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A very simply little
demonstration.

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And I would like you to take a
look at the two materials that

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are in the two vials I have
here.

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One of these that I am taping
is a beige-colored solid,

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and it is iron dichloride,
ferrous chloride.

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And then over here,
I have an organic molecule that

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is colorless,
just white.

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And what I am going to do is
first make a solution of the

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ferrous chloride in water.

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I am just using a very small
amount.

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And what you will see is that
the FeCl two dissolves

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up pretty nicely in water,
and it gives a solution that

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has very little color to it.
You might be able to see that

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it looks maybe pale yellow.
Can you see that?

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Okay.
To that solution of ferrous

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chloride in water,
I am now going to add this

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organic molecule.
I will draw the molecule for

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you before we finish today.
And I am just going to add a

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small amount of this colorless
organic molecule --

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-- to get a very nice intense
red color in the aqueous

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solution.
And the person I am going to be

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talking about today is the one
who really figured out what is

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going on in a reaction like
that.

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And this was a great mystery
for a long period of time,

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until Alfred Werner came along.

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I would encourage you while you
are at home, perhaps,

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or visiting friends over this
coming weekend,

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to go onto the Internet and go
to the Nobel Prize website,

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where you can read a very nice
short bibliography of Alfred

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Werner, because he won the Nobel
Prize in 1913.

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And he won this prize based on
a theory that he developed

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stemming from observations he
made regarding reactions of

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metal salts with various
substances.

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And I am going to point out
initially, here,

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that he studied the reaction of
cobalt three chloride.

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I just used iron dichloride.
This is cobalt trichloride

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reacting with six equivalents of
ammonia.

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And he observed that if to
aqueous solution of cobalt

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trichloride was added six
equivalence of NH three,

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ammonia, followed by silver
nitrate, that that resulted in

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no AgCl precipitate.
And that is rather astounding.

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Sorry.
We will get to "no,"

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but this one is "all."
How does he do this experiment?

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He just puts these things in
solution, adds silver nitrate,

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and either there is or is not a
precipitative silver chloride,

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which is very insoluble.
And that precipitate can be

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collected by filtration,
dried, and then weighed.

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And then, in comparison with
the mass of the added

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substances, you would know how
much of the chloride that was

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put into the reaction actually
came out as insoluble silver

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chloride precipitate.
And he did a series of

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experiments.
And so, if he uses cobalt

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trichloride and less ammonia,
namely five equivalents of

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ammonia, then he finds that that
leads instead to two-thirds of

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the possible AgCl precipitate.
And continuing down.

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If he uses now only four
equivalence of ammonia,

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then the addition of silver
nitrate provides one-third of

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the possible of the total
precipitated chloride.

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And then, finally,
if he drops down the number of

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equivalents of ammonia to three,
then we get none,

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zero of the AgCl precipitating.
And a further observation,

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to add to these four
observations,

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was that in no case,
here, did the solution give a

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reaction with hydrogen chloride.
What is the significance of

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that?
Hydrogen chloride,

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of course, is a strong Bronsted
acid.

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And, if you have a base in
solution, it should react with

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that Bronsted acid.
And what are we adding here?

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We are adding ammonia.
And ammonia is a base,

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isn't it?
But when you do the experiment

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like this and then test for any
reactivity with hydrogen

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chloride, there is no reactivity
with hydrogen chloride.

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So what is going on?
And, secondly,

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normally, this reaction with
silver nitrate is used to

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quantitatively precipitate
chloride from solution.

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So it is a quantitative
analytical test for chloride in

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solution.
And the less ammonia we add,

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the less silver chloride we are
getting as a precipitate.

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How are these facts related?
Well, Alfred Werner put it all

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together, and he correctly
formulated these complexes.

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Let me write this as follows.

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In that first instance,
when we have added six

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ammonias, Werner decided that
the reason that ammonia is not

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in solution in a form that is
reactive with hydrogen chloride

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is because the ammonia is
coordinated to the metal.

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And so he wrote the formula
this way.

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Cobalt NH three six times.
And this species

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is a trication.
And to balance those three

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positive charges,
we find that there must be

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three chloride ions outside of
what we are going to call the

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inner coordination sphere of the
cobalt complex.

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Now, there were lots of
different preparations that had

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been reported in the literature
back at this time of materials

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that seem to be composed of
metal ions and mixtures of

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chloride or ammonia or other
types of molecules.

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And this kind of a formulation
of them was completely unique.

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And I really think that in the
history of chemistry,

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you can compare Alfred Werner's
leap, his development of the

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coordination theory as very much
analogous to Kekule's

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description of planar benzene
with all equivalent C-C bond

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distances.
This is really a tremendous

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leap in our thinking about
molecules.

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And then, in the case where he
is adding five equivalents of

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ammonia, those five equivalents
all go onto the cobalt,

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and so does one chloride ion.
So he writes it that way.

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And that chloride ion is
balancing one of the three

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positive charges on the cobalt
plus three ion.

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So this overall,
now, has a two plus charge,

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and there are two chlorides
external to balance the charge

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on that.
And then in case three,

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we have added four NH three
per cobalt to solution.

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Four of them go on the metal
and two chlorides remain and

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interact with the metal in a way
that we will discuss shortly.

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And that system now has two of
the plus three charges on cobalt

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three balanced by chlorides that
are in the inner coordination

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sphere.
And only a single chloride,

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now, is needed externally to
balance that charge to give

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overall a neutral system.
And then, finally,

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when only three equivalents of
ammonia are added to solution,

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those three equivalents per
cobalt bind to the metal.

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And all three of the original
chlorides can be included in the

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primary coordination sphere,
balancing the three positive

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charges on the cobalt three ion
and giving overall a neutral

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coordination complex.
This is coordination theory.

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00:11:02,000 --> 00:11:06,000
And the dominance of organic
chemistry at that point in time

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was very great.
Most of the people who were

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thinking about these unusual
substances were thinking that

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they might have structures
analogous to those that organic

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molecules have.
And typical hydrocarbon

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molecules like n-pentane or
n-hexane have sequential joined

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CH two groups,
repeating CH two groups

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in a line.
And so the type of formula that

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you were seeing people write for
these molecules at that point in

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time was, for example,
a cobalt.

