06
That's quite close.
00:16:07
Trigonal planar?
00:16:08
AUDIENCE: 120.
00:16:09
CATHERINE DRENNAN: 120.
00:16:10
And linear?
00:16:11
AUDIENCE: 180.
00:16:11
CATHERINE DRENNAN: 180, right.
00:16:13
So you're going to need to
remember these for this unit,
00:16:16
but that's OK because you need
to remember them for the final
00:16:19
anyway.
00:16:19
So it gives you a nice review.
00:16:22
All right.
00:16:24
So we got every one.
00:16:27
We got them down.
00:16:28
Can look up your old notes.
00:16:30
Just review.
00:16:31
All right.
00:16:32
So coordination complexes
also have another name.
00:16:35
They can be called chelates.
00:16:37
Just another name for
coordination complex.
00:16:40
So chelates can be the thing.
00:16:42
But you can also
say that the ligand
00:16:44
will chelate as
another way of saying
00:16:47
that it will bind to a metal.
00:16:50
And it can bind more than
once with one or more sites
00:16:53
of attachment.
00:16:54
And the word "chelate"
comes from claws,
00:16:57
and I like that picture.
00:16:58
I feel like, yes, these
ligands coming in like claws
00:17:02
and binding that metal.
00:17:03
They're chelating that metal.
00:17:07
So there are different
names depending
00:17:09
on how many points of
attachment they have.
00:17:12
And we have what's
known as monodentate--
00:17:15
"dent" for dentist or tooth.
00:17:18
So that's one point
of attachment.
00:17:23
And I bet that without having
seen this material ever
00:17:26
before you can tell me
what the rest of these are.
00:17:29
What do you think
bidentate means?
00:17:31
AUDIENCE: Two.
00:17:32
CATHERINE DRENNAN: Two.
00:17:33
Tridentate?
00:17:34
AUDIENCE: Three.
00:17:35
CATHERINE DRENNAN: Tetradentate?
00:17:37
AUDIENCE: Four.
00:17:38
CATHERINE DRENNAN: Hexadentate?
00:17:39
AUDIENCE: Six.
00:17:40
CATHERINE DRENNAN: Six, right.
00:17:41
There's not one for five.
00:17:42
But this is good.
00:17:43
So don't lose a point on this.
00:17:44
I feel like sometimes
people lose a point
00:17:46
on this on the exam.
00:17:47
You knew it in class
before I taught it.
00:17:49
You don't want to like
somehow work backwards.
00:17:51
So this is easy
points right here.
00:17:54
Just remember on the
exam, wait a minute,
00:17:56
maybe I already know this.
00:17:59
All right.
00:17:59
So let's look at some
examples of chelating
00:18:03
ligands that bind with
multiple points of attachment.
00:18:07
And the first one--
we're kind of on a theme
00:18:09
today-- is vitamin B12 that
we're going to look at.
00:18:14
So this is called
the corrin ring.
00:18:17
Cobalt is in the
middle, and that ring
00:18:21
binds with four
points of attachment.
00:18:25
So it is a tetradentate
ligand, this corrin ring.
00:18:29
There is also an
upper ligand, which
00:18:31
is 5 prime-deoxyadenosine,
and a lower ligand that's
00:18:35
called dimethylbenzimidazole.
00:18:37
You don't need to
know their names.
00:18:38
Overall, it has six ligands
in octahedral geometry.
00:18:43
But the corrin ring is a
very nice biological example
00:18:46
of a multidentate ligand.
00:18:49
Heme would be the same.
00:18:52
I thought I would show you
this rotating around so you
00:18:54
get a better sense and tell
you that this structure
00:18:57
of this vitamin was determined
by Dorothy Hodgkin, who
00:19:01
won the Nobel Prize in
1964 for determining
00:19:04
the structure by
crystallography and also solving
00:19:07
the structure of penicillin.
