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Isomers in Coordination Chemistry
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0:01
- Isomers in coordination chemistry gonna be the topic of this lesson. My name is Chad and welcome to "Chad's Prep" where my goal is to take the stress out of learning science. Now, in addition to high school and college science prep, we also do DAT, OAT, and MCAP prep as well. I'll leave a link in the description below for where you can find those courses. Now, this lesson's part of my new general chemistry playlist, which is almost complete here. So, but for at least the next couple of weeks,
0:22
I'll still be releasing several lessons a week. So if you'd like to be notified every you have my post one, or when I get started on my next playlist and subscribe to the channel, click the bell notification. Isomers in coordination chemistry. So there's a range of detail in which this could be covered in your class. And some, you know, this might get towards the end of this semester and it might be left out altogether. And for some of you, they might just like pay
0:41
it a little bit of lip service. And for others of you, you might get a fair amount of detail on here, and you're my target audience. I'm gonna go into a fair amount of detail on these isomers. So it turns out for two structures to be isomers, they've gotta have all the same atoms. That's the first part. And it turns out there are two major classes of isomers. There are structural isomers and there are stereo isomers. And you kind of gotta know in principle, what's the difference here.
1:02
And so for structural isomers, we say they have a different bond connectivity. And in our complex ions, we're gonna find out that means usually that we're gonna have different atoms bonded directly to the central metal ion. So we're still gonna have the same overall atoms in the entire coordination compound or complex ion. But which ones are actually bonded to the central metal ion will change. So we call that a different bond connectivity. Now, on the other hand,
1:26
stereo isomers are gonna have the same bond connectivity. What's gonna be different is the three dimensional arrangement of the atoms. So all the same atoms will be bonded to that central metal, but how they're distributed three-dimensionally around that central metal ion, that's what's actually going to be different. So those are two, you know, different classes. Same, I'm sorry, different bond connectivity or same bond connectivity but different three-dimensional arrangement.
1:48
Now there are two types of structural isomers. There are also two types of stereo isomers. The two types of structural isomers are gonna be linkage isomers and then coordination sphere isomers. So two types of stereo isomers will be geometric isomers and then optical isomers. And we're gonna take some time to talk about all four of these different classes, and then how you can recognize, either by a formula or by the structure, which of these types of isomers you might actually have.
2:11
So let's start with linkage isomers here. Again, this type of structural isomer. So we're gonna have a different bond connectivity here. And so with linkage isomers, these only apply to certain ligands, and these ligands are those that might be able to link to the central metal ion through more than one atom. So if you recall, the hallmark of one of these ligands that are acting as a Lewis base is they've gotta have a lone pair of electrons. Well, it turns out that for thiocyanate,
2:36
we kind of draw out its structure.
2:45
Looks like this here. And it turns out because the sulfur has loan pairs and because the nitrogen has loan pairs, it turns out we've got an option here. It might be bonded through the central metal atom, through the sulfur atom in which case we see this here. Notice the bond is, in every case, to the sulfur. So when we call that thiocyanate as a ligand. So on the other hand, though, if nitrogen's got a loan pair and if it's bonded through the nitrogen as we see here,
3:10
so we call that isothiocyanate instead. So, but notice it's all the same atoms, but which ones are directly bonded to the central metal ion is what's different. Here, the iron is bonded to all the sulfur atoms. Here, the iron is bonded to all the nitrogen atoms instead. There's only a handful of ligands you might have to worry about this, and the other one you might see is cyanate here.
3:35
So, and again, because the two atoms on either end both have a lone pair of electrons, you got two options here, and we've got cyanophades bonded with the carbon or isocyano if it's bonded through the nitrogen. So same kind of thing here. It is named differently depending on which one. And when they put it in the formula, they will actually write it in the order in which it's bonded. So here, it's bonded through the sulfur so they wrote the sulfur first.
3:55
Over here, it's bonded through the nitrogen so the nitrogen's written first in the formula when identifying the ligand. So those are linkage isomers. So the next type of structural isomer is gonna be the coordination sphere isomer. Let's get a couple of those on the board. So the next type of structural isomer is what we're gonna call coordination sphere isomers. And it turns out that these come in two varieties as well. And what's really gonna be different
4:14
about these coordination sphere isomers is what's in the coordination sphere. So the ligands in the central metal ion. So, and then what's out of the coordination sphere, which is usually we think of as ions, but it turns out we can also have what are called hydrates, water molecules that are kind of held within the crystal structure in different places. So, but they're outside of the coordination sphere. So first off, we'll start with ionization isomers,
4:35
and this is just gonna be a difference of which ions are acting as ligands in the coordination sphere and which ones are just counter ions outside the coordination sphere. And so in this first example, we can see we've got four water ligands, four aqua ligands, and then one bromo, one chloral ligand, and then a chloride counter ion. But in the next one, what you'll find out is that this chloride and the bromo are gonna trade places so that we end up with two chloral ligands instead of just one.