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And then NH three,
NH three, NH three,

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NH three, somehow all stuck

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together in a way that does not
seem very intuitive to us today

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because we know so much more
now, partly due to the

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accomplishments of Alfred
Werner.

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This systematic set of
observations,

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the use of silver chloride's
insolubility as a means of

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precipitating it out so that you
could distinguish between

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external chloride from chloride
that is actually in the

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coordination complex.
And let me define coordination

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complex.

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The coordination complex is a
metal ion.

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Plus its ligands.

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00:12:47,000 --> 00:12:51,000
So there is another word that
you need to learn in this

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context.
Here, I would like to define

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the term ligand as an atom,
or a molecule,

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or an ion, that can bind
directly to a metal like cobalt

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in its primary coordination
sphere.

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And that means that they are
directly connected to the metal.

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And the amazing thing here and
what was so different from

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organic chemistry at this time
was the idea that a single metal

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ion can have a fairly large
number of ligands.

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In this particular case,
Werner analyzed his experiments

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with the assumption of
coordination number being equal

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to six.

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So here, it is six.
But coordination number is a

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variable that depends on the
metal itself and depends on the

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specific choice of the ligands.
Some molecules are known in

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which there are very low
coordination numbers.

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00:14:04,000 --> 00:14:09,000
A coordination number can be as
small as two or one in some very

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special instances for insoluble
molecules.

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And for very large metal ions,
sometimes the coordination

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number can be as great as about

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So 12 atoms or ions or
molecules directly connected to

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a central metal atom.
And ligands don't have to be as

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simple as chloride or ammonia.
Ligands can have some pretty

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interesting architectures.
And you can even dream up new

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ligands with which to decorate a
metal ion and with which to

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imbue it with special properties
for purposes like catalysis.

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We will be talking soon about
metaloenzymes.

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These are proteins as ligands
to metal complexes.

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And very many important enzymes
are metaloenzymes that have

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these elements from the 3D part
of the periodic table bonded.

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00:15:13,000 --> 00:15:18,000
Here is what we call the
d-block, --

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-- or transition elements.

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And, in the case of the 3D
series, you will know that we

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have metals like titanium,
vanadium, chromium,

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manganese, iron,
cobalt, nickel.

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These are called transition
elements because oftentimes in

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ions stemming from these
elements, as you go from left to

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right across the periodic table,
you are adding more electrons

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to an incompletely filled
d-shell.

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And at the end today,
we are going to talk a little

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00:16:09,000 --> 00:16:13,000
bit about the bonding properties
of transition elements.

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And that will hearken back to
what I said with my discussion

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00:16:17,000 --> 00:16:20,000
of carbon monoxide and why it is
a poison.

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And it is the interaction,
actually, with certain

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d-orbitals on the iron in
hemoglobin that makes CO a toxic

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00:16:28,000 --> 00:16:32,000
substance.
And so how does this work?

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How do ligands coordinate two
metals?

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00:16:37,000 --> 00:16:43,000
Well, one simple way is if you
have a ligand like ammonia that

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is a base, it can also be a
nucleophile, and the metal can

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00:16:50,000 --> 00:16:54,000
be the corresponding
electrophile.

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00:16:54,000 --> 00:17:00,000
I can draw that to represent a
lone pair of electrons on the

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nitrogen.
Now that we have studied

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00:17:04,000 --> 00:17:07,000
molecular orbital theory,
you will know that I can also

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00:17:07,000 --> 00:17:11,000
call this the highest occupied
molecular orbital of the NH

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00:17:11,000 --> 00:17:14,000
three molecule.
And it is the one responsible

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00:17:14,000 --> 00:17:18,000
for the basicity of the ammonia
molecule and the one responsible

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00:17:18,000 --> 00:17:21,000
for its ability to serve as a
ligand in coordination

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00:17:21,000 --> 00:17:25,000
complexes, like these.
And you might also suspect that

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00:17:25,000 --> 00:17:28,000
we might have some contributions
to this highest occupied

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00:17:28,000 --> 00:17:33,000
molecular orbital from hydrogen
1s linear combinations.

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00:17:33,000 --> 00:17:37,000
I will just draw that in to
make it a little bit more

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00:17:37,000 --> 00:17:41,000
accurate.
And so, you can think of this

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00:17:41,000 --> 00:17:46,000
as a big fat lone pair that will
coordinate to Lewis acids.

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00:17:46,000 --> 00:17:50,000
And the metal ion is a Lewis
acid.

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00:18:02,000 --> 00:18:06,000
But it is a very interesting
Lewis acid because,

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00:18:06,000 --> 00:18:11,000
unlike the BH three
molecule that has a single empty

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00:18:11,000 --> 00:18:16,000
orbital, this metal seems to be
able to act as a Lewis acid six

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00:18:16,000 --> 00:18:21,000
times and coordinate six bases
to it in forming this

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00:18:21,000 --> 00:18:26,000
coordination complex.
And if we go ahead and

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00:18:26,000 --> 00:18:32,000
crystallize molecules of this
sort and use X-ray diffraction

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00:18:32,000 --> 00:18:38,000
studies to determine the bond
angles and bond distances in

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00:18:38,000 --> 00:18:43,000
systems like this,
what we would find is that

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00:18:43,000 --> 00:18:49,000
these nitrogens are located at
the vertices of a nice,

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00:18:49,000 --> 00:18:54,000
regular octahedron.
So, in the case of our first

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00:18:54,000 --> 00:18:59,000
one, we can draw it out this
way.

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00:18:59,000 --> 00:19:03,000
This first one is what would
result if, to that aqueous

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00:19:03,000 --> 00:19:08,000
solution of cobalt trichloride,
we were to add six equivalents

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00:19:08,000 --> 00:19:11,000
of ammonia.
This is Werner's first system.

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00:19:11,000 --> 00:19:14,000
It is a molecule oriented like
this.

