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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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00:09:35 --> 00:09:40
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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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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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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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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12. --> 00:14:21


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

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How do ligands coordinate two
metals?

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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

216
00:16:50 --> 00:16:54
be the corresponding
electrophile.

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I can draw that to represent a
lone pair of electrons on the

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00:17:00 --> 00:17:04
nitrogen.
Now that we have studied

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

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call this the highest occupied
molecular orbital of the NH

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three molecule.
And it is the one responsible

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for the basicity of the ammonia
molecule and the one responsible

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for its ability to serve as a
ligand in coordination

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complexes, like these.
And you might also suspect that

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

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molecular orbital from hydrogen
1s linear combinations.

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I will just draw that in to
make it a little bit more

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accurate.
And so, you can think of this

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as a big fat lone pair that will
coordinate to Lewis acids.

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And the metal ion is a Lewis
acid.

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But it is a very interesting
Lewis acid because,

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

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orbital, this metal seems to be
able to act as a Lewis acid six

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

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

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crystallize molecules of this
sort and use X-ray diffraction

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

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

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these nitrogens are located at
the vertices of a nice,

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regular octahedron.
So, in the case of our first

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one, we can draw it out this
way.

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This first one is what would
result if, to that aqueous

244
00:19:03 --> 00:19:08
solution of cobalt trichloride,
we were to add six equivalents

245
00:19:08 --> 00:19:11
of ammonia.
This is Werner's first system.

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

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That lone pair that comes from
the highest occupied molecular

248
00:19:19 --> 00:19:24
orbital of ammonia is directed
right at the metal from each of

249
00:19:24 --> 00:19:29
the six ammonia ligands.
And this system does have a

250
00:19:29 --> 00:19:34
three plus charge that is
balanced by three chloride ions

251
00:19:34 --> 00:19:38
in solution.
This locating of six nitrogens

252
00:19:38 --> 00:19:42
in an array in space that
approximates a regular

253
00:19:42 --> 00:19:48
octahedron is what makes the
octahedron such a central aspect

254
00:19:48 --> 00:19:52
of the theory of transition
element chemistry.

255
00:19:52 --> 00:19:58
And, if you are going to design
molecules that do include these

256
00:19:58 --> 00:20:02
transition metal ions,
--

257
00:20:02 --> 00:20:05
-- whether you are going to do
it for their color,

258
00:20:05 --> 00:20:09
like the red color there of the
iron complex that we made a few

259
00:20:09 --> 00:20:13
moments ago, or whether you are
going to do it to take advantage

260
00:20:13 --> 00:20:17
of the properties associated
with unpaired electrons like

261
00:20:17 --> 00:20:20
magnetism, for example,
you would begin any such

262
00:20:20 --> 00:20:25
approach with the octahedron as
your starting point.

263
00:20:25 --> 00:20:38


264
00:20:38 --> 00:20:41
Let's go ahead and consider
some of the other examples

265
00:20:41 --> 00:20:44
provided to us by Alfred Werner.

266
00:20:44 --> 00:20:49


267
00:20:49 --> 00:20:53
If instead of six,
we are adding only five NH

268
00:20:53 --> 00:20:56
three molecules for
every cobalt,

269
00:20:56 --> 00:20:59
then what happens --

270
00:20:59 --> 00:21:04


271
00:21:04 --> 00:21:07
-- is indeed we do get an
octahedron, but one of the

272
00:21:07 --> 00:21:11
chlorides is not ionized.
It is bound directly to the

273
00:21:11 --> 00:21:14
metal, and it is serving as a
ligand.

274
00:21:14 --> 00:21:17
And this species,
therefore, has a two plus

275
00:21:17 --> 00:21:19
charge.
The cobalt ion is still

276
00:21:19 --> 00:21:23
considered here to be in the
plus three oxidation state.

277
00:21:23 --> 00:21:28
And this system is balanced by
two chloride ions that are

278
00:21:28 --> 00:21:32
floating around externally in
solution and that are not in the

279
00:21:32 --> 00:21:39
primary coordination sphere.
These atoms here that are part

280
00:21:39 --> 00:21:45
of ammonia molecules that are
bonded directly to the metal are

281
00:21:45 --> 00:21:49
in the inner coordination
sphere.

282
00:21:49 --> 00:21:55
That is, the inner or primary
coordination sphere.