00:19:09
This was the most
complicated molecule
00:19:12
to be solved by crystallography,
and a lot of people
00:19:15
said that technique
could never be
00:19:17
used to do something that big.
00:19:18
She showed that they were wrong.
00:19:20
In terms of determining the
structure of penicillin,
00:19:24
it was during the war.
00:19:25
And people wanted to
make more penicillin,
00:19:28
but they had no idea what
the structure was so they
00:19:30
didn't know what to make.
00:19:31
And she figured
out the structure.
00:19:33
And it's a
weird-looking molecule,
00:19:34
so no one would have guessed it
without knowing the structure.
00:19:37
So for her pioneering
work in crystallography
00:19:41
she won the Nobel Prize.
00:19:43
All right.
00:19:44
So vitamin B12 is one
example of a chelate.
00:19:49
Another that's probably more
that you probably hear about
00:19:52
the most-- it's almost
synonymous with the word
00:19:54
"chelate"-- is EDTA.
00:19:58
Here is the EDTA
molecule, and you
00:20:01
see that it has
lots of lone pairs
00:20:05
that are just dying
to grab onto a metal.
00:20:08
So we have six.
00:20:10
We have 1, 2, 3,
4, 5, 6, six things
00:20:15
that are capable of
chelating that metal.
00:20:19
And so here is what
the complex looks like.
00:20:21
So the red oxygen can chelate.
00:20:24
The green oxygen
here can chelate,
00:20:26
the nitrogen here in dark
blue, the other nitrogen
00:20:29
in dark blue here, light
blue oxygen here, and also
00:20:34
the purple oxygen.
So now why don't you
00:20:38
tell me what the
geometry of this
00:20:40
would be as a clicker question.
00:20:42
00:20:53
You ready?
00:20:53
AUDIENCE: Yeah.
00:20:54
CATHERINE DRENNAN: Yeah.
00:20:54
10 more seconds.
00:20:56
Should be fast hopefully.
00:20:57
00:21:09
Yeah, great, 86%.
00:21:12
It is octahedral.
00:21:14
And sometimes it's a little
bit hard to see that,
00:21:16
but I helped you out by
drawing those bonds in black
00:21:19
that you needed to look at.
00:21:21
So we have four that are in
the plane here, one above
00:21:25
and one below here.
00:21:28
So that is octahedral geometry.
00:21:30
Also, how many
points of attachment?
00:21:33
What kind of dentate
ligand is this?
00:21:35
00:21:37
It's hexadentate as well.
00:21:39
So it has six points
of attachment here.
00:21:44
All right.
00:21:44
So EDTA is a really
good metal chelator.
00:21:49
And part of the reason that
it is such an awesome metal
00:21:53
chelator is because of entropy.
00:21:57
So we're back to entropy again.
00:21:59
So the binding of EDTA to the
metal is entropically favored.
00:22:04
And the reason for this is
that metals that are, say,
00:22:07
in your body, like if you
happen to eat some lead paint,
00:22:11
and that lead is hanging out.
00:22:13
It's not just by itself.
00:22:14
It's coordinating hopefully
just to water and not
00:22:17
to proteins in your body.
00:22:19
But when you take some EDTA to
prevent your lead poisoning,
00:22:25
one molecule of EDTA
will bind to metal,
00:22:27
and all of these waters
are going to be released.
00:22:30
So I have over here some
lead with a whole bunch
00:22:34
of little waters.
00:22:35
This is quite an ordered system.
00:22:39
But if I take out all
of those waters here,
00:22:45
that's a lot more entropy
going on than what we had.
00:22:49
And then you have one
chelating ligand here,
00:22:52
and that's a pretty
simple system.
00:22:54
So this is ordered.
00:22:56
This is disordered.
00:22:58
So the binding of EDTA, one EDTA
releases six water molecules,
00:23:04
and that makes it very
favorable to do this.
00:23:11
And because of that,
chelating molecules,
00:23:14
or the chelate effect,
molecules that are chelates,
00:23:16
like metal bound to EDTA,
are unusually stable
00:23:20
because of this favorable
entropic effect,
00:23:22
this release of water.