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00:19:14,000 --> 00:19:19,000
That lone pair that comes from
the highest occupied molecular

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00:19:19,000 --> 00:19:24,000
orbital of ammonia is directed
right at the metal from each of

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00:19:24,000 --> 00:19:29,000
the six ammonia ligands.
And this system does have a

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00:19:29,000 --> 00:19:34,000
three plus charge that is
balanced by three chloride ions

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00:19:34,000 --> 00:19:38,000
in solution.
This locating of six nitrogens

236
00:19:38,000 --> 00:19:42,000
in an array in space that
approximates a regular

237
00:19:42,000 --> 00:19:48,000
octahedron is what makes the
octahedron such a central aspect

238
00:19:48,000 --> 00:19:52,000
of the theory of transition
element chemistry.

239
00:19:52,000 --> 00:19:58,000
And, if you are going to design
molecules that do include these

240
00:19:58,000 --> 00:20:02,000
transition metal ions,
--

241
00:20:02,000 --> 00:20:05,000
-- whether you are going to do
it for their color,

242
00:20:05,000 --> 00:20:09,000
like the red color there of the
iron complex that we made a few

243
00:20:09,000 --> 00:20:13,000
moments ago, or whether you are
going to do it to take advantage

244
00:20:13,000 --> 00:20:17,000
of the properties associated
with unpaired electrons like

245
00:20:17,000 --> 00:20:20,000
magnetism, for example,
you would begin any such

246
00:20:20,000 --> 00:20:25,000
approach with the octahedron as
your starting point.

247
00:20:38,000 --> 00:20:41,000
Let's go ahead and consider
some of the other examples

248
00:20:41,000 --> 00:20:44,000
provided to us by Alfred Werner.

249
00:20:49,000 --> 00:20:53,000
If instead of six,
we are adding only five NH

250
00:20:53,000 --> 00:20:56,000
three molecules for
every cobalt,

251
00:20:56,000 --> 00:20:59,000
then what happens --

252
00:21:04,000 --> 00:21:07,000
-- is indeed we do get an
octahedron, but one of the

253
00:21:07,000 --> 00:21:11,000
chlorides is not ionized.
It is bound directly to the

254
00:21:11,000 --> 00:21:14,000
metal, and it is serving as a
ligand.

255
00:21:14,000 --> 00:21:17,000
And this species,
therefore, has a two plus

256
00:21:17,000 --> 00:21:19,000
charge.
The cobalt ion is still

257
00:21:19,000 --> 00:21:23,000
considered here to be in the
plus three oxidation state.

258
00:21:23,000 --> 00:21:28,000
And this system is balanced by
two chloride ions that are

259
00:21:28,000 --> 00:21:32,000
floating around externally in
solution and that are not in the

260
00:21:32,000 --> 00:21:39,000
primary coordination sphere.
These atoms here that are part

261
00:21:39,000 --> 00:21:45,000
of ammonia molecules that are
bonded directly to the metal are

262
00:21:45,000 --> 00:21:49,000
in the inner coordination
sphere.

263
00:21:49,000 --> 00:21:55,000
That is, the inner or primary
coordination sphere.

264
00:22:08,000 --> 00:22:12,000
What do you think happens if
you take a metal salt and

265
00:22:12,000 --> 00:22:16,000
dissolve it in water?
I did that a moment ago with

266
00:22:16,000 --> 00:22:20,000
ferrous chloride.
I dissolved it in water.

267
00:22:20,000 --> 00:22:25,000
Water is a very polar solvent.
It promotes the formation of

268
00:22:25,000 --> 00:22:30,000
ions in solution because of its
great polarity.

269
00:22:30,000 --> 00:22:35,000
It is good at solvating ions,
water is, as a medium.

270
00:22:35,000 --> 00:22:39,000
If I take FeCl two
and add it to water,

271
00:22:39,000 --> 00:22:43,000
as I did a moment ago,
and it ionizes,

272
00:22:43,000 --> 00:22:47,000
what is happening to the iron?

273
00:22:54,000 --> 00:22:58,000
The iron is going to take up
water molecules into its inner

274
00:22:58,000 --> 00:23:02,000
coordination sphere.
When you dissolve FeCl two

275
00:23:02,000 --> 00:23:06,000
in solution,
which might often be written

276
00:23:06,000 --> 00:23:10,000
quite simply as FeCl
two aqueous,

277
00:23:10,000 --> 00:23:14,000
what you really have in
solution is the system in which

278
00:23:14,000 --> 00:23:19,000
six water molecules are bonded
to that iron.

279
00:23:26,000 --> 00:23:30,000
And, because I used FeCl two,
this system had a two

280
00:23:30,000 --> 00:23:33,000
plus charged balanced by two of
the chloride ions that

281
00:23:33,000 --> 00:23:38,000
dissociate from the iron and
ionize and go out into solution

282
00:23:38,000 --> 00:23:42,000
to be solvated separately from
the cation by water.

283
00:23:42,000 --> 00:23:46,000
That initial weakly-colored
solution contained iron in this

284
00:23:46,000 --> 00:23:49,000
form, hexaaquairon two.

285
00:23:49,000 --> 00:23:54,000
And I will tell you a little
bit about what made the color

286
00:23:54,000 --> 00:23:56,000
change in a moment.

287
00:24:04,000 --> 00:24:10,000
But first I would like to
discuss an issue that arises in

288
00:24:10,000 --> 00:24:15,000
the Werner system.
And this is the problem of

289
00:24:15,000 --> 00:24:17,000
isomerism.

290
00:24:24,000 --> 00:24:27,000
Werner found that you could
make different cobalt complexes

291
00:24:27,000 --> 00:24:30,000
that would have the same
chemical formula,

292
00:24:30,000 --> 00:24:33,000
but, for example,
one would be red and one would

293
00:24:33,000 --> 00:24:36,000
be green, or one would be
yellow, for example,

294
00:24:36,000 --> 00:24:39,000
even though they have the same
chemical formula.

295
00:24:39,000 --> 00:24:42,000
And that was because,
as he correctly reasoned,

296
00:24:42,000 --> 00:24:45,000
they were forming isomers.

297
00:24:49,000 --> 00:24:53,000
And this comes into play,
for example,

298
00:24:53,000 --> 00:24:58,000
when you add only four
equivalents of ammonia to

299
00:24:58,000 --> 00:25:01,000
solution.
And here is why.

