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PROFESSOR: OK, everyone,
pay attention to the

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clicker question.
 

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If you haven't responded,
now's a good time to

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click in your response.
 

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All right, let's just
take 10 more seconds.

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OK, we can do better than this.
 

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So, tetrahedral complexes.
 

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Do you recall tetrahedral
complexes with angles of

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what between the ligands?
 

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109 .
 

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5.
 

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The ligands' negative point
charges aren't facing any of

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the d orbitals perfectly.
 

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They're a little bit closer
to the orbitals that are 45

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degrees off-axis, so those
three are the most repelled.

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But they're not really
directly hitting any of them.

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So that's in contrast with the
octahedral system or square

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planar where the ligands
negative point charges are

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headed directly toward
some of the d orbitals.

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So because the ligands in a
tetrahedral case are not headed

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directly toward any of the d
orbitals, there is not a huge

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amount of crystal fields
splitting, so that's small.

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And so, when the splitting
is small, then you tend to

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have high spin systems.
 

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So you put in all of the
electrons singly to the fullest

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extent of the orbital as
possible before you pair.

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And so, since you put them in
singly for the fullest extent

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possible before you pair them
up, that will lead to a high

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spin system, which is
the maximum number of

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unpaired electrons.
 

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So today is then the last
lecture on transition metals,

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and we've been talking about
crystal field theory, and today

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we're going to talk about
colors and crystal

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field theory.
 

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So, colors, there are a lot of
beautiful colors in nature, and

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some of the beautiful colors
you find in nature have to do

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with transition metals or
other liganded states.

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So, we're going to start with
an example of how colors can

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change, how a molecule's color
can depend on its oxidation

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state, how a molecule's color
can depend on its

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liganded state.
 

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So, Dr. Taylor is going to be
doing this for you -- should we

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do it the demo and then look
at the questions or do

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the questions first?
 

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Demo first?
 

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PROFESSOR: OK.
 

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PROFESSOR: So, here are some of
the reactions up on the

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Powerpoint that you're going to
be looking at, and as the

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reaction proceeds, there's
going to be changes in

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oxidation state, and also
changes in ligand in state, and

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that will lead to
changes in color.

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[DEMONSTRATION]
 

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PROFESSOR: So, how would you
describe that first color?

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Any predictions?
 

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Have any of you seen this
before, what's going

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to happen next?
 

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[DEMONSTRATION]
 

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PROFESSOR: So this is called an
oscillating clock reaction, so

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as it runs through, it cycles
between the different colors.

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So, that should happily keep
going, and we can consider

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what's happening.
 

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So here's the overall reaction
here and it can be divided

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into two components.
 

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So, why don't you tell me, what
is happening to iodide in that

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first reaction, and
there it is again.

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All right, so let's
take 10 more seconds.

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Very good.
 

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So, if you looked up here, you
see that you have minus 2 for

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the oxygen, three of them,
minus 6, and it needs to equal

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minus 1, so you have plus 5.
 

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And then over here, we again
have plus 1 minus 2, and

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so that has to be plus 1.
 

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So that was actually a quiz
question, but you score three

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points if you answered, even if
you got it wrong, but we could

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have had that four points, most
people got that right anyway.

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But if you answered, you get an
extra few points on that one.

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All right, so very good.
 

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That's what's
happening to iodide.

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Now you notice that in this
reaction h o i is being

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produced, and then in the
second step it's

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being consumed.
 

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This later reaction can be
divided, then, into two

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additional reactions.
 

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And one of the reactions, you
can tell me again what is

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happening, what is being
oxidized and what is being

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reduced in this part
of the reaction.

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OK, let's take 10 more seconds.
 

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Excellent.
 

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Even a little higher
than before.

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All right, so you figured out
that this was the plus 1, this

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is minus 1, and they're
both going to 0.

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So you have oxidation
and reduction going on

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all involving iodide
in that reaction.

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OK, so as the reaction was
proceeding, you could see that

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it started out with clear and
then it went to sort of amber

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color, and that was the i 2,
the clear was i minus, and the

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00:09:53 --> 00:09:56
sort of darker blue color that
you saw is the complex with

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starch in the reaction.
 

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So that both of these guys have
colors independent, but then

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00:10:02 --> 00:10:04
when they're liganded to
something else, the

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00:10:04 --> 00:10:05
color is different.
 

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And that's very true with
coordination complexes that

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the metal by itself will
be strongly influenced

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00:10:13 --> 00:10:14
by what the ligands are.
 

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And depending on what type of
ligands it has, it can have a

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really entirely different
color than it had before.

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So, today we're going to talk
about why that's true, and how

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you can predict colors based on
what type of ligand is bound

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to a transition metal.
 

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So, a lot of transition metals
have really beautiful colors,

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and my laboratory studies
metals bound to proteins, and

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often the proteins will have
really beautiful colors because

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of the metal cofactor involved,
and that's one of the things

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that I liked about that
particular area of study

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is just how beautiful
these proteins can be.

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So, the color given off depends
on the nature of the metal, and

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depends on the nature
of the ligand.