00:23:24
So the release of water, the
release of increasing entropy,
00:23:27
drives that metal chelation,
and you sequester your metal,
00:23:32
which is really good if you're
trying to avoid lead poisoning.
00:23:36
So I think this is a nice
example of our friend entropy
00:23:40
driving a reaction.
00:23:41
So a lot of you did really
well on the exam talking about
00:23:44
factors of delta H and entropy
and when you'd have favorable
00:23:47
delta G's.
00:23:48
Here's another
nice example where
00:23:50
the chelate effect explains why
metal chelates are so unusually
00:23:54
stable.
00:23:56
All right.
00:23:57
So uses of EDTA.
00:23:58
I already just told you one.
00:24:01
Lead poisoning-- all ambulances
have EDTA in case someone
00:24:05
is eating some lead paint.
00:24:08
Another thing that EDTA is
used for, which I think is fun,
00:24:12
you should all go check if
you buy little packaged goods,
00:24:16
and they have a long list
of chemical ingredients.
00:24:18
Look for EDTA.
00:24:19
It's often there.
00:24:20
And it says it's "added
for freshness," which means
00:24:24
that bacteria need metals.
00:24:27
You have EDTA.
00:24:28
EDTA sequesters the metals.
00:24:30
The bacteria can't live on
the food that you're eating.
00:24:33
So instead of
saying, food additive
00:24:36
added to kill the bacteria
that were otherwise growing
00:24:39
on your food, they say
added for freshness.
00:24:42
And I do think that
is an improvement.
00:24:44
All right.
00:24:44
Another thing, we've already
talked about the importance
00:24:47
of cleaning bathtubs.
00:24:49
To chelate calcium
out of bathtub scum,
00:24:52
you have EDTA or
other metal chelates.
00:24:55
And then I have my
favorite other example
00:25:00
of the use of EDTA.
00:25:02
This favorite example is in
Hollywood, the movie Blade.
00:25:09
How do you kill a vampire?
00:25:11
Vampires drink what?
00:25:14
AUDIENCE: Blood.
00:25:14
CATHERINE DRENNAN: Blood.
00:25:15
Blood has?
00:25:16
AUDIENCE: Iron.
00:25:17
CATHERINE DRENNAN: Iron.
00:25:19
EDTA chelates?
00:25:20
AUDIENCE: Iron.
00:25:21
CATHERINE DRENNAN: Iron.
00:25:22
So you get a little
dart, and you
00:25:25
have-- you can kind of see
them maybe up here-- they're
00:25:27
filled with liquid.
00:25:29
That's EDTA.
00:25:31
You shoot the vampire with
EDTA, and the vampire just
00:25:37
disappears, just kind of
turns to sort of dust.
00:25:39
[LAUGHTER]
00:25:41
Because it's like mostly iron,
and the iron gets chelated.
00:25:43
But it happens right away.
00:25:45
But anyway, I think that's cool.
00:25:46
Yes, what a good way
to kill a vampire.
00:25:49
EDTA, it's brilliant.
00:25:51
Excellent use on
Hollywood's part for EDTA.
00:25:55
OK.
00:25:55
Metal chelates, all
sorts of potential values
00:25:59
that they have.
00:26:01
OK.
00:26:03
So when we're talking about
coordination complexes,
00:26:06
we're talking about geometries.
00:26:09
Sometimes the atoms can be
arranged in different ways.
00:26:14
And when you have these
geometric isomers,
00:26:18
they can have very
different properties.
00:26:20
So just look at an example here.
00:26:23
It's a platinum compound,
a platinum compound
00:26:26
who has two NH2 groups
and two chlorine groups.
00:26:30
And you could arrange those
in two different ways.
00:26:33
You could put the NH3 groups
on one side and the chlorine
00:26:37
groups on the other side,
and that would be cis.