300
00:25:01,000 --> 00:25:06,000
If I put the first chloride up
on top, as I have done here,

301
00:25:06,000 --> 00:25:11,000
there are two choices of where
to put the second one that are

302
00:25:11,000 --> 00:25:15,000
not the same.
I can either put a chloride

303
00:25:15,000 --> 00:25:20,000
here, such that we have a bond
angle of 90 degrees between the

304
00:25:20,000 --> 00:25:23,000
two chlorides.
And I will draw in our

305
00:25:23,000 --> 00:25:30,000
remaining ammonia molecules that
are coordinating to the cobalt.

306
00:25:30,000 --> 00:25:35,000
This is an isomer that we would
call "cis." Cis denotes a

307
00:25:35,000 --> 00:25:41,000
proximal arrangement of the two
chlorides with a 90 degree bond

308
00:25:41,000 --> 00:25:45,000
angle between them.
And then the alternative here

309
00:25:45,000 --> 00:25:51,000
would be to put the other
chloride 180 degrees away from

310
00:25:51,000 --> 00:25:55,000
the first one.
And that gives us what we call

311
00:25:55,000 --> 00:25:58,000
the trans iosomer.

312
00:26:03,000 --> 00:26:08,000
And note that both of these
would have a single plus charge.

313
00:26:08,000 --> 00:26:13,000
Trans means across.
So the two chloride ligands are

314
00:26:13,000 --> 00:26:17,000
located in a mutually trans
disposition here.

315
00:26:17,000 --> 00:26:23,000
Isomerism is very important.
I will discuss a couple other

316
00:26:23,000 --> 00:26:29,000
types of isomerism that you can
get and that Werner contributed

317
00:26:29,000 --> 00:26:34,000
to our understanding of very
greatly.

318
00:26:39,000 --> 00:26:43,000
And let me do that by
completing consideration of

319
00:26:43,000 --> 00:26:46,000
this.
You might ask yourself in the

320
00:26:46,000 --> 00:26:51,000
case where we added only three
ammonias to the solution is

321
00:26:51,000 --> 00:26:55,000
there a possibility for the
formation of isomers?

322
00:26:55,000 --> 00:26:59,000
And the answer again would be
yes, we can have two

323
00:26:59,000 --> 00:27:04,000
possibilities.
And this is for a neutral

324
00:27:04,000 --> 00:27:08,000
system that contains three
ammonias and three chloride

325
00:27:08,000 --> 00:27:11,000
ligands.
And let's say I put the first

326
00:27:11,000 --> 00:27:16,000
one here, the second one here,
the third one here.

327
00:27:16,000 --> 00:27:20,000
That is one of our possible
isomers of this neutral

328
00:27:20,000 --> 00:27:24,000
coordination complex.
And then the other possibility,

329
00:27:24,000 --> 00:27:28,000
the only other possibility is
with one there,

330
00:27:28,000 --> 00:27:33,000
one there, --
-- and then the third one here.

331
00:27:33,000 --> 00:27:36,000
And so you can try to draw
different structures.

332
00:27:36,000 --> 00:27:40,000
And you will see that these are
the only two possible structures

333
00:27:40,000 --> 00:27:44,000
that you can draw for a
combination of three ammonia

334
00:27:44,000 --> 00:27:47,000
ligands and three chloride
ligands surround a central

335
00:27:47,000 --> 00:27:50,000
cobalt three plus ion
in an octahedral

336
00:27:50,000 --> 00:27:52,000
array.

337
00:27:59,000 --> 00:28:01,000
And these have names,
too.

338
00:28:01,000 --> 00:28:05,000
This one is the so-called fac
isomer.

339
00:28:05,000 --> 00:28:10,000
And that fac is an abbreviation
of the word facial,

340
00:28:10,000 --> 00:28:17,000
because if you remember that
the octahedron is composed of a

341
00:28:17,000 --> 00:28:22,000
set of eight equilateral
triangles, then the polyhedron

342
00:28:22,000 --> 00:28:30,000
that we call the octahedron has
both vertices and faces.

343
00:28:30,000 --> 00:28:33,000
And these chlorides,
in this particular case,

344
00:28:33,000 --> 00:28:37,000
can be seen to define one of
the eight faces of the

345
00:28:37,000 --> 00:28:40,000
octahedron.
And so that is the facial

346
00:28:40,000 --> 00:28:43,000
isomer.
And then, the other type of

347
00:28:43,000 --> 00:28:47,000
isomer for this type of
structure is called mer.

348
00:28:47,000 --> 00:28:51,000
And that is an abbreviation of
the word meridional,

349
00:28:51,000 --> 00:28:56,000
which would be like the
meridians of longitude that you

350
00:28:56,000 --> 00:29:01,000
see on the globe.
They start at the top and run

351
00:29:01,000 --> 00:29:06,000
down through the equator and all
the way down to the South Pole.

352
00:29:06,000 --> 00:29:10,000
That is your meridional isomer.
These isomers,

353
00:29:10,000 --> 00:29:13,000
here, are called geometric
isomers.

354
00:29:13,000 --> 00:29:17,000
There are different types of
isomerism.

355
00:29:27,000 --> 00:29:33,000
Because the complexes that
differ only with regard to the

356
00:29:33,000 --> 00:29:40,000
spatial arrangement of the
ligands, but not with respect to

357
00:29:40,000 --> 00:29:47,000
the formula of the system,
these would be types of isomers

358
00:29:47,000 --> 00:29:53,000
known as geometric.
We have the possibility of CIS,

359
00:29:53,000 --> 00:30:00,000
TRANS, FAC, MER geometric
isomers for molecules that have

360
00:30:00,000 --> 00:30:06,000
the same formula.
And then, there is a further

361
00:30:06,000 --> 00:30:10,000
type of isomerism.
And here, again,

362
00:30:10,000 --> 00:30:10,000
the contributions of Alfred
Werner were exceedingly
important, because it was
thought that this next type of

363
00:30:11,000 --> 00:30:12,000
isomerism was restricted to
organic molecules.