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And so we can use crystal field
theory, which again, is a very

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00:11:01 --> 00:11:06
simplified theory, to try
to predict or explain

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the observed colors.
 

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And so again, this is not
always very precise, but given,

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if you're told information out
of color, you can rationalize

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why that would be true, and you
can also predict at least a

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range of color that you would
have under certain

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circumstances.
 

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00:11:23 --> 00:11:28
So, let's take a
look at this more.

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00:11:28 --> 00:11:30
So the ligands again, have
the ability to split

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00:11:30 --> 00:11:31
those d orbitals.
 

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00:11:31 --> 00:11:33
And when we're talking
about for metals, it's

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all about the d orbitals.
 

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00:11:35 --> 00:11:38
And we talked already about
strong field ligands and weak

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field ligands, and we're going
to talk more about that today,

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00:11:41 --> 00:11:43
but this time in
terms of color.

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So a strong field ligand, as we
discussed, creates a big

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00:11:47 --> 00:11:51
splitting in the d orbital
energy, whereas weak field

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00:11:51 --> 00:11:55
ligand, like the first question
you had today with tetrahedral

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00:11:55 --> 00:11:59
complexes, there's usually a
weak field there, so we have a

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00:11:59 --> 00:12:05
small energy separation
between the d orbitals.

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00:12:05 --> 00:12:07
So here is something that you
actually have to memorize --

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00:12:07 --> 00:12:11
there's not much memorization
in this course, but you do need

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to memorize these six ligands
in terms of their ability

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to split d orbitals.
 

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So, on the side, we have three
that are strong field ligands,

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cyanide, c o, and ammonia, and
so those are going to be strong

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field, so they're going to have
big splitting energy, and so

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they'll tend to be low spin.
 

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Then we have three that are
sort of in between that are

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intermediate field -- water,
hydroxide and f minus.

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And so, in comparing, those are
intermediate, so you're going

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to be asked questions such as
how does that compare to a

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00:12:48 --> 00:12:51
weak field, how does that
compare to a strong field.

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And then our weak field ligands
or a lot of our halides down

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here, i minus, b r
minus, c l minus.

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00:12:59 --> 00:13:03
And so those are weak field
ligands, so you'll have a small

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splitting, so they'll tend to
be in high spin complexes.

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00:13:09 --> 00:13:16
So let's take a look
at some examples.

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So we talked about iron before
in complexes, and now we can

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consider two cases where we
have iron plus 3, so the same

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00:13:26 --> 00:13:30
metal in the same oxidation
state, but it has

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different ligands.
 

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It So in one case you have a
high spin system with six water

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00:13:35 --> 00:13:38
ligands, and then the other in
a low spin system with

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00:13:38 --> 00:13:40
six cyanide ligands.
 

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00:13:40 --> 00:13:43
So first, before you do
anything else with this, you

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always have to think about
what the d count is.

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So, to do the d count, we're
going to look at where iron is

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in the periodic table, and
we're going to see

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it's in group 8.
 

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And then, we'll have 8
minus 3, the oxidation

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00:14:01 --> 00:14:08
number is d 5 system.
 

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00:14:08 --> 00:14:12
So now we have two diagrams
here, one has a big splitting,

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one has a small splitting, and
why don't you fill in for me in

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a clicker question, what the
high spin system

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00:14:18 --> 00:14:19
would look like.
 

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OK, let's just take
10 more seconds.

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00:15:15 --> 00:15:17
Very good.
 

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00:15:17 --> 00:15:20
I think that's one of our
highest numbers in a while.

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00:15:20 --> 00:15:23
That's right, so we're going to
fill to the fullest extent

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00:15:23 --> 00:15:30
possible before we pair
any of the electrons.

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So again, here we put in
electrons down here, and then

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00:15:34 --> 00:15:37
go up here, because the
splitting is small, so it

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00:15:37 --> 00:15:41
doesn't take that much energy
to put an electron in the upper

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00:15:41 --> 00:15:43
orbitals, it takes more energy
to pair the electrons for

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00:15:43 --> 00:15:45
this weak field system.
 

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00:15:45 --> 00:15:49
And so, this is a high spin
case, we have a maximum number

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00:15:49 --> 00:15:51
of unpaired electrons.
 

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00:15:51 --> 00:15:55
So, over here when we have a
much bigger splitting energy,

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00:15:55 --> 00:15:58
it's going to take a lot more
energy to put electrons up

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00:15:58 --> 00:16:01
there, and so we're going to
fill up all of these orbitals

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00:16:01 --> 00:16:05
down here until we have to
put an electron up there.

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00:16:05 --> 00:16:09
So, if we do that, put in the
three, and now we're going to

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00:16:09 --> 00:16:12
pair, because it takes less
energy to pair than it does to

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00:16:12 --> 00:16:16
put an electron up there,
so we do four and five.

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00:16:16 --> 00:16:22
And so then here is our system
where we have a strong field,

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00:16:22 --> 00:16:26
and so here's a weak field and
it's going to be high spin,

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00:16:26 --> 00:16:28
maximum number of unpaired
electrons, here we have the

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00:16:28 --> 00:16:32
strong field and it'll be low
spin, the minimum number

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00:16:32 --> 00:16:35
of unpaired electrons.
 