283
00:21:55 --> 00:22:08


284
00:22:08 --> 00:22:12
What do you think happens if
you take a metal salt and

285
00:22:12 --> 00:22:16
dissolve it in water?
I did that a moment ago with

286
00:22:16 --> 00:22:20
ferrous chloride.
I dissolved it in water.

287
00:22:20 --> 00:22:25
Water is a very polar solvent.
It promotes the formation of

288
00:22:25 --> 00:22:30
ions in solution because of its
great polarity.

289
00:22:30 --> 00:22:35
It is good at solvating ions,
water is, as a medium.

290
00:22:35 --> 00:22:39
If I take FeCl two
and add it to water,

291
00:22:39 --> 00:22:43
as I did a moment ago,
and it ionizes,

292
00:22:43 --> 00:22:47
what is happening to the iron?

293
00:22:47 --> 00:22:54


294
00:22:54 --> 00:22:58
The iron is going to take up
water molecules into its inner

295
00:22:58 --> 00:23:02
coordination sphere.
When you dissolve FeCl two

296
00:23:02 --> 00:23:06
in solution,
which might often be written

297
00:23:06 --> 00:23:10
quite simply as FeCl
two aqueous,

298
00:23:10 --> 00:23:14
what you really have in
solution is the system in which

299
00:23:14 --> 00:23:19
six water molecules are bonded
to that iron.

300
00:23:19 --> 00:23:26


301
00:23:26 --> 00:23:30
And, because I used FeCl two,
this system had a two

302
00:23:30 --> 00:23:33
plus charged balanced by two of
the chloride ions that

303
00:23:33 --> 00:23:38
dissociate from the iron and
ionize and go out into solution

304
00:23:38 --> 00:23:42
to be solvated separately from
the cation by water.

305
00:23:42 --> 00:23:46
That initial weakly-colored
solution contained iron in this

306
00:23:46 --> 00:23:49
form, hexaaquairon two.

307
00:23:49 --> 00:23:54
And I will tell you a little
bit about what made the color

308
00:23:54 --> 00:23:56
change in a moment.

309
00:23:56 --> 00:24:04


310
00:24:04 --> 00:24:10
But first I would like to
discuss an issue that arises in

311
00:24:10 --> 00:24:15
the Werner system.
And this is the problem of

312
00:24:15 --> 00:24:17
isomerism.

313
00:24:17 --> 00:24:24


314
00:24:24 --> 00:24:27
Werner found that you could
make different cobalt complexes

315
00:24:27 --> 00:24:30
that would have the same
chemical formula,

316
00:24:30 --> 00:24:33
but, for example,
one would be red and one would

317
00:24:33 --> 00:24:36
be green, or one would be
yellow, for example,

318
00:24:36 --> 00:24:39
even though they have the same
chemical formula.

319
00:24:39 --> 00:24:42
And that was because,
as he correctly reasoned,

320
00:24:42 --> 00:24:45
they were forming isomers.

321
00:24:45 --> 00:24:49


322
00:24:49 --> 00:24:53
And this comes into play,
for example,

323
00:24:53 --> 00:24:58
when you add only four
equivalents of ammonia to

324
00:24:58 --> 00:25:01
solution.
And here is why.

325
00:25:01 --> 00:25:06
If I put the first chloride up
on top, as I have done here,

326
00:25:06 --> 00:25:11
there are two choices of where
to put the second one that are

327
00:25:11 --> 00:25:15
not the same.
I can either put a chloride

328
00:25:15 --> 00:25:20
here, such that we have a bond
angle of 90 degrees between the

329
00:25:20 --> 00:25:23
two chlorides.
And I will draw in our

330
00:25:23 --> 00:25:30
remaining ammonia molecules that
are coordinating to the cobalt.

331
00:25:30 --> 00:25:35
This is an isomer that we would
call "cis." Cis denotes a

332
00:25:35 --> 00:25:41
proximal arrangement of the two
chlorides with a 90 degree bond

333
00:25:41 --> 00:25:45
angle between them.
And then the alternative here

334
00:25:45 --> 00:25:51
would be to put the other
chloride 180 degrees away from

335
00:25:51 --> 00:25:55
the first one.
And that gives us what we call

336
00:25:55 --> 00:25:58
the trans iosomer.

337
00:25:58 --> 00:26:03


338
00:26:03 --> 00:26:08
And note that both of these
would have a single plus charge.

339
00:26:08 --> 00:26:13
Trans means across.
So the two chloride ligands are

340
00:26:13 --> 00:26:17
located in a mutually trans
disposition here.