364
00:30:12,000 --> 00:30:13,000
And this is stereoisomerism.

365
00:30:30,000 --> 00:30:34,000
Stereoisomerism is a little
more subtle than geometric

366
00:30:34,000 --> 00:30:37,000
isomerism.
And it is a little more subtle

367
00:30:37,000 --> 00:30:42,000
because two molecules that are
stereoisomers of each other are

368
00:30:42,000 --> 00:30:46,000
related in the same way that
your left hand and your right

369
00:30:46,000 --> 00:30:50,000
hand are related.
They are non-superimposable

370
00:30:50,000 --> 00:30:53,000
mirror images.
If you can find a way to

371
00:30:53,000 --> 00:30:57,000
separate molecules that are
chiral, then you can have a

372
00:30:57,000 --> 00:31:02,000
sample that can do interesting
things, like rotate the plane of

373
00:31:02,000 --> 00:31:07,000
polarized light.
This happens when you have

374
00:31:07,000 --> 00:31:11,000
chiral molecules.
And if a molecule is chiral,

375
00:31:11,000 --> 00:31:16,000
that is to say it is
non-superimposable on its mirror

376
00:31:16,000 --> 00:31:17,000
image.

377
00:31:34,000 --> 00:31:36,000
And in order to see whether a
molecule is or is not

378
00:31:36,000 --> 00:31:40,000
superimposable on its mirror
image, you really need to get

379
00:31:40,000 --> 00:31:43,000
good at visualizing things in
three-dimensions and at rotating

380
00:31:43,000 --> 00:31:46,000
molecules around in your mind.
You can also do it on the

381
00:31:46,000 --> 00:31:49,000
computer.
And doing it on the computer

382
00:31:49,000 --> 00:31:51,000
will help you prepare for doing
it on the exam,

383
00:31:51,000 --> 00:31:53,000
where you have to do it in your
mind.

384
00:31:53,000 --> 00:31:57,000
But if you like architecture,
and you like visualizing things

385
00:31:57,000 --> 00:32:00,000
in three-dimensions,
you should know that that is a

386
00:32:00,000 --> 00:32:04,000
lot of what we do in chemistry.
You should think about these

387
00:32:04,000 --> 00:32:09,000
molecules, these 3D structures,
in ways that allow you to test

388
00:32:09,000 --> 00:32:11,000
for a property like
stereoisomerism.

389
00:32:11,000 --> 00:32:16,000
And I mentioned that it was
thought that stereoisomerism was

390
00:32:16,000 --> 00:32:19,000
a property associated with
organic molecules.

391
00:32:19,000 --> 00:32:23,000
And organic molecules were
compounds of carbon that were

392
00:32:23,000 --> 00:32:27,000
thought to be associated very
fundamentally with life and

393
00:32:27,000 --> 00:32:31,000
living things.
And so the fact that Werner in

394
00:32:31,000 --> 00:32:35,000
one of his most amazing
accomplishments was ultimately

395
00:32:35,000 --> 00:32:39,000
able to synthesize a
coordination complex that

396
00:32:39,000 --> 00:32:44,000
contained no carbon at all but
exhibited stereoisomerism just

397
00:32:44,000 --> 00:32:49,000
shattered that theory and really
helped to bring science onto a

398
00:32:49,000 --> 00:32:53,000
much more firm footing.
And that parallelism between

399
00:32:53,000 --> 00:32:58,000
organic and inorganic chemistry,
I think, has stemmed from this

400
00:32:58,000 --> 00:33:02,000
aspect of its history.
And so let's look at an example

401
00:33:02,000 --> 00:33:06,000
of a molecule that is chiral --

402
00:33:10,000 --> 00:33:12,000
-- that could be made from
cobalt.

403
00:33:12,000 --> 00:33:16,000
And if you imagine carrying out
a reaction like we were talking

404
00:33:16,000 --> 00:33:19,000
about up above but not even
giving it enough ammonia to

405
00:33:19,000 --> 00:33:23,000
displace all the water molecules
then you could have an

406
00:33:23,000 --> 00:33:25,000
intermediate like this.

407
00:33:40,000 --> 00:33:44,000
And in this type of species
what I've got are two water

408
00:33:44,000 --> 00:33:47,000
molecules, two ammonia
molecules, two chlorides.

409
00:33:47,000 --> 00:33:52,000
And so if this is cobalt three,
we would have a single positive

410
00:33:52,000 --> 00:33:56,000
charge on that ion.
And what I can represent here

411
00:33:56,000 --> 00:34:00,000
by a dashed line would be a
mirror plane.

412
00:34:05,000 --> 00:34:08,000
That is our mirror.
And we are going to reflect

413
00:34:08,000 --> 00:34:12,000
this molecule through that
mirror plane to see what its

414
00:34:12,000 --> 00:34:16,000
mirror image would look like.
And then, if you can rotate it

415
00:34:16,000 --> 00:34:19,000
around in your mind,
we can determine whether it is

416
00:34:19,000 --> 00:34:22,000
or is not superimposable on that
mirror image.

417
00:34:22,000 --> 00:34:26,000
I am generating the mirror
image by reflecting this water

418
00:34:26,000 --> 00:34:31,000
to this position.
This ammonia back here reflects

419
00:34:31,000 --> 00:34:35,000
over to here.
The top ammonia reflects still

420
00:34:35,000 --> 00:34:39,000
into the top position.
This water behind the board

421
00:34:39,000 --> 00:34:42,000
reflects behind the board.
And over here,

422
00:34:42,000 --> 00:34:47,000
this chloride coming out in
front of the board reflects over

423
00:34:47,000 --> 00:34:50,000
to here.
And we have one more chloride,

424
00:34:50,000 --> 00:34:54,000
down on the bottom.
That molecule is now our mirror

425
00:34:54,000 --> 00:34:57,000
image.
And let's go ahead and rotate

426
00:34:57,000 --> 00:35:02,000
it, like this.
Because it is a little hard,

427
00:35:02,000 --> 00:35:06,000
I am going to highlight the
position of the two ammonia

428
00:35:06,000 --> 00:35:09,000
ligands.
And to see if this mirror image

429
00:35:09,000 --> 00:35:13,000
is superimposable on the
structure we started with,

430
00:35:13,000 --> 00:35:17,000
I am going to rotate this
around so that we can put the

431
00:35:17,000 --> 00:35:21,000
two ammonia ligands coincident
with the two shown here

432
00:35:21,000 --> 00:35:26,000
underlined in green on the left.
We are going to do a rotation.