4:59
And then the bromide ends up as the counter ion instead. And so which ions are inside and outside the coordination sphere, which ones are acting as ligands versus which ones are acting as the counter ions. That's what's the gonna be the difference in these ionization isomers. And again, what atoms are directly bonded to that central metal ion are indeed different. So again, this is a type of structural isomer. So, and it's not that the same ligands are bonded as part of the coordination sphere,
5:24
like we saw in linkage isomers just with different atoms being bonded. But here we have what's actually bonded to the central metal ion as far as the ligands goes is different and then what's outside the coordination sphere and they're just trading places. So the other type we're gonna have, the other type of coordination sphere isomer is what's gonna be what's called a hydration isomer. So when we have hydrates, and so there are certain molecules that just happen to absorb water.
5:45
And oftentimes we put little packets of silica gel and stuff like that in your luggage or new clothes or things of as sort 'cause it absorbs water. They're hygroscopic, we say. So it turns out, some of these coordination compounds are also potentially hygroscopic, and they might absorb water. And we can't really put it on the structure here in any, you know, identifiable way because it's really in the, gonna be put in different places within the entire crystal structure
6:08
of a coordination compound. So, but the way we represent it was if we put a dot H2O, and it might be one water, it might be two waters, it might be seven waters. It just some variable number of waters associated with the structure. And so in this first example, we see we've got four aqua ligands and two chloral ligands in the coordination sphere of bonded chromium. Then one chloride counter iron and one water that is part of a hydrate here. But again, still not part of the hydrate.
6:32
I'm sorry, still not part of the coordination sphere. But in the second example, now we've got five waters. One chloral ligand and then two chloral counter ions. And so effectively what's happened is that one of the chloral ligands has traded places with the water here to get this second one here. And so we do have a different bond connectivity, that's structural isomers. What's in and outside the coordination sphere is different. So the coordination sphere isomers,
6:55
and then it's actually waters that are being traded inside and outside the coordination sphere so we'll call them hydration isomers. And so now we've covered all of the structural isomers and students don't usually struggle as much with these as they do with the stereo isomers, which we're gonna devote some time to talk about now. All right. So let's talk about geometric isomers. This is our first of two types of stereo isomers. And again, a stereo isomer,
7:20
just recall that has the same bond connectivity but a different three-dimensional arrangement of the atoms. And geometric isomers can also be called cis-trans isomers. And this different three-dimensional arrangement really just comes down to when two groups are either 90 degrees apart or 180 degrees apart. And when they're 90 degrees apart, we call it cis. And when they're 180 degrees apart, we call it trans. So that's gonna be kind of the deal here.
7:41
And it turns out there's two different geometries where this is possible. So with a coordination number of four, it's possible in square planar complexes, because two groups can either be 90 degrees apart like they are here or 180 degrees apart like they are here. Now, before I can ask you to recognize that these are 90 versus 180, there's a little little tool here we're using when drawing these three dimensional structures on, well, I guess these technically are planar,
8:04
but we're trying to draw them with some perspective coming off the whiteboard here. And the idea is that these, we call these wedged bonds, these darker ones right here. And when you use a wedged bond, it means it's coming out of the board. So this bromine would actually be out in front of the board here and this chlorine out in front of the board over here as well. And then we refer to these as dashed bonds and they mean they're actually going into the board away from you.
8:27
So, and again, this is just a way of representing some three-dimensional perspective. And so in this case, these two chlorines again are 90 degrees apart. So, but in the next one, these would be exactly 180 degrees apart. This chlorine would be coming out of the board and off to the right. This one would be going into the board and off to the left, and they'd be 180 degrees apart. And so when they're 90 degrees apart, we call it cis. And when they're 180 degrees apart, we call it trans.
8:49
Now you've gotta be a little careful when you've got a coordination number of four. So, and it turns out only certain metals like platinum can even have a coordination number of four, or at least commonly will have a coordination number of four, but you gotta also remember that there's two geometries possible, square planar and tetrahedral. Had these been tetrahedral, well, on a tetrahedral structure, all the bond angles are 109.5. There's no like 90 versus 180. It's just all 109.5.