341
00:26:17 --> 00:26:23
Isomerism is very important.
I will discuss a couple other

342
00:26:23 --> 00:26:29
types of isomerism that you can
get and that Werner contributed

343
00:26:29 --> 00:26:34
to our understanding of very
greatly.

344
00:26:34 --> 00:26:39


345
00:26:39 --> 00:26:43
And let me do that by
completing consideration of

346
00:26:43 --> 00:26:46
this.
You might ask yourself in the

347
00:26:46 --> 00:26:51
case where we added only three
ammonias to the solution is

348
00:26:51 --> 00:26:55
there a possibility for the
formation of isomers?

349
00:26:55 --> 00:26:59
And the answer again would be
yes, we can have two

350
00:26:59 --> 00:27:04
possibilities.
And this is for a neutral

351
00:27:04 --> 00:27:08
system that contains three
ammonias and three chloride

352
00:27:08 --> 00:27:11
ligands.
And let's say I put the first

353
00:27:11 --> 00:27:16
one here, the second one here,
the third one here.

354
00:27:16 --> 00:27:20
That is one of our possible
isomers of this neutral

355
00:27:20 --> 00:27:24
coordination complex.
And then the other possibility,

356
00:27:24 --> 00:27:28
the only other possibility is
with one there,

357
00:27:28 --> 00:27:33
one there, --
-- and then the third one here.

358
00:27:33 --> 00:27:36
And so you can try to draw
different structures.

359
00:27:36 --> 00:27:40
And you will see that these are
the only two possible structures

360
00:27:40 --> 00:27:44
that you can draw for a
combination of three ammonia

361
00:27:44 --> 00:27:47
ligands and three chloride
ligands surround a central

362
00:27:47 --> 00:27:50
cobalt three plus ion
in an octahedral

363
00:27:50 --> 00:27:52
array.

364
00:27:52 --> 00:27:59


365
00:27:59 --> 00:28:01
And these have names,
too.

366
00:28:01 --> 00:28:05
This one is the so-called fac
isomer.

367
00:28:05 --> 00:28:10
And that fac is an abbreviation
of the word facial,

368
00:28:10 --> 00:28:17
because if you remember that
the octahedron is composed of a

369
00:28:17 --> 00:28:22
set of eight equilateral
triangles, then the polyhedron

370
00:28:22 --> 00:28:30
that we call the octahedron has
both vertices and faces.

371
00:28:30 --> 00:28:33
And these chlorides,
in this particular case,

372
00:28:33 --> 00:28:37
can be seen to define one of
the eight faces of the

373
00:28:37 --> 00:28:40
octahedron.
And so that is the facial

374
00:28:40 --> 00:28:43
isomer.
And then, the other type of

375
00:28:43 --> 00:28:47
isomer for this type of
structure is called mer.

376
00:28:47 --> 00:28:51
And that is an abbreviation of
the word meridional,

377
00:28:51 --> 00:28:56
which would be like the
meridians of longitude that you

378
00:28:56 --> 00:29:01
see on the globe.
They start at the top and run

379
00:29:01 --> 00:29:06
down through the equator and all
the way down to the South Pole.

380
00:29:06 --> 00:29:10
That is your meridional isomer.
These isomers,

381
00:29:10 --> 00:29:13
here, are called geometric
isomers.

382
00:29:13 --> 00:29:17
There are different types of
isomerism.

383
00:29:17 --> 00:29:27


384
00:29:27 --> 00:29:33
Because the complexes that
differ only with regard to the

385
00:29:33 --> 00:29:40
spatial arrangement of the
ligands, but not with respect to

386
00:29:40 --> 00:29:47
the formula of the system,
these would be types of isomers

387
00:29:47 --> 00:29:53
known as geometric.
We have the possibility of CIS,

388
00:29:53 --> 00:30:00
TRANS, FAC, MER geometric
isomers for molecules that have

389
00:30:00 --> 00:30:06
the same formula.
And then, there is a further

390
00:30:06 --> 00:30:10
type of isomerism.
And here, again,

391
00:30:10 --> 00:30:10
the contributions of Alfred
Werner were exceedingly

392
00:30:10 --> 00:30:11
important, because it was
thought that this next type of

393
00:30:11 --> 00:30:12
isomerism was restricted to
organic molecules.

394
00:30:12 --> 00:30:13
And this is stereoisomerism.