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00:16:35 --> 00:16:40
And we're doing this, again,
because we know that cyanide is

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00:16:40 --> 00:16:44
a strong field ligand, whereas
water is an intermediate

209
00:16:44 --> 00:16:49
field ligand, it's a lot
weaker than cyanide.

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00:16:49 --> 00:16:51
OK, now we can continue doing
some of the things that

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00:16:51 --> 00:16:52
we've done before.
 

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00:16:52 --> 00:16:56
So just to review this
material, we can write the d

213
00:16:56 --> 00:16:58
n electron configurations.
 

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00:16:58 --> 00:17:02
What are the orbitals
called that are down here?

215
00:17:02 --> 00:17:04
Yup, t 2 g.
 

216
00:17:04 --> 00:17:08
And how many electrons do
we have there? three.

217
00:17:08 --> 00:17:14
And then the e g system up
here with two electrons.

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00:17:14 --> 00:17:17
And over here what do we have?
 

219
00:17:17 --> 00:17:21
Yup, t 2 g to the 5.
 

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00:17:21 --> 00:17:24
So, a review, electron
configurations, these are just

221
00:17:24 --> 00:17:28
shorthand notations, which
tell people what these

222
00:17:28 --> 00:17:32
diagrams look like.
 

223
00:17:32 --> 00:17:36
Then, we also talked
about this before.

224
00:17:36 --> 00:17:42
So, what does this
term stand for?

225
00:17:42 --> 00:17:47
Crystal field stabilization
energy, right.

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00:17:47 --> 00:17:49
So now why don't you tell
me what it is for the

227
00:17:49 --> 00:18:15
high spin system.
 

228
00:18:15 --> 00:18:29
OK, let's just take
10 more seconds.

229
00:18:29 --> 00:18:34
Yup, zero.
 

230
00:18:34 --> 00:18:40
So if we look at that, we have
three electrons down here, so

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00:18:40 --> 00:18:46
that's minus 2/5, two electrons
up here, which is plus 3/5, so

232
00:18:46 --> 00:18:53
3 times minus 2/5 is minus
6/5, plus 6/5 gives you 0.

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00:18:53 --> 00:18:56
So here is a case where
you really don't have

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much stabilization.
 

235
00:18:58 --> 00:19:02
It would be equivalent then,
so there's zero stabilization

236
00:19:02 --> 00:19:06
because you have three
electrons down and two up.

237
00:19:06 --> 00:19:11
All right, so what about
the low spin system now?

238
00:19:11 --> 00:19:13
So if we can look at that,
what are we going to

239
00:19:13 --> 00:19:18
have for this system?
 

240
00:19:18 --> 00:19:24
So, we have 5 times minus 2/5
or minus 10/5, and we also have

241
00:19:24 --> 00:19:30
two pairing energy terms there,
which we can write in, because

242
00:19:30 --> 00:19:33
there are two sets
that are paired.

243
00:19:33 --> 00:19:38
So this is mostly review on
what we've had before, and we

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00:19:38 --> 00:19:40
haven't talked about some of
these things in a little while,

245
00:19:40 --> 00:19:43
so we go over it again, and now
we're going to take it a next

246
00:19:43 --> 00:19:48
step and think about what sort
of wavelength would be absorbed

247
00:19:48 --> 00:19:51
if you're going to promote some
of these electrons to

248
00:19:51 --> 00:19:54
unfilled orbitals.
 

249
00:19:54 --> 00:19:57
So what about the light
absorbed by these octahedral

250
00:19:57 --> 00:20:00
coordination complexes?
 

251
00:20:00 --> 00:20:02
So, if you remember back to the
beginning of the course, a

252
00:20:02 --> 00:20:07
physics course or high school,
a substance absorbs photons of

253
00:20:07 --> 00:20:11
light if the energy of the
photons match the energy

254
00:20:11 --> 00:20:14
required to excite those
electrons to a higher

255
00:20:14 --> 00:20:16
energy level.
 

256
00:20:16 --> 00:20:20
And so now we are doing some
review from the first part of

257
00:20:20 --> 00:20:21
the course, which
is always good.

258
00:20:21 --> 00:20:25
As I mentioned, everything kind
of comes together and we need

259
00:20:25 --> 00:20:28
to go over everything for the
final, but there's also

260
00:20:28 --> 00:20:30
connections between all
the different parts

261
00:20:30 --> 00:20:32
of the semester.
 

262
00:20:32 --> 00:20:34
So this should look
familiar to you.

263
00:20:34 --> 00:20:37
So, the energy of the absorbed
light equals Planck's constant

264
00:20:37 --> 00:20:40
times the frequency
of that light.

265
00:20:40 --> 00:20:44
But now we can make that equal
to another term, a term we've

266
00:20:44 --> 00:20:47
been talking about in this
unit, and that is our

267
00:20:47 --> 00:20:50
octahedral crystal field
splitting energy.

268
00:20:50 --> 00:20:53
Because the energy that's going
to be required to bump an

269
00:20:53 --> 00:20:57
electron from here to here is
that energy, that

270
00:20:57 --> 00:20:58
splitting energy.
 