00:26:40
These are cis to each other.
00:26:42
Or you could put a
transconfiguration, where
00:26:45
chlorine is here, and
then another chlorine
00:26:47
is trans on the other side.
00:26:50
And the same with this.
00:26:52
So cisplatinum here is a
potent anti-cancer drug.
00:26:56
And it has to be cisplatinum
because it binds to DNA,
00:27:01
and the two bases of DNA
displace these chlorines.
00:27:05
So if they're not on the same
side, it can't bind to the DNA.
00:27:08
And so this prevents
the cancer cells
00:27:12
from being repaired
from damaging agents.
00:27:15
Transplatinum does absolutely
nothing that anyone knows.
00:27:18
So it's exactly the
same composition,
00:27:22
but because they are different
isomers from each other--
00:27:26
and I have, let's see, ah,
over here-- different isomers
00:27:31
of each other-- and so
chlorines on the same side,
00:27:35
cis versus the trans--
have completely different
00:27:38
properties.
00:27:40
So cisplatinum got a lot
of fame because it cured
00:27:43
Lance Armstrong of cancer.
00:27:45
Lance Armstrong now, of course,
is a much more controversial
00:27:48
figure than he was at the time.
00:27:50
But still he created
an amazing charity
00:27:53
that hopefully is still doing
well despite some of his fall
00:27:57
from fame.
00:27:59
OK.
00:27:59
So another type of
isomer are called
00:28:04
optical isomers, also
called enantiomers or chiral
00:28:09
molecules.
00:28:10
And these are one, again, you
have the same composition,
00:28:14
but they are non-superimposable.
00:28:17
They are, in fact, mirror
images of each other.
00:28:20
So if my head was
a mirror, these
00:28:22
would be mirror
images of each other.
00:28:24
And I could try very
hard to superimpose them,
00:28:27
bringing the blue
molecules over here,
00:28:30
but then the green and
the red don't match.
00:28:32
You can come and try.
00:28:34
These are, in fact,
non-superimposable mirror
00:28:37
images from each other.
00:28:38
And sometimes they can have
very similar properties.
00:28:42
It depends.
00:28:43
But if you put
molecules like that that
00:28:45
are known as chiral,
chiral molecules,
00:28:48
i.e. enantiomers--
non-superimposable mirror
00:28:51
images.
00:28:52
The human body is very much
of a chiral environment.
00:28:55
You have enzymes
designed to bind things
00:28:57
in a particular way.
00:28:58
So they can have very,
very different properties.
00:29:01
OK.
00:29:02
So we have to do some d-electron
counting before we end today.
00:29:08
And I love this because
it's really pretty simple
00:29:13
to count d-electrons.
00:29:14
And so we're going to just
take a look at some examples.
00:29:18
And for doing this
part, we're going
00:29:20
to start using our friend
the periodic table again.
00:29:26
And we need to find oxidation
numbers, which we just
00:29:30
talked about in the last unit.
00:29:33
So if we have a coordination
complex with cobalt,
00:29:37
and this cobalt has those
six NH3 groups and our plus 3
00:29:44
charge-- so this is the complex
that we have been talking
00:29:47
about-- let's now figure
out what the oxidation
00:29:51
number of this is.
00:29:53
And so this NH3 is neutral,
so that's given as a hint.
00:29:59
Many of our ligands are
going to be neutral ligands.
00:30:04
So if that is 0, what is
the charge on the cobalt?
00:30:09
AUDIENCE: Plus 3.
00:30:10
CATHERINE DRENNAN: Plus 3.
00:30:12
Now we're going to use
the rules of d-count.
00:30:17
So we have a d-count.
00:30:19
We need to look up the group
number from the periodic table,
00:30:25
which, in this case, is 9.
00:30:27
Then we have minus the oxidation
number, so we have 9 minus 3,
00:30:34
or 6.
00:30:36
And so this is a d6 system.
00:30:39
And that is all there is
to doing these counts.