433
00:35:26,000 --> 00:35:31,000
And I need to rotate this.
I am going to rotate here,

434
00:35:31,000 --> 00:35:34,000
around the cobalt-chlorine bond
access.

435
00:35:34,000 --> 00:35:39,000
And I am actually going to go
in the negative direction to

436
00:35:39,000 --> 00:35:42,000
generate the following
structure.

437
00:35:42,000 --> 00:35:48,000
This puts this ammonia up top,
and it will put this one down

438
00:35:48,000 --> 00:35:51,000
below.
We have NH three and NH three

439
00:35:51,000 --> 00:35:54,000
here.
Let me underline them.

440
00:35:54,000 --> 00:35:59,000
So those are in positions,
coincident.

441
00:35:59,000 --> 00:36:04,000
And this rotation also will
carry that chloride from the

442
00:36:04,000 --> 00:36:07,000
bottom up here,
into what I may call an

443
00:36:07,000 --> 00:36:12,000
equatorial position.
And it puts a water molecule

444
00:36:12,000 --> 00:36:15,000
down.
And that rotation about this

445
00:36:15,000 --> 00:36:21,000
cobalt chlorine bond left the
cobalt and chlorine on that bond

446
00:36:21,000 --> 00:36:24,000
axis unrotated.
And then in the back,

447
00:36:24,000 --> 00:36:29,000
we have this OH two molecule.

448
00:36:29,000 --> 00:36:32,000
And what you can see is,
if you now bring this over,

449
00:36:32,000 --> 00:36:35,000
what we have,
in fact, is a situation where

450
00:36:35,000 --> 00:36:39,000
we are not currently
superimposable with that choice.

451
00:36:39,000 --> 00:36:43,000
I generated the mirror image.
I have rotated it by 90 degrees

452
00:36:43,000 --> 00:36:47,000
around the cobalt-chlorine bond
axis to bring these two ammonias

453
00:36:47,000 --> 00:36:51,000
coincident with these two.
So you can see that,

454
00:36:51,000 --> 00:36:54,000
whereas we have a water
molecule on the bottom here,

455
00:36:54,000 --> 00:37:00,000
we have a chloride over here.
So that is not superimposable.

456
00:37:00,000 --> 00:37:06,000
But we can do one more rotation
to check the other possibility,

457
00:37:06,000 --> 00:37:12,000
and that rotation will be a
rotation by 180 degrees around

458
00:37:12,000 --> 00:37:15,000
an axis, here,
that bisects the

459
00:37:15,000 --> 00:37:20,000
nitrogen-cobalt-nitrogen bond
angle of 90 degrees.

460
00:37:20,000 --> 00:37:25,000
We will rotate 180 degrees
around that axis,

461
00:37:25,000 --> 00:37:29,000
and that will bring our
ammonias, again,

462
00:37:29,000 --> 00:37:35,000
into a position so as to be
coincident.

463
00:37:38,000 --> 00:37:43,000
And rotating around that axis
brings a water around front here

464
00:37:43,000 --> 00:37:48,000
and puts a chloride in back,
rotating around there,

465
00:37:48,000 --> 00:37:53,000
and it swaps this chloride with
that water molecule.

466
00:37:53,000 --> 00:38:00,000
So we now have chloride down
and OH two over here.

467
00:38:00,000 --> 00:38:03,000
And so if we take this,
we identify our ammonia

468
00:38:03,000 --> 00:38:07,000
positions by green underlining,
they're coincident,

469
00:38:07,000 --> 00:38:09,000
here.
And now where we have a

470
00:38:09,000 --> 00:38:13,000
chloride coming out,
we have a water coming out,

471
00:38:13,000 --> 00:38:17,000
so our mirror image is not
superimposable on the structure

472
00:38:17,000 --> 00:38:21,000
that we generated it from
through the process of

473
00:38:21,000 --> 00:38:24,000
reflection through that mirror
plane.

474
00:38:24,000 --> 00:38:28,000
And so, what we can say is that
this molecule and this one

475
00:38:28,000 --> 00:38:33,000
constitute a pair of
stereoisomers.

476
00:38:33,000 --> 00:38:36,000
And because this condition was
satisfied that the mirror image

477
00:38:36,000 --> 00:38:40,000
was not superimposable on the
structure we generated it from,

478
00:38:40,000 --> 00:38:44,000
the molecule is chiral.
And you will see that I have

479
00:38:44,000 --> 00:38:47,000
chosen a molecule that contains
no carbon, and yet it is chiral

480
00:38:47,000 --> 00:38:51,000
and it has stereoisomers.
And that was thought impossible

481
00:38:51,000 --> 00:38:54,000
prior to the time of Werner.

482
00:39:00,000 --> 00:39:04,000
Let me show you another example
of a molecule that is chiral.

483
00:39:04,000 --> 00:39:07,000
And I am going to use this
example, also,

484
00:39:07,000 --> 00:39:13,000
to illustrate another important
feature that ligands can have.

485
00:39:18,000 --> 00:39:23,000
And that is that they can have
more than one atom that can bond

486
00:39:23,000 --> 00:39:26,000
to the metal at the same time.

487
00:39:29,000 --> 00:39:33,000
I am drawing a cobalt ion three
plus complex that

488
00:39:33,000 --> 00:39:37,000
has six nitrogens directly
bonded to the cobalt.

489
00:39:37,000 --> 00:39:40,000
But now, look what I am going
to do.

490
00:39:40,000 --> 00:39:45,000
I am going to put some organic
material in here and link these

491
00:39:45,000 --> 00:39:49,000
nitrogens by a CH two CH two
unit,

492
00:39:49,000 --> 00:39:52,000
a CH2-CH2 chain here.
So it is CH2-CH2.

