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Chemistry: Coordination Compound, Isomer Structure


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  • Type: Video Tutorial
  • Length: 15:27
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 166 MB
  • Posted: 07/14/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Transition Metals (9 lessons, $14.85)
Chemistry: Coordination Compounds (3 lessons, $5.94)

This lesson was selected from a broader, comprehensive course, Chemistry, taught by Professor Harman, Professor Yee, and Professor Sammakia. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more.

Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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We've talked about coordination complexes and how to name them. Let's spend some time now talking about the types of geometries you are likely to see with these materials. Like organic compounds, one of the types of geometries you'll see for coordination complexes is a tetrahedral geometry. You'll also see an octahedral geometry, is a very common motif for these materials. Remember octahedral complexes have 90 degree angles between the bonds, approximately, whereas tetrahedral, more close to 109. You'll also see a fair amount of materials that are in a square-planar geometry. And that's 4 ligands all in the same plane with bond angles close to 90 degrees. And you will occasionally see 5-coordinate complexes, or 7-coordinate complexes, even 8-coordinate systems.
So with transition metals, in particular, you can see very large numbers of things attached to the central atom, especially when we're talking about deep in the Periodic Table where the sizes are larger. So I would encourage you to review what you've seen before about the geometry of molecules, because the same exact types of arguments would apply to what you're seeing here.
What I want to focus on now are isomers. Isomers are molecules that have the same number of atoms making them up, the same molecular formula, but nonetheless, they're different materials. They have different chemical or physical properties. So they're similar in the sense that they have the same atoms making them up, but again they are fundamentally different. So let's spend a little time looking at different kinds of isomers.
Now when I use the word isomer; that corresponds to a relationship in the same way that I may be the brother of somebody else, or the father of Dustin and Lora. When I use that term, I'm always describing a relationship. It doesn't make any sense for me to just say father, because that doesn't describe what I'm a father to. We use that term isomer in the same way. So you wouldn't talk about a molecule being an isomer. You'd talk about it being an isomer of something else.
So our first definition here will be constitutional isomers. These are molecules that differ in their fundamental constitution. What I mean by that is how they're connected together. What makes them isomers again is we have the same number of atoms, the same molecular formula, but the way they're connected is different. So for example, if I have a molecule here consisting of a red, a green, a brown, a black and a white ball, I'm going to now make a constitutional isomer of this material by disconnecting this bond and connecting the white ball to the red ball instead of the black ball. What I've done is change the order in which the atoms are connected. Who is connected to whom, if you will? But of course I have the same set of atoms making that up. So the relationship between this molecule and this molecule is, again, that they are constitutional isomers of each other.
Now where do we see this? Well an example in organic chemistry comes with alcohols. This is ethanol, which we've discussed before. An isomer of ethanol is dimethyl ether, for example. It has a different functional group in this case, but the important point is that it's a carbon, connected to an oxygen, connected to a carbon now, instead of a hydrogen, connected to an oxygen, connected to a carbon. So again how the atoms are connected is different, so constitutional isomers.
An example that shows up in coordination complexes is something called linkage isomers. Linkage isomers is just a subset of constitutional isomers. So really there's nothing special about this term, except that it describes a particular situation where a ligand connected to a metal has been connected in a different way to that metal. The linkage, if you will, is different. So whereas here, the bond between the ligand and the metal is through sulfur, the bond between the ligand and the metal here is through nitrogen. So these are referred to as linkage isomers of each other. And that's just a subset of the idea of a constitutional isomer. Down here is another example of where we have a nitrite ligand connected either through nitrogen or through oxygen, once again linkage isomers.
Now another type of isomer that exists, this relationship between molecules, is stereoisomer. Molecules are stereoisomers of each other if they are constitutionally identical, as well as having the same molecular formula, but they still are different molecules. And they differ by their geometry, the geometry of the molecule. Let me back up a step and say that stereoisomers actually come in two subclasses--usually it's divided that way--diastereomers and optical isomers, or enantiomers. Those two words often are used interchangeably. Diastereomers often are called geometrical isomers also. So again, these two names can be interchanged practically.
Let's talk about this type of stereoisomer first. A diastereomer, or geometrical isomer, is describing a relationship between molecules that have the same constitution, but differ in the geometry within the molecule. So for instance, consider this molecule butene. This is in fact 2 butene. And butene tells us it's a four-carbon molecule with a carbon-carbon double bond, but I want you to look at the orientation of these two methyl groups. These carbons both are on the same side of the molecule. You can imagine a different molecule in which the carbons are connected on opposite sides of the molecule. Notice that this methyl group points away now from this other methyl group. These molecules are not interchangeable, meaning that we can't easily rotate on this bond to swing that methyl group down here, so they're distinct molecules. You can put them in separate bottles. They don't inter-convert with each other, but again they have the same connectivity, in other words they have the same constitution, but they do differ in their geometry.
Examples in terms of coordination compounds are these two molecules shown here. And you'll notice in this complex on the left, the chloride and the bromide are 90 degrees apart from each other, whereas down here the chloride and bromide are 180 degrees apart from each other. So once again they have a different geometrical relationship. So they're geometrical isomers, but they have the same constitution. And once again I point out, you can't easily inter-convert one into the other by rotating on a bond or something like that. So geometrical isomers are diastereomers. And I should mention quickly that diastereomers have different chemical properties, different physical properties, so they really are completely different molecules.