00:30:44
So let's just try another one.
00:30:47
So we heard about nickel.
00:30:49
We'll do nickel.
00:30:51
Nickel is coordinated
by carbon monoxide,
00:30:56
and there are four of those.
00:31:00
So what is my charge on
the nickel going to be,
00:31:04
my oxidation number
of the nickel?
00:31:06
So what's my overall
charge of this complex?
00:31:09
AUDIENCE: 0.
00:31:09
CATHERINE DRENNAN: 0.
00:31:11
CO is also going to be 0.
00:31:15
There's no charge on CO.
00:31:17
So what is the oxidation
number of nickel?
00:31:20
AUDIENCE: 0.
00:31:20
CATHERINE DRENNAN: 0.
00:31:22
So then we can do our d-count.
00:31:26
The d-count, what is the
group number for nickel?
00:31:30
AUDIENCE: 10.
00:31:31
CATHERINE DRENNAN: What is it?
00:31:31
AUDIENCE: 10.
00:31:32
CATHERINE DRENNAN: 10.
00:31:34
This is the kind of math that
always makes me very happy.
00:31:37
10 minus 0 is 10.
00:31:41
So that is a d10 system.
00:31:46
All right.
00:31:47
We'll do one more over here.
00:31:48
And the next one is
a clicker question.
00:31:50
00:32:01
Gives me time to write
00:32:24
AUDIENCE: Whenever you're ready.
00:32:26
We're out of time.
00:32:27
CATHERINE DRENNAN: Yep.
00:32:27
All right.
00:32:28
Let's just do 10 more seconds.
00:32:29
00:32:43
Yep.
00:32:45
So here our overall
charge is minus 1.
00:32:49
We have the chlorines
are minus 1.
00:32:54
NH3 is 0.
00:32:56
Water is 0.
00:32:58
So this has to be plus 2 because
plus 2 minus 3 is minus 1.
00:33:05
We have 9 minus 2 is
7, so it's a d7 system.
00:33:14
All right.
00:33:14
So Wednesday, d orbitals.
00:33:18
I cannot wait.
00:33:20
00:33:31
Yes.
00:33:32
All right, 10 seconds.
00:33:33
00:33:50
OK.
00:33:51
Does someone want to tell me
why that's the right answer?
00:33:56
00:34:02
Anybody?
00:34:02
00:34:07
We got a nice dangly
thing for your keys or ID.
00:34:13
No?
00:34:13
00:34:16
All right.
00:34:19
So here we're thinking about
whether things are better
00:34:22
reducing agents or
better oxidizing agents.
00:34:25
And here we're given two
different redox potentials--
00:34:29
minus 600 and minus 300.
00:34:33
So the one that is going
to be the lower number
00:34:38
is going to be better at
reducing other things.
00:34:41
It wants to be oxidized itself.
00:34:45
And then we can think
about whether it's
00:34:48
a favorable process in terms
of whether the thing that
00:34:53
likes to reduce is actually
doing the reducing.
00:34:56
That's going to make it
a spontaneous process.
00:34:59
All right.
00:34:59
So these are the
kinds of questions
00:35:03
for the oxidation-reduction
unit that we just finished.
00:35:06
And this will be on exam 4,
which, amazingly, we just
00:35:10
finished an exam, and
there's another one.
00:35:13
So exam 4 is two
weeks from today.
00:35:17
All right.
00:35:18
From Friday, sorry,
two weeks from Friday.
00:35:21
All right.
00:35:21
So today we're going to continue
with this unit on transition
00:35:24
metals.
00:35:25
The next exam is going to
have oxidation-reduction
00:35:27
and transition metals and
a little bit of kinetics.
00:35:30
Kinetics is our last unit.
00:35:31
So we're getting very close
to the end of the semester.
00:35:33
00:35:36
So we're finishing up the
handout from last time.
00:35:38
Again, we're back to
the periodic table.
00:35:41
We're thinking about
transition metals.