271
00:20:58 --> 00:21:04
So that's going to be
equal to this term here.

272
00:21:04 --> 00:21:08
OK, so what does this mean in
terms of the wavelengths of

273
00:21:08 --> 00:21:14
lights absorbed by different
coordination complexes.

274
00:21:14 --> 00:21:16
So we can think about that.
 

275
00:21:16 --> 00:21:20
So if you have high frequency
of light is absorbed, the

276
00:21:20 --> 00:21:25
wavelength of the absorbed
light is going to be short.

277
00:21:25 --> 00:21:28
And we know that relationship
-- again, think back to the

278
00:21:28 --> 00:21:30
beginning of the course and
also probably to physics and to

279
00:21:30 --> 00:21:34
high school, we know a very
handy equation for telling us

280
00:21:34 --> 00:21:38
about the relationship of
frequency and wavelength of

281
00:21:38 --> 00:21:41
light, so we have the speed of
light equals the wavelength

282
00:21:41 --> 00:21:43
times the frequency.
 

283
00:21:43 --> 00:21:46
So if you have a high frequency
of light absorbed, the absorbed

284
00:21:46 --> 00:21:51
wavelength is going
to be short.

285
00:21:51 --> 00:21:54
So, let's look at a couple of
examples, the example here,

286
00:21:54 --> 00:21:56
going back to our example.
 

287
00:21:56 --> 00:22:00
So we had this high spin system
with water, and now I'm telling

288
00:22:00 --> 00:22:06
you that the splitting energy
is 171 kilojoules per mole, and

289
00:22:06 --> 00:22:10
when you have cyanide as your
ligand, your splitting energy

290
00:22:10 --> 00:22:13
is 392 kilojoules per mole.
 

291
00:22:13 --> 00:22:15
Again, this was a stronger
field ligand, so we have a

292
00:22:15 --> 00:22:17
bigger splitting energy.
 

293
00:22:17 --> 00:22:21
And this was an intermediate
field ligand, certainly weaker

294
00:22:21 --> 00:22:25
than cyanide, so this has a
smaller splitting energy.

295
00:22:25 --> 00:22:29
So, from these values now, we
can calculate the wavelength

296
00:22:29 --> 00:22:32
of absorbed light.
 

297
00:22:32 --> 00:22:36
So for the high spin system
first, we can rearrange these

298
00:22:36 --> 00:22:40
equations, which you know well,
to come up with the rearranged

299
00:22:40 --> 00:22:44
equation, the wavelength equals
Planck's constant times the

300
00:22:44 --> 00:22:48
speed of light, and divided by
e, and this time our e is that

301
00:22:48 --> 00:22:51
crystal field splitting energy.
 

302
00:22:51 --> 00:22:54
So we can put in these terms.
 

303
00:22:54 --> 00:22:59
So we have Planck's constant
times the speed of light over

304
00:22:59 --> 00:23:03
our octahedral field splitting
energy, oh, but then we

305
00:23:03 --> 00:23:04
have some other terms here.
 

306
00:23:04 --> 00:23:07
Now one thing that you have
to pay attention to in

307
00:23:07 --> 00:23:09
this unit is your units.
 

308
00:23:09 --> 00:23:14
So, splitting energies are
often given in kilojoules per

309
00:23:14 --> 00:23:18
mole, whereas you often see
Planck's constant in joules, so

310
00:23:18 --> 00:23:21
we want to make sure that we
convert one or the other, and

311
00:23:21 --> 00:23:25
here it's set up to convert
the kilojoules to joules.

312
00:23:25 --> 00:23:28
And also, we want our final
unit, we're talking about

313
00:23:28 --> 00:23:32
wavelengths, in meters or
nanometers, so we need to

314
00:23:32 --> 00:23:35
get rid of this mole term,
and we use Avagadro's

315
00:23:35 --> 00:23:38
number to do that.
 

316
00:23:38 --> 00:23:40
So now we should be able
to cancel our units and

317
00:23:40 --> 00:23:42
get the correct units.
 

318
00:23:42 --> 00:23:47
So we should be able to cancel
the seconds over here, and we

319
00:23:47 --> 00:23:50
should also be able to go
in and cancel our moles.

320
00:23:50 --> 00:23:55
We should be able to cancel the
joules and the kilojoules out,

321
00:23:55 --> 00:23:59
and that should leave us just
with this over here,

322
00:23:59 --> 00:24:01
which is meters.
 

323
00:24:01 --> 00:24:02
So this is 7 .
 

324
00:24:02 --> 00:24:06
0 0 times 10 to the
minus 7 meters.

325
00:24:06 --> 00:24:11
Does that make sense in terms
of a wavelength of light?

326
00:24:11 --> 00:24:16
Because that would convert
to what nanometers?

327
00:24:16 --> 00:24:18
700 nanometers.
 

328
00:24:18 --> 00:24:22
So if you do something strange
and you forget Avagadro's

329
00:24:22 --> 00:24:24
number, you're going to come up
with a very interesting

330
00:24:24 --> 00:24:25
wavelength.
 