395
00:30:13 --> 00:30:30


396
00:30:30 --> 00:30:34
Stereoisomerism is a little
more subtle than geometric

397
00:30:34 --> 00:30:37
isomerism.
And it is a little more subtle

398
00:30:37 --> 00:30:42
because two molecules that are
stereoisomers of each other are

399
00:30:42 --> 00:30:46
related in the same way that
your left hand and your right

400
00:30:46 --> 00:30:50
hand are related.
They are non-superimposable

401
00:30:50 --> 00:30:53
mirror images.
If you can find a way to

402
00:30:53 --> 00:30:57
separate molecules that are
chiral, then you can have a

403
00:30:57 --> 00:31:02
sample that can do interesting
things, like rotate the plane of

404
00:31:02 --> 00:31:07
polarized light.
This happens when you have

405
00:31:07 --> 00:31:11
chiral molecules.
And if a molecule is chiral,

406
00:31:11 --> 00:31:16
that is to say it is
non-superimposable on its mirror

407
00:31:16 --> 00:31:17
image.

408
00:31:17 --> 00:31:34


409
00:31:34 --> 00:31:36
And in order to see whether a
molecule is or is not

410
00:31:36 --> 00:31:40
superimposable on its mirror
image, you really need to get

411
00:31:40 --> 00:31:43
good at visualizing things in
three-dimensions and at rotating

412
00:31:43 --> 00:31:46
molecules around in your mind.
You can also do it on the

413
00:31:46 --> 00:31:49
computer.
And doing it on the computer

414
00:31:49 --> 00:31:51
will help you prepare for doing
it on the exam,

415
00:31:51 --> 00:31:53
where you have to do it in your
mind.

416
00:31:53 --> 00:31:57
But if you like architecture,
and you like visualizing things

417
00:31:57 --> 00:32:00
in three-dimensions,
you should know that that is a

418
00:32:00 --> 00:32:04
lot of what we do in chemistry.
You should think about these

419
00:32:04 --> 00:32:09
molecules, these 3D structures,
in ways that allow you to test

420
00:32:09 --> 00:32:11
for a property like
stereoisomerism.

421
00:32:11 --> 00:32:16
And I mentioned that it was
thought that stereoisomerism was

422
00:32:16 --> 00:32:19
a property associated with
organic molecules.

423
00:32:19 --> 00:32:23
And organic molecules were
compounds of carbon that were

424
00:32:23 --> 00:32:27
thought to be associated very
fundamentally with life and

425
00:32:27 --> 00:32:31
living things.
And so the fact that Werner in

426
00:32:31 --> 00:32:35
one of his most amazing
accomplishments was ultimately

427
00:32:35 --> 00:32:39
able to synthesize a
coordination complex that

428
00:32:39 --> 00:32:44
contained no carbon at all but
exhibited stereoisomerism just

429
00:32:44 --> 00:32:49
shattered that theory and really
helped to bring science onto a

430
00:32:49 --> 00:32:53
much more firm footing.
And that parallelism between

431
00:32:53 --> 00:32:58
organic and inorganic chemistry,
I think, has stemmed from this

432
00:32:58 --> 00:33:02
aspect of its history.
And so let's look at an example

433
00:33:02 --> 00:33:06
of a molecule that is chiral --

434
00:33:06 --> 00:33:10


435
00:33:10 --> 00:33:12
-- that could be made from
cobalt.

436
00:33:12 --> 00:33:16
And if you imagine carrying out
a reaction like we were talking

437
00:33:16 --> 00:33:19
about up above but not even
giving it enough ammonia to

438
00:33:19 --> 00:33:23
displace all the water molecules
then you could have an

439
00:33:23 --> 00:33:25
intermediate like this.

440
00:33:25 --> 00:33:40


441
00:33:40 --> 00:33:44
And in this type of species
what I've got are two water

442
00:33:44 --> 00:33:47
molecules, two ammonia
molecules, two chlorides.

443
00:33:47 --> 00:33:52
And so if this is cobalt three,
we would have a single positive

444
00:33:52 --> 00:33:56
charge on that ion.
And what I can represent here

445
00:33:56 --> 00:34:00
by a dashed line would be a
mirror plane.

446
00:34:00 --> 00:34:05


447
00:34:05 --> 00:34:08
That is our mirror.
And we are going to reflect

448
00:34:08 --> 00:34:12
this molecule through that
mirror plane to see what its

449
00:34:12 --> 00:34:16
mirror image would look like.
And then, if you can rotate it

450
00:34:16 --> 00:34:19
around in your mind,
we can determine whether it is

451
00:34:19 --> 00:34:22
or is not superimposable on that
mirror image.