9:14
And so in a tetrahedral geometry, cis and trans isomers, geometric isomers are not even possible. So, but in a square planar geometry, they totally are. But the key is you gotta have exactly two of the same ligand. Not three, not whatever. So, and it's just two of the same ligand. Well, here I had two chlorines and two bromines. But the truth is I only had to have two of just one of 'em. Like, let's say I take one of these bromines and replace it with an iodine. Both here and say here.
9:40
Well, I'd still have a relationship between these chlorines being either 90 degrees apart or 180 degrees apart. And so the key is, you know, this is one example of way of getting exactly two of a ligand being 90 or 180, but it's not the only way. All right, the other ways we're gonna see this is two examples in octahedral structures. And in an octahedral structure, any two adjacent ligands are 90 degrees apart. So like these two are 90 degrees apart. These two are 90 degrees apart.
10:05
These two are 90 degrees apart. These two are 90 degrees apart. But you also then have the ones that are exactly on opposite sides that are 180 apart. So like these two are 180 apart. These two are 180 degrees apart. And then these two are 180 degrees apart. And so as a result, you now have this possibility again of having cis and trans. And so if you have exactly two of the same ligand again in an octahedral complex, they might end up 90 degrees apart in adjacent positions like this one,
10:30
which case we'd call it the cis isomer, or they might end up 180 degrees apart like in this example, in which case we'd call it the trans isomer. And notice in naming it, we just slapped the phrase cis or trans, the prefix, right on the front of the formula. And we put it right on the front of the name, it turns out, as well.
10:49
All right, we got one more here and let me get that up on the board. Okay, so the last place these geometric isomers are gonna show up here is in this case, we've got two bidentate ligands and then two monodentate ligands. And those two monodentate ligands can be the same. They can be different. And technically, I could have just had four monodentate ligands instead of these two bidentate. But this is probably the more common way it shows up. But the key is that the last two ligands you have,
11:12
whether they're same or different, you have the option, they could be 90 degrees apart like here, in which case that would be the cis version, cis isomer, or they could be 180 degrees apart as is the case here, and that's the trans version. So that's kind of the deal. And these are your three places, three most common places by far where you're gonna find these geometric isomers. Again, either in a square planar complex with two of, exactly two of the same ligand
11:37
or in one of these different variants of these octahedral complexes. Let's take a look at optical isomers. So now we'll finally talk about optical isomers, and these are the trickiest of the different isomers we'll talk about. And when I spoke the last lesson about drinking some coffee before getting to this lesson, this is the reason why. It's all about these optical isomers. So as a reminder, optical isomers are a type of stereo isomers. So they have the same bond connectivity,
12:01
but a different three-dimensional arrangement. And it turns out the reason they're called optical isomers is 'cause these types of compounds will actually rotate plain polarized light. And you're like, "Yeah, that makes perfect sense to me, Chad." No, you're like, what in the world's plain polarized light? And so we gotta talk about that. Well, it turns out light has wave-like properties and it can have like a vertical orientation or a horizontal orientation
12:22
or a diagonal orientation or anything in between. And so we would call that unpolarized light when it's just kind of got all these different orientations. But if you shine it through a polarizing filter, that polarizing filter, and this is like kind of the principle in which polarizing sunglasses work on. So when you shine it through that polarizing filter, all the different orientations but largely one are blocked. So let's just say we have a polarizing filter that blocks
12:42
out everything except the vertically oriented light. Okay, so that's plain polarized light. Well, it turns out if you shine that plain polarized light through a solution of most compounds, it just comes out vertical still on the other side. It hasn't been rotated in any way, shape, or form. However, if you shine it through a solution of something that is optically active, so which turns out these optical isomers, each of them individually is what we call optically active.
13:09
The light's not gonna come out vertical anymore. It's gonna get rotated one way or another. And these different optical isomers, it turns out they rotate light by exactly the same amount, but in opposite directions. Kind of a weird thing. So, but in some way, shape, or form, so this plain polarized light actually interacts with these lovely optical isomers. So it turns out these compounds that have optical isomers get the adjective, chiral.
13:38
So chiral compounds rotate light and they have optical isomers. So whereas achiral compounds don't rotate light and they don't have optical isomers. So all the compounds we're talking about in this section are going to be chiral molecules. And the way you recognize a molecule that is chiral, and this is gonna be a little bit strange, is that you have a molecule and its perfect mirror image and they're not identical. And the word we use is nonsuperimposable.
14:03
So when you have nonsuperimposable mirror images, those are optical isomers. And it turns out individually they're gonna rotate light as we'll see here in a minute. So, but if you take a look here, these are perfect mirror images just reflected right across a mirror plane here. So, and you might be like, "Wait a minute, Chad, that doesn't look like mirror images." So, but they are. And notice the iodine here's on the left. I'm sorry, on the right. Here, it's on the left.