493
00:39:52,000 --> 00:39:57,000
These carbons that I am
representing as vertices here

494
00:39:57,000 --> 00:40:03,000
each have two additional
hydrogens that I am not showing.

495
00:40:03,000 --> 00:40:06,000
And that is typical organic
shorthand.

496
00:40:06,000 --> 00:40:12,000
And I am going to suggest that
this molecule would be generated

497
00:40:12,000 --> 00:40:16,000
by adding three of these ligands
to the metal center.

498
00:40:16,000 --> 00:40:21,000
And for each nitrogen,
if you consider it as being

499
00:40:21,000 --> 00:40:25,000
derived from ammonia,
one of the hydrogens of the

500
00:40:25,000 --> 00:40:31,000
ammonia is replaced with a
nitrogen-carbon bond.

501
00:40:31,000 --> 00:40:37,000
And we have used this organic
moiety here to tether two

502
00:40:37,000 --> 00:40:42,000
nitrogens together.
This is a very popular and

503
00:40:42,000 --> 00:40:47,000
ancient ligand in coordination
chemistry.

504
00:40:47,000 --> 00:40:54,000
And, by drawing in simplistic
form the two lone pairs on the

505
00:40:54,000 --> 00:40:59,000
two nitrogens,
you can see that this set of

506
00:40:59,000 --> 00:41:07,000
four atoms is able to organize
itself so as to simultaneously

507
00:41:07,000 --> 00:41:14,000
point two lone pairs at the same
metal center.

508
00:41:14,000 --> 00:41:17,000
That is permitted by this
bridge.

509
00:41:17,000 --> 00:41:22,000
This particular ligand is
called ethylenediamine.

510
00:41:27,000 --> 00:41:32,000
And it is called en for short,
ethylenediamine.

511
00:41:32,000 --> 00:41:37,000
And it is an example of a
bidentate ligand.

512
00:41:44,000 --> 00:41:53,000
And that means that it has two
teeth with which to bite down on

513
00:41:53,000 --> 00:42:00,000
the metal center.
It is a double Lewis base.

514
00:42:00,000 --> 00:42:05,000
And when it binds to the metal
center, we call that the process

515
00:42:05,000 --> 00:42:08,000
of chelation.
When a bidentate or a

516
00:42:08,000 --> 00:42:11,000
multidentate,
which would be maybe a

517
00:42:11,000 --> 00:42:16,000
tridentate or a tetradentate
ligand, binds to a metal through

518
00:42:16,000 --> 00:42:20,000
multiple points,
we call that a ligand chelate.

519
00:42:20,000 --> 00:42:25,000
And we call the process one of
chelation that forms these ring

520
00:42:25,000 --> 00:42:30,000
structures with the metal as
part of the ring produced

521
00:42:30,000 --> 00:42:37,000
through multipoint binding of
the ligand to the metal center.

522
00:42:37,000 --> 00:42:41,000
And you are going to see that
it is possible to have all kinds

523
00:42:41,000 --> 00:42:45,000
of different architectures for
ligands in proteins or in

524
00:42:45,000 --> 00:42:48,000
synthetic systems.
And the reason that I carried

525
00:42:48,000 --> 00:42:53,000
out over here earlier is one in
which I added three equivalents

526
00:42:53,000 --> 00:42:57,000
of a bidentate ligand to this
solution of iron two plus.

527
00:42:57,000 --> 00:43:01,000
And, when that occurred,

528
00:43:01,000 --> 00:43:07,000
this bidentate ligand displaced
the water molecules from the

529
00:43:07,000 --> 00:43:11,000
inner coordination sphere of the
metal.

530
00:43:11,000 --> 00:43:16,000
And the bidentate ligand that I
used was this one.

531
00:43:16,000 --> 00:43:22,000
This is a very common chelating
ligand, a planar aromatic

532
00:43:22,000 --> 00:43:25,000
ligand.
And you can see that,

533
00:43:25,000 --> 00:43:31,000
like ethylenediamine,
its architecture promotes the

534
00:43:31,000 --> 00:43:39,000
pointing of a pair of electrons
toward the same point in space.

535
00:43:39,000 --> 00:43:43,000
So that this ligand can bind
itself to a metal center through

536
00:43:43,000 --> 00:43:46,000
two nitrogen lone pairs
simultaneously.

537
00:43:46,000 --> 00:43:51,000
And it is the interaction of
the d-electrons on the iron

538
00:43:51,000 --> 00:43:55,000
center with the unsaturated pi
system of this organic ligand

539
00:43:55,000 --> 00:44:00,000
that produces the red color in
ways that we are going to

540
00:44:00,000 --> 00:44:05,000
explore in more detail in one of
our next lectures.

541
00:44:05,000 --> 00:44:08,000
But, before we do that,
we are going to need to

542
00:44:08,000 --> 00:44:10,000
understand something about
d-orbitals.

543
00:44:10,000 --> 00:44:14,000
And, as you have learned,
when you are forming molecular

544
00:44:14,000 --> 00:44:18,000
orbitals in systems that consist
of either s or p orbitals,

545
00:44:18,000 --> 00:44:22,000
you needed to know something
about the nodal properties of

546
00:44:22,000 --> 00:44:26,000
those atomic orbitals in order
to build proper molecular

547
00:44:26,000 --> 00:44:30,000
orbitals.
And that will certainly be the

548
00:44:30,000 --> 00:44:35,000
case for these more interesting
elements that have d orbitals.

549
00:44:35,000 --> 00:44:40,000
Not just s and p valance
orbitals, but also a set of

550
00:44:40,000 --> 00:44:43,000
d-orbitals.
And I call those the 3d

551
00:44:43,000 --> 00:44:48,000
elements because their principle
quantum number for those

552
00:44:48,000 --> 00:44:52,000
elements is three.
And what we need to now know is

553
00:44:52,000 --> 00:44:57,000
what do these orbitals have,
as far as nodal properties,

554
00:44:57,000 --> 00:45:02,000
depending on the other quantum
numbers?

555
00:45:02,000 --> 00:45:06,000
And I will draw a set of
coordinate axes,

556
00:45:06,000 --> 00:45:10,000
here, on which to map these
orbitals.