331
00:24:25 --> 00:24:28
So that's a good way to check
to make sure that you've

332
00:24:28 --> 00:24:31
done the problem correctly.
 

333
00:24:31 --> 00:24:35
So, 700 nanometers, anyone
remember what light that is,

334
00:24:35 --> 00:24:38
what color that corresponds to?
 

335
00:24:38 --> 00:24:41
Red, so it's
absorbing red light.

336
00:24:41 --> 00:24:44
All right, so now let's just do
the same thing for the low spin

337
00:24:44 --> 00:24:48
system with the cyanide ligand,
and we're going to plug in

338
00:24:48 --> 00:24:55
our 392 here, and we
get 305 over here.

339
00:24:55 --> 00:25:03
And so, that is a much
shorter wavelength of light.

340
00:25:03 --> 00:25:07
So again, light absorbed for
the compound with water, 700

341
00:25:07 --> 00:25:13
nanometers, and the compound of
iron with cyanide, 305, and so

342
00:25:13 --> 00:25:18
we're absorbing a red light
over with the water compound,

343
00:25:18 --> 00:25:24
and sort of purple or violet
light is being absorbed

344
00:25:24 --> 00:25:26
in the cyanide complex.
 

345
00:25:26 --> 00:25:29
And we're talk in a minute
about the light that is being

346
00:25:29 --> 00:25:32
transmitted, which is
complementary to the color

347
00:25:32 --> 00:25:35
of the light absorbed.
 

348
00:25:35 --> 00:25:38
So by knowing something about
splitting energies, by knowing

349
00:25:38 --> 00:25:42
something about the types of
ligands, then you can know

350
00:25:42 --> 00:25:45
something about colors.
 

351
00:25:45 --> 00:25:49
So now, for another example,
we're going to look at the

352
00:25:49 --> 00:25:55
different colors of two
chromium complexes.

353
00:25:55 --> 00:26:01
So first, what is the oxidation
number of chromium in

354
00:26:01 --> 00:26:06
the water complex here?
 

355
00:26:06 --> 00:26:08
What is it?
 

356
00:26:08 --> 00:26:12
And what about over here
with n h 3 ligands.

357
00:26:12 --> 00:26:14
Plus 3.
 

358
00:26:14 --> 00:26:18
So, plus 3 and plus 3.
 

359
00:26:18 --> 00:26:19
And what is our d count?
 

360
00:26:19 --> 00:26:23
You know where chromium
is, in what group?

361
00:26:23 --> 00:26:24
Six.
 

362
00:26:24 --> 00:26:29
6 minus 3 is 3, so we
have a d 3 system.

363
00:26:29 --> 00:26:32
What does c n mean again?
 

364
00:26:32 --> 00:26:37
Coordination number, so what
is that for both of these?

365
00:26:37 --> 00:26:39
Six.
 

366
00:26:39 --> 00:26:42
So there's Six things
coordinated to the chromium,

367
00:26:42 --> 00:26:48
and so we have, again,
an octahedral system.

368
00:26:48 --> 00:26:55
And what type of
ligand is water?

369
00:26:55 --> 00:26:56
Intermediate.
 

370
00:26:56 --> 00:27:00
And what about n h 3?
 

371
00:27:00 --> 00:27:01
Strong.
 

372
00:27:01 --> 00:27:06
So this is strong, and water
is and intermediate ligand,

373
00:27:06 --> 00:27:10
and certainly, it is
weaker than n h 3.

374
00:27:10 --> 00:27:14
So, we expect one system over
here for a strong ligand, and

375
00:27:14 --> 00:27:17
then something that's weaker
than that strong ligand.

376
00:27:17 --> 00:27:26
All right, so here are two
diagrams, one with a big

377
00:27:26 --> 00:27:30
splitting energy, and one with
a smaller splitting energy.

378
00:27:30 --> 00:27:34
Are the diagrams going to
look same or different?

379
00:27:34 --> 00:27:35
The same.
 

380
00:27:35 --> 00:27:39
Because we only have three
electrons, so they're

381
00:27:39 --> 00:27:40
going to be the same.
 

382
00:27:40 --> 00:27:43
In both cases, we put in the
three electrons in the lowest

383
00:27:43 --> 00:27:46
orbitals, and then there's no
decision to be made, because

384
00:27:46 --> 00:27:50
there isn't that fourth
electron, so we don't have to

385
00:27:50 --> 00:27:53
decide which place to put this.
 

386
00:27:53 --> 00:27:56
So these diagrams are going to
look the same, and before when

387
00:27:56 --> 00:27:59
we were doing this in this
unit, we said OK, we're

388
00:27:59 --> 00:28:00
done, they look the same.
 

389
00:28:00 --> 00:28:04
But now, we realize that these
are really not the same

390
00:28:04 --> 00:28:06
compounds and that they're
going to have different

391
00:28:06 --> 00:28:09
properties, even though their
diagrams are going

392
00:28:09 --> 00:28:10
to look the same.
 