14:25
Here are the bromines on the left and here are the bromines on the right. But again, don't forget what these wedged bonds and dashed bonds mean. So it turns out that our wedged bonds and dashed bonds there. So the chlorine's not on the left of the fluorine. It's actually the chlorine's right in front of the fluorine. So the chlorine's coming out of the plane with a wedge bond. The fluorine's going back into the plane with a dashed bond. And so if you look at it sideways,
14:45
you got one in the front, one in the back. And the one in the front is directly in front of the one in the back, but I can't draw it like that. I'd put the wedge in right on top of the dash and the chlorine right on top of the fluorine. And it would just look nonsensical. And so we just offset 'em a little bit, but that's what it means. And so when it's reflected while the chlorine is out here and it's still gonna be out here on the reflection and the fluorine's in the back,
15:07
and it's still gonna be in the back on the reflection. And whether you write 'em on the left and the right is irrelevant because they're not on the left or the right, they're right in front and behind each other. All right, so these are identical mirror, these are perfect mirror images of each other, but they're not identical. They're nonsuperimposable. Notice if you wanted to superimpose these, you'd, you know, if you wanna get this bromine right where this bromine is
15:27
and this iodine right where this iodine is, you wanna flip the whole thing over. But if you flip the whole thing over, well, that chlorine that is now pointing out at you, flip it over would be pointing away and it would be a dashed bond, not a wedge bond still. Same thing with the fluorine, it's a dashed bond, and it's going away from us right now. So, and once again, if we flip this whole thing over, well, then it would be pointing out of the board and would be a wedge bond, not a dashed bond.
15:50
So these are not the same structure. They're mirror images, but they're not the same. And when you have that being the case, we call these chiral. And it turns out we refer to the relationship between these two molecules, we refer to them as being enantiomers of each other.
16:07
Very funky word there. So chiral compounds have enantiomers, achiral compounds don't have enantiomers. Chiral compounds are optically active and have optical isomers. And these optical isomers, we refer to them as being enantiomers of each other. So there's some vocab words we had to learn here.
16:26
It turns out that either one of these compounds, one's gonna rotate light one direction and one's gonna rotate light to the other direction. So by the same amount, and we can't predict it by looking at it. You'd have to put 'em in what's called a polarimeter and just see which way they rotate light. And it turns out we actually name them quite often differently based on which way they rotate light. We talk about one rotating it in the positive direction,
16:47
one rotating it in the negative direction. And so you might have the plus isomer versus the minus isomer. So oddly enough. Now turns out for a coordination number of four, these optical isomers are possible for tetrahedral complexes but not for square planar. But it turns out that they only occur in these tetrahedral complexes when the central metal ion is bonded to four different ligands. All four have to be different. So if any two of these are the same, there's no optical activity.
17:15
So, and it turns out when you, what you'd find out is that when any two of these end up being the same on a tetrahedral structure, that, the result is that the molecule in its mirror image would end up being superimposable, absolutely identical if you rotate them around.
17:29
The last thing I did wanna say here is that if these rotate light in opposite directions, well, it turns out if you mix these two together in solution and you have an exactly 50, 50 mixture, it gets a special name.
17:45
We call it a racemic mixture.
17:49
So, and it turns out if, you know, 1/2 the molecules in your solution or 1/2 the ions in this case in your solution wanna rotate light one direction and 1/2 of 'em wanna rotate light in the other direction, well, then this light passes through the entire solution. It should on average encounter the same number of each type of molecule. And as a result, not be rotated in any net fashion. And so it turns out these racemic mixtures, which is a 50, 50 mixture of the two different enantiomers,
18:12
that's, that mixture ends up being optically inactive. It doesn't rotate the light. So either one of these separate rotates light. But 50, 50 mixture, it has to be exactly 50, 50. You can't have an excess of one over the other. So over that exact 50, 50 mixture is a racemic mixture. It is optically inactive.
18:31
All right. So you get to take an organic chemistry in the future, you'll find out that there are oftentimes, you'll have carbon being at the center of a tetrahedral structure. And quite commonly, you'll visit this kind of chiral structure with enantiomers and racemic mixtures. All this vocabulary is something you will visit again. So, but there's two examples in octahedral complexes we gotta talk about, and you won't see these two in organic chemistry at all.