Now the last term that we're going to be introduced to is stereoisomers called optical isomers, or enantiomers. Once again this is a subclass of the relationship stereoisomer. But this particular subclass has the relationship that they have the same geometry. Well if they have the same geometry, aren't they indeed the same thing? I want you to consider something. Take a look at these two molecules, and let me orient them in such a way that you can readily see that they have a special relationship. They are indeed mirror images of each other. But what I want to convince you of is just because they are mirror images, it doesn't make them, in fact, the same thing. For if I try to superimpose this molecule on this one, I run into some problems. If I try to line up the green ball and the red ball, for instance, the brown ball and the white ball, those don't correspond. If I tried, on the other hand, to line up the brown balls and the white balls, let's see if I can do that. I have a heck of a time doing that. See there's no way I can get them lined up now. But the green and the red no longer correspond to each other. So in fact, these are two fundamentally different molecules, and the only way that they can inter-convert is to break a bond. I would have to actually break this bond and reform it somewhere else to convert this one into this one.
All right, so what if they're mirror images? I'll concede that they are different, but does that have any significance? Well in fact, it has a lot of significance. By the same token that they are different hands of each other, in other words this being a mirror image of this. We often talk about it being handed, in that its mirror image is non-superimposable. So again they're referred to as handed molecules. This hand will interact differently with another molecule that's handed. Now what am I talking about? Well let me show you a silly analogy for a moment. Think about how my right hand will interact with this right hand. And I apologize for the low class of this tutorial using a false hand, but you'll notice that the right hand interacts with the right hand in a fundamentally different way than the right hand interacts with my left hand. So that would, by analogy, correspond to a different chemical reactivity with the different hand. So what I'm telling you is that molecules that are mirror images can react very differently with something else that's handed.
Let me give you a wonderful example of that. The scent that's responsible for what you consider to be a spearmint flavor or smell, is the exact same molecule that's responsible for the flavor or the scent that you get from caraway seeds. These are exactly the same molecules giving rise to these different sensations, except for the fact that they're mirror images of each other. And in your nose, you have receptors that are handed, or chiral, in that they have a particular handedness associated with it that interacts differently with the two different molecules, the two different enantiomers, or the two different optical isomers. I'm trying to use again both words so you get comfortable with both terms. There again, those are interchangeable.
What are examples? Well the one I just showed you is here. These two molecules would be optical isomers of each other, or enantiomers of each other in that they're mirror images, and they're non-superimposable. They are indeed different things, just like we showed here. Down here are two octahedral complexes that are also non-superimposable. Their mirror images, rather, are non-superimposable. So these two are considered enantiomers, or optical isomers. Once again, they can behave differently, if you are considering how they interact with something that's chiral.
Now quickly, where does this term optical come from? Well it turns out that you can take light, and you can pass it through a polarizer. A polarizer is actually what you have in polaroid lenses in sunglasses, for instance. And what the lens does is it polarizes the light. It plane polarizes it, meaning it gives it a direction if you will, a particular orientation. If you pass that light through this molecule, let's say, a sample of these molecules, it will cause that light to rotate a little bit. Whereas if you pass it through a collection of these molecules, it will cause the light to rotate in the opposite sense. So again this term optical isomers, historically, comes from the fact that the molecules interact differently with plane polarized light.
Now last question is how can you tell if a molecule has an optical isomer? In other words, this is a molecule, like many other molecules. I can take this molecule, and I can make a mirror image of it. But in fact the mirror image of this molecule will be itself. How do I know that? And how do I know that I can't do that with the molecules that I showed you earlier? Well in this molecule, I can identify a plane that passes through the molecule that, in fact, has the mirror image on one side of the plane, of the other. So again, there's a mirror plane of symmetry in this molecule passing right through the middle there. And you'll notice the orange and yellow balls are in that plane. And that means that the mirror image of this molecule will contain both halves, which are themselves, mirror images. So if you think about that for a moment, that ensures that the mirror image of this molecule will, in fact, be itself. And so unless I have a molecule that doesn't have a mirror plane, I won't have a molecule that can have an optical isomer. Imagine quickly what would happen if I take this off, and I replace it now with a green ball instead of a white ball. And now, nor matter what I do, I won't be able to find a mirror plane, a symmetry through this molecule. That tells me, since I can't find any plane of symmetry, that this molecule, if I take its mirror image, it will be non-superimposable with this one. In other words, they'll be optical isomers of each other.
Okay, so where are we? Just summing up quickly, molecules that are isomers of each other break into two categories, constitutional isomers that differ in how they're put together, how the atoms are connected to each other. Versus stereoisomers, where the atoms are connected the same, but there is still a difference. And that difference is within the geometry of the molecule. That geometry can be in the absolute sense, in that they can have fundamentally different orientations of the bonds, in which they're diastereomers of each other. Or they can have the relationship that they are optical isomers of each other, that they are mirror images of each other. One way to think about this is, if they're stereoisomers, and they're not mirror images of each other, they have to be diastereomers. This is kind of the catchall. Every other type of isomer relationship within the realm of stereoisomers, other than mirror images, would be diastereomers.
So okay we've spent a lot of time now classifying these different terms, but these relationships are really important, because it tells us about a molecule's differing ability to act chemically or physically, even though they have the same molecular formula.
Transition Elements
Coordination Compounds
Structures of Coordination Compounds and Isomers Page [3 of 3]

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