393
00:28:10 --> 00:28:13
Because the energy that it's
going to take to excite an

394
00:28:13 --> 00:28:17
electron here is much smaller
than over here, and that's

395
00:28:17 --> 00:28:20
going to result in a different
wavelength of light being

396
00:28:20 --> 00:28:23
absorbed in these two different
cases, which will mean a

397
00:28:23 --> 00:28:28
different wavelength of
light being transmitted.

398
00:28:28 --> 00:28:31
So, again, here we have a
weaker field and here we

399
00:28:31 --> 00:28:35
have a stronger field.
 

400
00:28:35 --> 00:28:40
So again, we can go through and
think about these two cases.

401
00:28:40 --> 00:28:45
So when we have a smaller term
here, that means a lower energy

402
00:28:45 --> 00:28:47
-- this is a splitting energy,
and so we'll have a

403
00:28:47 --> 00:28:49
lower frequency.
 

404
00:28:49 --> 00:28:53
When we have a larger case
or higher energy, we

405
00:28:53 --> 00:28:57
have a higher frequency.
 

406
00:28:57 --> 00:29:02
Again, if you have a lower
frequency absorbed, we have a

407
00:29:02 --> 00:29:06
longer wavelength absorbed,
and in this case, the higher

408
00:29:06 --> 00:29:12
frequency translates into a
shorter wavelength absorbed.

409
00:29:12 --> 00:29:15
The color of the transmitted
light is complementary to the

410
00:29:15 --> 00:29:18
color of the absorbed light.
 

411
00:29:18 --> 00:29:24
So now we can think about --
I always want to ask, when

412
00:29:24 --> 00:29:26
do people learn about
complementary colors?

413
00:29:26 --> 00:29:33
Is that 6th grade, earlier?
 

414
00:29:33 --> 00:29:38
I don't really remember, but I
think it's pretty early on.

415
00:29:38 --> 00:29:42
And people always ask me, are
you going to have that on the

416
00:29:42 --> 00:29:45
equation sheet, or do I have
to actually remember my

417
00:29:45 --> 00:29:46
complementary colors?
 

418
00:29:46 --> 00:29:52
And I tell people that I will
put some version of this on, so

419
00:29:52 --> 00:29:58
you don't have to review your
kindergarten notes for this

420
00:29:58 --> 00:30:00
class, if that's when
you learned it.

421
00:30:00 --> 00:30:05
It's pretty early, I don't
know when it is exactly.

422
00:30:05 --> 00:30:09
So, the color of the light is
going to be complementary about

423
00:30:09 --> 00:30:14
-- this is very approximate.
 

424
00:30:14 --> 00:30:19
So here, if the transmitted
light is shorter because we

425
00:30:19 --> 00:30:24
have this weaker splitting,
then we are expecting we're

426
00:30:24 --> 00:30:26
going to have this shorter
wavelength, the transmitted

427
00:30:26 --> 00:30:29
light, and experimentally,
if you make this compound

428
00:30:29 --> 00:30:31
you'll see it's violet.
 

429
00:30:31 --> 00:30:35
In this other case, we have a
stronger field ligand, and so

430
00:30:35 --> 00:30:38
you have a larger energy,
higher frequency absorbed,

431
00:30:38 --> 00:30:41
shorter wavelength absorbed,
then you're going to have

432
00:30:41 --> 00:30:44
a longer wavelength from
your transmitted light.

433
00:30:44 --> 00:30:48
And that this compound, if you
make it, is actually yellow.

434
00:30:48 --> 00:30:52
So if we go back to our colors
for a minute, you see that

435
00:30:52 --> 00:30:55
when you have the shorter
wavelength, we have a violet,

436
00:30:55 --> 00:30:58
and that is a short wavelength.
 

437
00:30:58 --> 00:31:03
And in this case for the strong
field ligand, we are going to

438
00:31:03 --> 00:31:05
have transmitted light of a
longer wavelength

439
00:31:05 --> 00:31:07
and it's yellow.
 

440
00:31:07 --> 00:31:11
So it's the same oxidation
state of chromium, it's the

441
00:31:11 --> 00:31:15
same -- it's an octahedral
complex, six ligands in both

442
00:31:15 --> 00:31:20
cases, same octahedral crystal
field diagrams, but yet one

443
00:31:20 --> 00:31:23
compound has a violet
color, and the other

444
00:31:23 --> 00:31:30
one has a yellow color.
 

445
00:31:30 --> 00:31:35
All right, so one can also be
asked to calculate a crystal

446
00:31:35 --> 00:31:39
field splitting energy in
kilojoules per mole, given

447
00:31:39 --> 00:31:41
the appropriate information.
 

448
00:31:41 --> 00:31:45
So we've looked at when a
splitting energy is given, and

449
00:31:45 --> 00:31:48
we've been asked to calculate
wavelength absorbed, you can

450
00:31:48 --> 00:31:52
also be asked to go in
the other direction.

451
00:31:52 --> 00:31:55
And so here we have another
chromium complex to work with.

452
00:31:55 --> 00:31:57
And we're told that the
wavelength of the most

453
00:31:57 --> 00:32:04
intensely absorbed light is
740, and so what would you

454
00:32:04 --> 00:32:10
predict the color
of this to be?