452
00:34:22 --> 00:34:26
I am generating the mirror
image by reflecting this water

453
00:34:26 --> 00:34:31
to this position.
This ammonia back here reflects

454
00:34:31 --> 00:34:35
over to here.
The top ammonia reflects still

455
00:34:35 --> 00:34:39
into the top position.
This water behind the board

456
00:34:39 --> 00:34:42
reflects behind the board.
And over here,

457
00:34:42 --> 00:34:47
this chloride coming out in
front of the board reflects over

458
00:34:47 --> 00:34:50
to here.
And we have one more chloride,

459
00:34:50 --> 00:34:54
down on the bottom.
That molecule is now our mirror

460
00:34:54 --> 00:34:57
image.
And let's go ahead and rotate

461
00:34:57 --> 00:35:02
it, like this.
Because it is a little hard,

462
00:35:02 --> 00:35:06
I am going to highlight the
position of the two ammonia

463
00:35:06 --> 00:35:09
ligands.
And to see if this mirror image

464
00:35:09 --> 00:35:13
is superimposable on the
structure we started with,

465
00:35:13 --> 00:35:17
I am going to rotate this
around so that we can put the

466
00:35:17 --> 00:35:21
two ammonia ligands coincident
with the two shown here

467
00:35:21 --> 00:35:26
underlined in green on the left.
We are going to do a rotation.

468
00:35:26 --> 00:35:31
And I need to rotate this.
I am going to rotate here,

469
00:35:31 --> 00:35:34
around the cobalt-chlorine bond
access.

470
00:35:34 --> 00:35:39
And I am actually going to go
in the negative direction to

471
00:35:39 --> 00:35:42
generate the following
structure.

472
00:35:42 --> 00:35:48
This puts this ammonia up top,
and it will put this one down

473
00:35:48 --> 00:35:51
below.
We have NH three and NH three

474
00:35:51 --> 00:35:54
here.
Let me underline them.

475
00:35:54 --> 00:35:59
So those are in positions,
coincident.

476
00:35:59 --> 00:36:04
And this rotation also will
carry that chloride from the

477
00:36:04 --> 00:36:07
bottom up here,
into what I may call an

478
00:36:07 --> 00:36:12
equatorial position.
And it puts a water molecule

479
00:36:12 --> 00:36:15
down.
And that rotation about this

480
00:36:15 --> 00:36:21
cobalt chlorine bond left the
cobalt and chlorine on that bond

481
00:36:21 --> 00:36:24
axis unrotated.
And then in the back,

482
00:36:24 --> 00:36:29
we have this OH two molecule.

483
00:36:29 --> 00:36:32
And what you can see is,
if you now bring this over,

484
00:36:32 --> 00:36:35
what we have,
in fact, is a situation where

485
00:36:35 --> 00:36:39
we are not currently
superimposable with that choice.

486
00:36:39 --> 00:36:43
I generated the mirror image.
I have rotated it by 90 degrees

487
00:36:43 --> 00:36:47
around the cobalt-chlorine bond
axis to bring these two ammonias

488
00:36:47 --> 00:36:51
coincident with these two.
So you can see that,

489
00:36:51 --> 00:36:54
whereas we have a water
molecule on the bottom here,

490
00:36:54 --> 00:37:00
we have a chloride over here.
So that is not superimposable.

491
00:37:00 --> 00:37:06
But we can do one more rotation
to check the other possibility,

492
00:37:06 --> 00:37:12
and that rotation will be a
rotation by 180 degrees around

493
00:37:12 --> 00:37:15
an axis, here,
that bisects the

494
00:37:15 --> 00:37:20
nitrogen-cobalt-nitrogen bond
angle of 90 degrees.

495
00:37:20 --> 00:37:25
We will rotate 180 degrees
around that axis,

496
00:37:25 --> 00:37:29
and that will bring our
ammonias, again,

497
00:37:29 --> 00:37:35
into a position so as to be
coincident.

498
00:37:35 --> 00:37:38


499
00:37:38 --> 00:37:43
And rotating around that axis
brings a water around front here

500
00:37:43 --> 00:37:48
and puts a chloride in back,
rotating around there,

501
00:37:48 --> 00:37:53
and it swaps this chloride with
that water molecule.

502
00:37:53 --> 00:38:00
So we now have chloride down
and OH two over here.