00:35:43
We're thinking about that middle
part of the periodic table,
00:35:46
and so we're thinking
about d orbitals.
00:35:48
00:35:52
So there are five d orbitals.
00:35:54
How many s orbitals are there?
00:35:58
AUDIENCE: One.
00:35:58
CATHERINE DRENNAN: One.
00:35:59
How many p orbitals are there?
00:36:01
AUDIENCE: Three.
00:36:01
CATHERINE DRENNAN: Three.
00:36:02
And so d orbitals have five.
00:36:04
And we're not going to
talk about really anything
00:36:06
beyond d orbitals in this class.
00:36:08
And frankly, not
very many people do.
00:36:10
But d orbitals are amazing,
so we have to fit them in.
00:36:13
All right.
00:36:13
So there are five d orbitals.
00:36:15
And they're up
here, and you need
00:36:18
to be able to draw their shapes.
00:36:20
And the bar for drawing the
shapes is actually pretty low.
00:36:25
So these are my
drawings that I made.
00:36:28
And so you can probably
do just about as well.
00:36:32
All right.
00:36:32
So the one that has
the most unusual shape
00:36:36
is the dz squared.
00:36:40
And so it has its maximum
amplitude along the z-axis.
00:36:44
And for this unit,
our z-axis is always
00:36:46
going to be up and down here.
00:36:49
y is in the plane of the screen,
and x is coming out toward you
00:36:53
and also going into the screen.
00:36:55
And so dz squared has its
maximum amplitude along z,
00:37:00
and it also has a
doughnut in the xy-plane.
00:37:05
And so I also brought
a little model of this.
00:37:09
So here's dz squared.
00:37:11
We have maximum amplitude
along the z-axis, up and down.
00:37:16
And we have our little
doughnut in our xy-plane.
00:37:22
So then we have dx squared
minus y squared, which
00:37:25
has maximum amplitude
along x and along y.
00:37:31
And that would look like this.
00:37:35
So we have our
maximum amplitudes
00:37:37
that are right on axis.
00:37:39
So if this is y-axis and x
is coming out toward you,
00:37:43
those orbitals are
pointing right along
00:37:46
that coordinate frame.
00:37:49
The other three orbitals
look a little bit
00:37:52
like dx squared minus y squared,
but they're not on-axis.
00:37:57
They're off-axis.
00:37:58
They're in between the axes.
00:38:00
So we have dyz.
00:38:03
It has its maximum
amplitude 45 degrees off
00:38:07
of the y and the z-axis.
00:38:11
So if this is
z-axis here, there's
00:38:14
no maximum amplitude along here.
00:38:16
It's 45 degrees off.
00:38:18
So it's right in the middle
between the z and the y.
00:38:24
So dxz has its maximum amplitude
45 degrees between x and z.
00:38:33
So that would be
pointing the other way.
00:38:37
And so I tried to
draw this keeping
00:38:39
the reference frame the same.
00:38:42
It's a little hard
to see the orbitals,
00:38:44
but it would be kind of this.
00:38:45
So we rotate that
around, and so that's
00:38:48
what that would look like.
00:38:50
And then dxy we have maximum
amplitude 45 degrees in
00:38:56
between the x and the y.
00:38:59
So x coming out, y in the plane.
00:39:02
And so this is, again, a
little bit hard to draw.
00:39:05
If I drew it absolutely
perfectly and not tilted
00:39:07
at all, you kind of
wouldn't see anything.
00:39:09
But that's what that
would look like.
00:39:12
So again, the names
of this, it tells you
00:39:14
about the relationship
between that orbital,
00:39:17
that maximum amplitude, and
the axis that we have defined.
00:39:23
So this is very
important to know
00:39:27
that these guys are in
between the axes, right
00:39:30
in the middle, 45 degrees.
00:39:32
And you'll see why in a few
minutes why that's important.
00:39:36
OK.
00:39:36
So just to practice, here are
some slightly better pictures
00:39:41
of the orbitals.