455
00:32:10 --> 00:32:12
It would be greenish.
 

456
00:32:12 --> 00:32:14
So that would be what
you would predict.

457
00:32:14 --> 00:32:18
Again, chemistry is an
experimental science, but based

458
00:32:18 --> 00:32:21
on having a complementary
color to the one absorbed,

459
00:32:21 --> 00:32:22
that would be a guess.
 

460
00:32:22 --> 00:32:31
All right, so we can actually
calculate the frequency

461
00:32:31 --> 00:32:34
of the light absorbed.
 

462
00:32:34 --> 00:32:38
So we were given the
wavelength, and use speed of

463
00:32:38 --> 00:32:41
light, and plug in your
wavelength and you can come up

464
00:32:41 --> 00:32:48
with a frequency, 4.05 times 10
to the 14 per second.

465
00:32:48 --> 00:32:51
Then we can calculate from that
the crystal field splitting

466
00:32:51 --> 00:32:55
energy, and so we use Planck's
constant, and we have our

467
00:32:55 --> 00:32:59
frequency, and we calculate 2 .
 

468
00:32:59 --> 00:33:02
6 8 times 10 to the
minus 19 joules.

469
00:33:02 --> 00:33:07
Am I done with the problem?
 

470
00:33:07 --> 00:33:09
What does the problem ask for?
 

471
00:33:09 --> 00:33:15
It asks for it in kilojoules
per mole, and so, we're not

472
00:33:15 --> 00:33:18
done, we need to convert
to kilojoules per mole.

473
00:33:18 --> 00:33:20
And I'm making this point,
because often this is where

474
00:33:20 --> 00:33:24
people lose points on the final
exam, and that's not where

475
00:33:24 --> 00:33:25
you want to lose points.
 

476
00:33:25 --> 00:33:28
You want to lose them on a
really hard problem, not

477
00:33:28 --> 00:33:29
on something like this.
 

478
00:33:29 --> 00:33:33
So, most of the time you're
asked for kilojoules per mole,

479
00:33:33 --> 00:33:36
so make sure that if that's
what asked for, that's

480
00:33:36 --> 00:33:37
what you provide.
 

481
00:33:37 --> 00:33:41
So here, we can just do the
conversion of units, and then

482
00:33:41 --> 00:33:44
we're going to use Avagadro's
number to give us

483
00:33:44 --> 00:33:45
that per mole.
 

484
00:33:45 --> 00:33:49
And so, this translates into a
160 kilojoules per mole, which

485
00:33:49 --> 00:33:52
you might recognize is more
similar to the other numbers

486
00:33:52 --> 00:34:00
that you saw for octahedral
crystal field splitting energy.

487
00:34:00 --> 00:34:01
All right.
 

488
00:34:01 --> 00:34:06
Sadly, there are some
coordination complexes

489
00:34:06 --> 00:34:11
that do not have colors.
 

490
00:34:11 --> 00:34:19
Why would that be?
 

491
00:34:19 --> 00:34:21
Why would something
not have a color?

492
00:34:21 --> 00:34:32
It has d orbitals, it's
a transition metal.

493
00:34:32 --> 00:34:34
So what would be true about
all of the d orbitals?

494
00:34:34 --> 00:34:43
Yeah, so one example, is if
they're all full, and that is

495
00:34:43 --> 00:34:46
the most common thing that we
see, so it's not possible to

496
00:34:46 --> 00:34:51
have a d to d transition
in the visible range.

497
00:34:51 --> 00:34:53
So there are a number of
examples of metals who

498
00:34:53 --> 00:34:56
have this situation.
 

499
00:34:56 --> 00:35:01
Zinc and cadmium are two of the
most common that give you

500
00:35:01 --> 00:35:05
problems in biological systems.
 

501
00:35:05 --> 00:35:06
So why is this?
 

502
00:35:06 --> 00:35:11
Well, that's because they're
over here in group twelve.

503
00:35:11 --> 00:35:15
But their most common
oxidation states are plus 2.

504
00:35:15 --> 00:35:21
So, if you have 12 minus 2 you
have a d 10 system, and that's

505
00:35:21 --> 00:35:24
the case for both of
these systems here.

506
00:35:24 --> 00:35:27
And so all the d
orbitals are filled.

507
00:35:27 --> 00:35:32
Now, zinc is a really important
metal in biological systems,

508
00:35:32 --> 00:35:37
and because it has all these
d orbitals filled, it

509
00:35:37 --> 00:35:38
doesn't have a color.
 

510
00:35:38 --> 00:35:44
And so it's very hard to tell
if an enzyme molecule has zinc.

511
00:35:44 --> 00:35:47
And I think one of the problems
that you have on this

512
00:35:47 --> 00:35:50
problem-set talks about how
zinc is important in a

513
00:35:50 --> 00:35:54
biological system by altering
the p k a of a residue

514
00:35:54 --> 00:35:56
that coordinates to it.
 