503
00:38:00 --> 00:38:03
And so if we take this,
we identify our ammonia

504
00:38:03 --> 00:38:07
positions by green underlining,
they're coincident,

505
00:38:07 --> 00:38:09
here.
And now where we have a

506
00:38:09 --> 00:38:13
chloride coming out,
we have a water coming out,

507
00:38:13 --> 00:38:17
so our mirror image is not
superimposable on the structure

508
00:38:17 --> 00:38:21
that we generated it from
through the process of

509
00:38:21 --> 00:38:24
reflection through that mirror
plane.

510
00:38:24 --> 00:38:28
And so, what we can say is that
this molecule and this one

511
00:38:28 --> 00:38:33
constitute a pair of
stereoisomers.

512
00:38:33 --> 00:38:36
And because this condition was
satisfied that the mirror image

513
00:38:36 --> 00:38:40
was not superimposable on the
structure we generated it from,

514
00:38:40 --> 00:38:44
the molecule is chiral.
And you will see that I have

515
00:38:44 --> 00:38:47
chosen a molecule that contains
no carbon, and yet it is chiral

516
00:38:47 --> 00:38:51
and it has stereoisomers.
And that was thought impossible

517
00:38:51 --> 00:38:54
prior to the time of Werner.

518
00:38:54 --> 00:39:00


519
00:39:00 --> 00:39:04
Let me show you another example
of a molecule that is chiral.

520
00:39:04 --> 00:39:07
And I am going to use this
example, also,

521
00:39:07 --> 00:39:13
to illustrate another important
feature that ligands can have.

522
00:39:13 --> 00:39:18


523
00:39:18 --> 00:39:23
And that is that they can have
more than one atom that can bond

524
00:39:23 --> 00:39:26
to the metal at the same time.

525
00:39:26 --> 00:39:29


526
00:39:29 --> 00:39:33
I am drawing a cobalt ion three
plus complex that

527
00:39:33 --> 00:39:37
has six nitrogens directly
bonded to the cobalt.

528
00:39:37 --> 00:39:40
But now, look what I am going
to do.

529
00:39:40 --> 00:39:45
I am going to put some organic
material in here and link these

530
00:39:45 --> 00:39:49
nitrogens by a CH two CH two
unit,

531
00:39:49 --> 00:39:52
a CH2-CH2 chain here.
So it is CH2-CH2.

532
00:39:52 --> 00:39:57
These carbons that I am
representing as vertices here

533
00:39:57 --> 00:40:03
each have two additional
hydrogens that I am not showing.

534
00:40:03 --> 00:40:06
And that is typical organic
shorthand.

535
00:40:06 --> 00:40:12
And I am going to suggest that
this molecule would be generated

536
00:40:12 --> 00:40:16
by adding three of these ligands
to the metal center.

537
00:40:16 --> 00:40:21
And for each nitrogen,
if you consider it as being

538
00:40:21 --> 00:40:25
derived from ammonia,
one of the hydrogens of the

539
00:40:25 --> 00:40:31
ammonia is replaced with a
nitrogen-carbon bond.

540
00:40:31 --> 00:40:37
And we have used this organic
moiety here to tether two

541
00:40:37 --> 00:40:42
nitrogens together.
This is a very popular and

542
00:40:42 --> 00:40:47
ancient ligand in coordination
chemistry.

543
00:40:47 --> 00:40:54
And, by drawing in simplistic
form the two lone pairs on the

544
00:40:54 --> 00:40:59
two nitrogens,
you can see that this set of

545
00:40:59 --> 00:41:07
four atoms is able to organize
itself so as to simultaneously

546
00:41:07 --> 00:41:14
point two lone pairs at the same
metal center.

547
00:41:14 --> 00:41:17
That is permitted by this
bridge.

548
00:41:17 --> 00:41:22
This particular ligand is
called ethylenediamine.

549
00:41:22 --> 00:41:27


550
00:41:27 --> 00:41:32
And it is called en for short,
ethylenediamine.

551
00:41:32 --> 00:41:37
And it is an example of a
bidentate ligand.

552
00:41:37 --> 00:41:44


553
00:41:44 --> 00:41:53
And that means that it has two
teeth with which to bite down on

554
00:41:53 --> 00:42:00
the metal center.
It is a double Lewis base.

555
00:42:00 --> 00:42:05
And when it binds to the metal
center, we call that the process

556
00:42:05 --> 00:42:08
of chelation.
When a bidentate or a

557
00:42:08 --> 00:42:11
multidentate,
which would be maybe a

558
00:42:11 --> 00:42:16
tridentate or a tetradentate
ligand, binds to a metal through

559
00:42:16 --> 00:42:20
multiple points,
we call that a ligand chelate.