557
00:45:23,000 --> 00:45:26,000
It should be pretty
straightforward for you to keep

558
00:45:26,000 --> 00:45:30,000
straight the nodal properties of
the d orbitals of which there is

559
00:45:30,000 --> 00:45:34,000
a set of five.
We had one s orbital for a

560
00:45:34,000 --> 00:45:39,000
given valance shell,
and we had a set of three p

561
00:45:39,000 --> 00:45:45,000
orbitals, and there is a set of
five d orbitals for the d-block

562
00:45:45,000 --> 00:45:49,000
elements.
And they can have different

563
00:45:49,000 --> 00:45:53,000
values for the quantum number m.
One is zero.

564
00:45:53,000 --> 00:45:56,000
One is plus one.
One is plus two.

565
00:45:56,000 --> 00:46:03,000
And m can be minus one,
and m can equal minus two.

566
00:46:03,000 --> 00:46:08,000
And this quantum number
determines the angular nodal

567
00:46:08,000 --> 00:46:13,000
properties of the d orbital in
question.

568
00:46:13,000 --> 00:46:17,000
Here, let's draw a fairly
simple one.

569
00:46:17,000 --> 00:46:21,000
Let's say that we have x,
y, and z.

570
00:46:21,000 --> 00:46:29,000
Then what we might have is a d
orbital that looks like this.

571
00:46:34,000 --> 00:46:37,000
d orbitals often have four
lobes.

572
00:46:37,000 --> 00:46:45,000
In fact, you will see that we
represent four of the d orbitals

573
00:46:45,000 --> 00:46:51,000
this way and not the fifth.
And let me use this pink to

574
00:46:51,000 --> 00:46:58,000
represent the negative phase.
And so this orbital here is

575
00:46:58,000 --> 00:47:02,000
(d)xz.
And that means that it has

576
00:47:02,000 --> 00:47:05,000
nodes.
You have two planes that are

577
00:47:05,000 --> 00:47:10,000
nodes for a (d)xz orbital.
And one of these is the

578
00:47:10,000 --> 00:47:13,000
x,y-plane.
And then the other one is the

579
00:47:13,000 --> 00:47:17,000
y,z-plane.
Those are planes when you go

580
00:47:17,000 --> 00:47:22,000
from one side through one of
those planes to the other side.

581
00:47:22,000 --> 00:47:28,000
The wave function changes sign.
And, just like each p orbital

582
00:47:28,000 --> 00:47:34,000
has a single nodal plane,
each d orbital has two.

583
00:47:34,000 --> 00:47:38,000
And this is (d)xz.
And we can also have one that

584
00:47:38,000 --> 00:47:42,000
we call d x squared minus y
squared.

585
00:47:42,000 --> 00:47:47,000
And that one lies right along
the coordinate axes like this,

586
00:47:47,000 --> 00:47:52,000
with the four lobes being
skewered by the x-axis and the

587
00:47:52,000 --> 00:47:55,000
y-axis.
And we have negative phase

588
00:47:55,000 --> 00:48:00,000
located along y for the d x
squared minus y squared.

589
00:48:00,000 --> 00:48:04,000
And you can see that the nodal

590
00:48:04,000 --> 00:48:10,000
surfaces here both contain z.
The nodes contain the z-axis

591
00:48:10,000 --> 00:48:13,000
and bisect the x and y
coordinate axes.

592
00:48:13,000 --> 00:48:19,000
There is one plane up here that
contains the z-axis and one over

593
00:48:19,000 --> 00:48:23,000
there located at 90 degrees to
the first one.

594
00:48:23,000 --> 00:48:29,000
Those are the nodal planes for
d x squared minus y squared.

595
00:48:29,000 --> 00:48:34,000
In addition to that (d)xz

596
00:48:34,000 --> 00:48:40,000
orbital, I have a (d)yz orbital,
which is located with its lobes

597
00:48:40,000 --> 00:48:44,000
lying between the y and z
coordinate axes.

598
00:48:44,000 --> 00:48:49,000
And it will have phasing as
indicated here in pink.

599
00:48:49,000 --> 00:48:53,000
That is (d)yz.
And it looks exactly like

600
00:48:53,000 --> 00:48:56,000
(d)xz.
And it is just rotated by 90

601
00:48:56,000 --> 00:49:02,000
degrees around the z-axis
relative to (d)xz.

602
00:49:02,000 --> 00:49:06,000
And then, finally,
we have one that looks just

603
00:49:06,000 --> 00:49:11,000
like d x squared minus
y squared.

604
00:49:11,000 --> 00:49:15,000
And this one is (d)xy.
And, like d x squared minus y

605
00:49:15,000 --> 00:49:20,000
squared, the (d)xy orbital lies
in the x,y-plane.

606
00:49:20,000 --> 00:49:26,000
And its lobes point between the
axes, as shown here with that

607
00:49:26,000 --> 00:49:30,000
phasing.
And then, finally --

608
00:49:30,000 --> 00:49:33,000
And we will return to this
point next week.

609
00:49:33,000 --> 00:49:38,000
Our m equals zero orbital is
our d z squared **d(z^2)**.

610
00:49:38,000 --> 00:49:44,000
And d z squared lies along and
is skewered by the z-axis.

611
00:49:44,000 --> 00:49:49,000
It looks like a p orbital,
except the sign is the same on

612
00:49:49,000 --> 00:49:53,000
top and on bottom.
And then it has this beautiful

613
00:49:53,000 --> 00:49:57,000
torus here that is in the
x,y-plane like that,

614
00:49:57,000 --> 00:50:03,000
so that its nodal surfaces are
actually conical rather than

615
00:50:03,070 --> 00:50:07,000
planes.
That is our set of five d

616
00:50:07,000 --> 00:50:12,000
orbitals with which we are going
to do a lot more to understand

617
00:50:12,000 --> 00:50:16,000
the chemistry and coordination
complexes.

618
00:50:16,000 --> 00:50:21,000
Have a great break,
and please don't forget to read

619
00:50:21,205 --> 00:50:24,000
about Alfred Werner.