515
00:35:56 --> 00:36:01
And that's often its job, and
so biochemists are often faced

516
00:36:01 --> 00:36:04
with the problem of trying to
figure out if their protein has

517
00:36:04 --> 00:36:08
zinc, but they have no color of
the protein, also they might

518
00:36:08 --> 00:36:12
try to look for a paramagnetic
or diamagnetic system.

519
00:36:12 --> 00:36:15
They're not -- you know if they
see a paramagnetic system, they

520
00:36:15 --> 00:36:19
say oh, unpaired electrons, we
know we must have metal

521
00:36:19 --> 00:36:21
involved, but there's no sort
of spectroscopic

522
00:36:21 --> 00:36:23
probe for zinc.
 

523
00:36:23 --> 00:36:26
And so, often someone will
determine a crystal structure,

524
00:36:26 --> 00:36:30
and it'll be a huge surprise
that there's zinc associated

525
00:36:30 --> 00:36:32
with this protein.
 

526
00:36:32 --> 00:36:36
Do you think there's a lot of
proteins that use cadmium as

527
00:36:36 --> 00:36:39
of part of their mechanism?
 

528
00:36:39 --> 00:36:42
What do you know about cadmium?
 

529
00:36:42 --> 00:36:44
Yeah, cadmium is poisonous.
 

530
00:36:44 --> 00:36:47
Old barbecue grills were
sometimes, they used to

531
00:36:47 --> 00:36:50
coat things with cadmium
on a barbecue grill.

532
00:36:50 --> 00:36:54
Yeah, that was not very smart.
 

533
00:36:54 --> 00:36:56
So, cadmium poisoning is a
problem, and people have been

534
00:36:56 --> 00:36:59
trying to figure out the
mechanism of that, but again,

535
00:36:59 --> 00:37:01
it's hard to study cadmium
because it has no

536
00:37:01 --> 00:37:07
spectroscopic signal.
 

537
00:37:07 --> 00:37:13
All right, so getting back now
to just kind of review over

538
00:37:13 --> 00:37:16
what we've talked about
in terms of colors.

539
00:37:16 --> 00:37:19
So we have our weak field
ligands, again, you need to

540
00:37:19 --> 00:37:20
memorize what they are.
 

541
00:37:20 --> 00:37:23
You have your intermediate
field ligands, which you need

542
00:37:23 --> 00:37:27
to memorize, and also your
strong field ligands.

543
00:37:27 --> 00:37:30
So these weak field ligands are
going to have a small splitting

544
00:37:30 --> 00:37:34
energy, and that means that in
terms of how the complex

545
00:37:34 --> 00:37:39
absorbs, low energy, low
frequency, long wavelength, and

546
00:37:39 --> 00:37:41
that the color transmitted
will be complementary.

547
00:37:41 --> 00:37:48
And usually what this means is
that it'll be sort of in the

548
00:37:48 --> 00:37:51
end of the spectra, so it's
often hard to say well, red

549
00:37:51 --> 00:37:53
will definitely be green.
 

550
00:37:53 --> 00:37:57
So it's not a perfect
agreement, but you can usually

551
00:37:57 --> 00:37:59
say well, it's probably going
to be in the blue-violet

552
00:37:59 --> 00:38:00
or green end.
 

553
00:38:00 --> 00:38:05
So in sort of one
part of the spectra.

554
00:38:05 --> 00:38:08
Strong field ligands, again,
have a huge splitting energy.

555
00:38:08 --> 00:38:11
So you're going to have big
energy, high frequency, short

556
00:38:11 --> 00:38:14
wavelength, and so it's going
to transmit, then, in the

557
00:38:14 --> 00:38:17
complementary, so it
should be in the yellow,

558
00:38:17 --> 00:38:19
orange or red end.
 

559
00:38:19 --> 00:38:22
And you will be asked in some
of the problems, in again,

560
00:38:22 --> 00:38:25
problem-set 9 due Wednesday,
and you should be able to

561
00:38:25 --> 00:38:28
finish that up pretty quickly
tonight after this lecture,

562
00:38:28 --> 00:38:30
and there are some
problems on this.

563
00:38:30 --> 00:38:36
So, for cobalt complexes,
you get pretty much the

564
00:38:36 --> 00:38:39
entire range of colors.
 

565
00:38:39 --> 00:38:44
So I'm just going to end with
one more biological example.

566
00:38:44 --> 00:38:49
And here are some pictures of
actual colors, and so this is

567
00:38:49 --> 00:38:53
cobalt coordinated
to vitamin B12.

568
00:38:53 --> 00:38:57
So one ligand gives you this
brilliant red color, another

569
00:38:57 --> 00:39:00
gives you an orange color,
and a third ligand

570
00:39:00 --> 00:39:02
gives you a pink color.
 

571
00:39:02 --> 00:39:06
So you can tell the oxidation
state of vitamin B12 by the

572
00:39:06 --> 00:39:10
colors of the molecule.
 

573
00:39:10 --> 00:39:12
All right.
 

574
00:39:12 --> 00:39:14
Now you have all the
information to finish your

575
00:39:14 --> 00:39:18
problem-set, and that's the
end of transition metals.

576
00:39:18 --> 00:39:21
On Wednesday we start kinetics.
 

577
00:39:21 --> 00:39:22