560
00:42:20 --> 00:42:25
And we call the process one of
chelation that forms these ring

561
00:42:25 --> 00:42:30
structures with the metal as
part of the ring produced

562
00:42:30 --> 00:42:37
through multipoint binding of
the ligand to the metal center.

563
00:42:37 --> 00:42:41
And you are going to see that
it is possible to have all kinds

564
00:42:41 --> 00:42:45
of different architectures for
ligands in proteins or in

565
00:42:45 --> 00:42:48
synthetic systems.
And the reason that I carried

566
00:42:48 --> 00:42:53
out over here earlier is one in
which I added three equivalents

567
00:42:53 --> 00:42:57
of a bidentate ligand to this
solution of iron two plus.

568
00:42:57 --> 00:43:01
And, when that occurred,

569
00:43:01 --> 00:43:07
this bidentate ligand displaced
the water molecules from the

570
00:43:07 --> 00:43:11
inner coordination sphere of the
metal.

571
00:43:11 --> 00:43:16
And the bidentate ligand that I
used was this one.

572
00:43:16 --> 00:43:22
This is a very common chelating
ligand, a planar aromatic

573
00:43:22 --> 00:43:25
ligand.
And you can see that,

574
00:43:25 --> 00:43:31
like ethylenediamine,
its architecture promotes the

575
00:43:31 --> 00:43:39
pointing of a pair of electrons
toward the same point in space.

576
00:43:39 --> 00:43:43
So that this ligand can bind
itself to a metal center through

577
00:43:43 --> 00:43:46
two nitrogen lone pairs
simultaneously.

578
00:43:46 --> 00:43:51
And it is the interaction of
the d-electrons on the iron

579
00:43:51 --> 00:43:55
center with the unsaturated pi
system of this organic ligand

580
00:43:55 --> 00:44:00
that produces the red color in
ways that we are going to

581
00:44:00 --> 00:44:05
explore in more detail in one of
our next lectures.

582
00:44:05 --> 00:44:08
But, before we do that,
we are going to need to

583
00:44:08 --> 00:44:10
understand something about
d-orbitals.

584
00:44:10 --> 00:44:14
And, as you have learned,
when you are forming molecular

585
00:44:14 --> 00:44:18
orbitals in systems that consist
of either s or p orbitals,

586
00:44:18 --> 00:44:22
you needed to know something
about the nodal properties of

587
00:44:22 --> 00:44:26
those atomic orbitals in order
to build proper molecular

588
00:44:26 --> 00:44:30
orbitals.
And that will certainly be the

589
00:44:30 --> 00:44:35
case for these more interesting
elements that have d orbitals.

590
00:44:35 --> 00:44:40
Not just s and p valance
orbitals, but also a set of

591
00:44:40 --> 00:44:43
d-orbitals.
And I call those the 3d

592
00:44:43 --> 00:44:48
elements because their principle
quantum number for those

593
00:44:48 --> 00:44:52
elements is three.
And what we need to now know is

594
00:44:52 --> 00:44:57
what do these orbitals have,
as far as nodal properties,

595
00:44:57 --> 00:45:02
depending on the other quantum
numbers?

596
00:45:02 --> 00:45:06
And I will draw a set of
coordinate axes,

597
00:45:06 --> 00:45:10
here, on which to map these
orbitals.

598
00:45:10 --> 00:45:23


599
00:45:23 --> 00:45:26
It should be pretty
straightforward for you to keep

600
00:45:26 --> 00:45:30
straight the nodal properties of
the d orbitals of which there is

601
00:45:30 --> 00:45:34
a set of five.
We had one s orbital for a

602
00:45:34 --> 00:45:39
given valance shell,
and we had a set of three p

603
00:45:39 --> 00:45:45
orbitals, and there is a set of
five d orbitals for the d-block

604
00:45:45 --> 00:45:49
elements.
And they can have different

605
00:45:49 --> 00:45:53
values for the quantum number m.
One is zero.

606
00:45:53 --> 00:45:56
One is plus one.
One is plus two.

607
00:45:56 --> 00:46:03
And m can be minus one,
and m can equal minus two.

608
00:46:03 --> 00:46:08
And this quantum number
determines the angular nodal

609
00:46:08 --> 00:46:13
properties of the d orbital in
question.

610
00:46:13 --> 00:46:17
Here, let's draw a fairly
simple one.