18:55
So but the same rules will still apply. We'll still talk about optical isomers and enantiomers and chiral. So however, recognizing when these are possible is gonna be a little bit trickier. So the second example of an optical isomer here is gonna be when you've got three bidentate ligands in an octahedral complex here. And so here, I've got Fe ethylene diamine three here. So Fe en , two plus and octahedral three bidentate ligands. And if you take the exact mirror image,
19:22
and you can see that these are mirror images of each other, it turns out these are not superimposable. And I'm having to tell you this because looking at this and remembering what wedges mean and what dashes mean, if you can magically see this, it must mean like you are a crazy artist or something who can see three-dimensional things. And so I've drawn this and I've taken the time to draw it, but I don't actually expect most of you to see this. I struggle to see this. So, but if you built
19:44
a three-dimensional model of this structure, you'd find out that there are two mirror image versions of this that are not identical, they're nonsuperimposable. And so these are optical isomers, and this is the second case where you should recognize it from a formula when you've got exactly three bidentate ligands on an octahedral complex. So I drew it for you again. But most likely, you're gonna recognize it from a formula just 'cause you've memorized when these optical isomers can exist.
20:10
But these are mirror images, they're nonsuperimposable. We could describe either one of these as being chiral. They're optically active. These are enantiomers of each other. And if you mix them in a 50, 50 mixture, that would be racemic mixture. All right, so the last example, we're gonna encounter optical isomers here. So once again, we're gonna come in an octahedral complex. When you have exactly two of a bidentate ligand and two of a monodentate ligand. If we take a look at these two,
20:35
so they're perfect mirror images of each other here. I've got the two chlorines on the wedge and dash on the left here. They're on the wedge dash positions on the right. They're mirror images of each other. But if you flipped this guy over to make them match up, the chlorines would indeed match up in that sense. But you'd find out that your ethylene diomines would not match up. So if you flip this over 180 degrees, well, this dash then is gonna end
20:59
up with a wedge and you'd find out that the top one would actually be in connected right here with old Fe between these two and then between these two instead. And so these perfect mirror images are not identical. And once again, this is difficult to see, especially from a two-dimensional drawing. And if we built a three-dimensional model, you could see it. So, however, what a best way to kind of come to approach this is, is just pure memorization again. So, and again, these actually,
21:25
these two chlorines had to be 90 degrees apart as well. They had to be cis. So octahedral complex cis, two bidentates, two monodentates. That's gonna have optical isomers. The two mirror images are going to be different from each other. Not identical, nonsuperimposable. They are enantiomers. This guy's chiral, this guy's chiral, this guy's optical active, this guy's optical active. And relative to each other, we can call them enantiomers or we can call them optical isomers.
21:52
I just wanna point out the trans version here. This guy is not chiral. This guy is not optically active. He does not have an enantiomer. He does not have an optical isomer. If you were to draw the perfect mirror image of this guy, it would look exactly like this. Now if you just try and reflect it, well, if you do a perfect reflection, the chlorine would still point up. This one would still point down and these would look exactly like this. It would look exactly the same
22:14
'cause it's perfectly superimposable, perfectly identical. And so the trans version of this does not have optical isomers. So, but notice we can get tricky here. So if I said, how are these two related? You should say optical isomers or enantiomers. But notice I could say, well, how are these two related? Well, these two are isomers as well. They're cis and trans isomers. That is geometric isomers. A type of stereo isomer. But again, they're not optical isomers.
22:43
If I said, how are these two related? Same thing. Again, these are cis trans isomers. And again, they're gonna be geometric isomer, a type of stereo isomers, but they're not gonna be optical isomers. So lots of different possible relationships now of all these different isomers. And again, it is usually the stereo isomers that give students a little more fits. And again, seeing the three dimensionality is the biggest pain of the butt. So know some definitions.
23:05
So, and then as far as the stereo isomers go, know kind of based on how the formula looks, based on what I've looked, you know, listed here with examples on the board of what I've put on the study guide here. But know, based on looking at a formula, should that be capable of optical isomers or geometric isomer or something like this. And in this case with that exactly two of a ligand in an octahedral complex, you should be like, "Yeah, that's gonna be capable of cis and trans isomerism,
23:30
geometrical isomer, geometrical isomerism." But then also look and say, "Two bidentates, two monodentates? Chad taught me that that's also gonna be capable of optical isomerism as well." Now, if you found this lesson helpful, a like and a comment, let me know are pretty much the best things you can do to support the channel. And I've stated about my goal here is to take the stress out of learning science. But my secret goal has also meant that when you say you're such a Chad,
23:54
that you mean something other than what you currently would mean when you say that. I just wanna remind you about my general chemistry master course that includes final exam rapid reviews, practice final exams in case you're studying for finals. And also if you've got a preference on what I do for my next playlist, biochem, redoing physics, or biology are all in the running. Let me know in the comment section below for that as well. Happy studying.
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