00:39:42
And this is the coordinate
frame over here,
00:39:48
and now we have the
orbitals inside that.
00:39:50
So again, z is going up, y is
in the plane of the screen,
00:39:55
and x is going back and
also coming out toward us.
00:39:58
So which is this d orbital?
00:40:01
You can just yell it out.
00:40:03
AUDIENCE: dz squared.
00:40:03
CATHERINE DRENNAN: dz.
00:40:04
Yeah, that's easy to remember.
00:40:06
That's the unique-looking one.
00:40:09
What about this one?
00:40:11
First think about the plane.
00:40:13
So it's the xy-plane.
00:40:15
And then, is it on or off-axis?
00:40:17
So which one is this?
00:40:18
AUDIENCE: [INAUDIBLE]
00:40:19
CATHERINE DRENNAN: Yeah.
00:40:20
So this one is on-axis.
00:40:21
You can see the
maximum amplitude
00:40:23
of the orbital pointing
right along those axes.
00:40:26
So it's right in the corners
of that square there.
00:40:31
And then what about
this one down here?
00:40:33
AUDIENCE: [INAUDIBLE]
00:40:34
CATHERINE DRENNAN: Yep.
00:40:35
So that would be dxy.
00:40:37
So it's in the xy-plane, but
it's 45 degrees off the axes.
00:40:41
So it's in between
the axes here.
00:40:44
And what about that one?
00:40:46
AUDIENCE: .
00:40:47
CATHERINE DRENNAN: Right.
00:40:48
So it's along both z and y here.
00:40:55
And then this last
one, which is drawn
00:40:57
to kind of come out toward you,
so that is along x as well.
00:41:01
So that's dxz, and
it's going up along z.
00:41:05
So you can look at the
coordinate frame, which we'll
00:41:07
try to keep consistent,
and ask yourself,
00:41:11
is it on-axis or off-axis,
and which plane is it in?
00:41:14
And that will allow you to name
them and also to draw them.
00:41:19
So just to kind of give you more
of a three-dimensional sense,
00:41:23
there's these little movies
that I'll show you now.
00:41:26
And so you can get a better
sense of that awesome doughnut.
00:41:29
It's going to make you hungry.
00:41:30
They even colored it like a
really nice original doughnut
00:41:34
that you would get
at Dunkin' Donuts.
00:41:36
So the doughnut is
in the xy-plane,
00:41:42
and these other
lobes are along z.
00:41:47
So now we have dx
squared minus y squared,
00:41:51
and you can see that the
maximum amplitudes, again,
00:41:54
are along the axes.
00:41:56
Key-- they're along
the axes here.
00:41:59
I don't know why it
comes out towards you
00:42:01
and-- I didn't, yeah.
00:42:03
But it gives you a good
three-dimensional sense
00:42:06
of this.
00:42:07
All right.
00:42:08
So dxy now, again,
in the xy-plane.
00:42:13
But instead of being on-axis,
it's 45 degrees off-axis.
00:42:16
So you can see, I think,
in this really nicely,
00:42:19
it's right between the axes,
but it's not touching them.
00:42:23
The axes sort of
separate these orbitals.
00:42:26
00:42:28
And then we have xz.
00:42:33
So now we're going up along
the z-axis and in the x-plane.
00:42:37
And here it comes at you again,
45 degrees in between z and x.
00:42:44
And then our last
one, we have yz.
00:42:51
00:42:54
So the shapes of
those later three,
00:42:57
actually even four of
them, are the same.
00:42:59
It's just a matter if
they're on or off-axis
00:43:01
and which plane they're in.
00:43:03
So this is not too hard to draw.
00:43:06
All right.
00:43:06
So why is this important?
00:43:08
Why should we care exactly
how the orbitals are oriented?
00:43:13
And the reason that you
should care about that
00:43:15
is because it can explain a
lot of the special properties
00:43:19
of transition metals.
00:43:21