611
00:46:17 --> 00:46:21
Let's say that we have x,
y, and z.

612
00:46:21 --> 00:46:29
Then what we might have is a d
orbital that looks like this.

613
00:46:29 --> 00:46:34


614
00:46:34 --> 00:46:37
d orbitals often have four
lobes.

615
00:46:37 --> 00:46:45
In fact, you will see that we
represent four of the d orbitals

616
00:46:45 --> 00:46:51
this way and not the fifth.
And let me use this pink to

617
00:46:51 --> 00:46:58
represent the negative phase.
And so this orbital here is

618
00:46:58 --> 00:47:02
(d)xz.
And that means that it has

619
00:47:02 --> 00:47:05
nodes.
You have two planes that are

620
00:47:05 --> 00:47:10
nodes for a (d)xz orbital.
And one of these is the

621
00:47:10 --> 00:47:13
x,y-plane.
And then the other one is the

622
00:47:13 --> 00:47:17
y,z-plane.
Those are planes when you go

623
00:47:17 --> 00:47:22
from one side through one of
those planes to the other side.

624
00:47:22 --> 00:47:28
The wave function changes sign.
And, just like each p orbital

625
00:47:28 --> 00:47:34
has a single nodal plane,
each d orbital has two.

626
00:47:34 --> 00:47:38
And this is (d)xz.
And we can also have one that

627
00:47:38 --> 00:47:42
we call d x squared minus y
squared.

628
00:47:42 --> 00:47:47
And that one lies right along
the coordinate axes like this,

629
00:47:47 --> 00:47:52
with the four lobes being
skewered by the x-axis and the

630
00:47:52 --> 00:47:55
y-axis.
And we have negative phase

631
00:47:55 --> 00:48:00
located along y for the d x
squared minus y squared.

632
00:48:00 --> 00:48:04
And you can see that the nodal

633
00:48:04 --> 00:48:10
surfaces here both contain z.
The nodes contain the z-axis

634
00:48:10 --> 00:48:13
and bisect the x and y
coordinate axes.

635
00:48:13 --> 00:48:19
There is one plane up here that
contains the z-axis and one over

636
00:48:19 --> 00:48:23
there located at 90 degrees to
the first one.

637
00:48:23 --> 00:48:29
Those are the nodal planes for
d x squared minus y squared.

638
00:48:29 --> 00:48:34
In addition to that (d)xz

639
00:48:34 --> 00:48:40
orbital, I have a (d)yz orbital,
which is located with its lobes

640
00:48:40 --> 00:48:44
lying between the y and z
coordinate axes.

641
00:48:44 --> 00:48:49
And it will have phasing as
indicated here in pink.

642
00:48:49 --> 00:48:53
That is (d)yz.
And it looks exactly like

643
00:48:53 --> 00:48:56
(d)xz.
And it is just rotated by 90

644
00:48:56 --> 00:49:02
degrees around the z-axis
relative to (d)xz.

645
00:49:02 --> 00:49:06
And then, finally,
we have one that looks just

646
00:49:06 --> 00:49:11
like d x squared minus
y squared.

647
00:49:11 --> 00:49:15
And this one is (d)xy.
And, like d x squared minus y

648
00:49:15 --> 00:49:20
squared, the (d)xy orbital lies
in the x,y-plane.

649
00:49:20 --> 00:49:26
And its lobes point between the
axes, as shown here with that

650
00:49:26 --> 00:49:30
phasing.
And then, finally --

651
00:49:30 --> 00:49:33
And we will return to this
point next week.

652
00:49:33 --> 00:49:38
Our m equals zero orbital is
our d z squared **d(z^2)**.

653
00:49:38 --> 00:49:44
And d z squared lies along and
is skewered by the z-axis.

654
00:49:44 --> 00:49:49
It looks like a p orbital,
except the sign is the same on

655
00:49:49 --> 00:49:53
top and on bottom.
And then it has this beautiful

656
00:49:53 --> 00:49:57
torus here that is in the
x,y-plane like that,

657
00:49:57 --> 00:50:03
so that its nodal surfaces are
actually conical rather than

658
0:50:03.07 --> 00:50:07
planes.
That is our set of five d

659
0:50:07 --> 00:50:12
orbitals with which we are going
to do a lot more to understand

660
0:50:12 --> 00:50:16
the chemistry and coordination
complexes.

661
0:50:16 --> 00:50:21
Have a great break,
and please don't forget to read

662
0:50:21.205 --> 50:24
about Alfred Werner.