Isomerism in Coordination Complexes - Online Tutor, Practice Problems & Exam Prep

• Isomers are molecules with the same molecular formula but differ in connectivity or spatial orientation.
• Structural isomers include coordination isomers, where anionic ligands and counter ions switch places, and linkage isomers, where the connectivity between the ligand and the metal varies due to different donor atoms.
• Stereoisomers differ in the spatial arrangement of atoms; the connections remain the same but their positions change.
• Geometric isomers, occurring in complexes like MX2Y2 and MX2Y4, differ by the relative positions of ligands around the metal, classified as cis (same side) or trans (opposite sides).
• Coordination isomers and linkage isomers are subtypes of structural isomers, with the former involving exchange between ligands and counter ions, and the latter involving different connectivity due to resonance in ligands like thionocyanate.

Isomers

Now recall that isomers are molecules with these same molecular formula but different connectivity or spatial orientation. Here in the middle we have our coordination complex which is comprised of our coordination complex ion and this counter ion of bromide. With a structural isomer you'd have the same exact formula, overall formula, but different connections. The way we could do this is we could take one of the chlorine's here and this bromide and have them swap places.

So now our bromine would be here and our chloride ion would be out here. Both structures have the same formula, but they have different connectivity because now we're connected to the bromine instead of two chlorines. With sterile isomers, we're talking about differences in spatial orientation. So the connections are the same, they're just represented differently in terms of spacing. So what I could do here is I have two chlorines on the same side here and two ammonia on the same side.

I can just swamp that. Now I have an ammonia on this side and have a chlorine on this side. The connections are still the same. Our metal cation is still connected to two chlorides and two ammonium ammonia molecules. It's just now they're not on the same side with each other. So this will count as a stereo isomer. So keep this in mind when we're talking about isomers overall, we'll take taking a look at differences in connectivity or in spatial orientation.

Structural Isomers

Now when we talk about structural isomers, we're going to say they consist of two types. We have coordination isomers or linkage isomers. In coordination isomers, we're going to say these are molecules where the anionic ligand and a counter ion and have switched places. So if we take a look here at these two, what can make them coordination isomers? Well, we have an anionic ligand. Let's say our ligand here that's aniotic is the bromide ion and it's going to switch places with a counter ion.

Now the charges of them both have to be the same. So if I have a -1 here, I need to have a -1 ligand attached to the metal. So let's put a chlorine here for us to make its isomer. I just have those switch places now. My bromide ion has come here and my chloride ion is out here. So we swapped in an aniotic ligand for another one.

Now linkage isomers. These are molecules where the connectivity between the ligand and the metal is different. So one example we have here is with thinosinate. Remember, thinosanate has resonance involved and because of that we could either have the nitrogen be the negative end or the sulfur being the negative end. When we talked about donor atoms, we said that the negatively charged atom within a ligand would act as the donor atom, and because nitrogen or sulfur can be negatively charged, either one could be the donor atom.

This will result in linkage isomers 1 where we have these sulfurs directly connecting to our silver ion or one where the nitrogen directly connects to our silver ion. So these will be two linkage isomers and as a result of the ligand itself possessing resonance meaning multiple elements could be the donor atom. So just keep in mind when we talk about structural isomers, we're talking either about coordination isomers or linkage isomers.

Isomerism in Coordination Complexes Example

Draw one coordination isomer and one linkage isomer of the following complex. Alright, so here we have our coordination complex. If we want to draw a coordination isomer, that means we have to substitute a counter ion for a ligand that's attached to the metal. Here we could do that. We still have our platinum. Here it would still be connected to the ammonias and it's the thiocyanate ion that's negatively charged, like the bromide counter ion that's negatively charged.

So I would just switch out the final cyanide or thiocyanate anion and bring in the bromide and then your file signing ion is on the outside. This will represent a coordination isomer for our linkage isomer. The ammonias are still there and what we're doing here. Remember, thiocyanate has resonance involved. Here we show the sulfur directly connecting to the metal cation, what I could easily have the nitrogen directly connecting to it instead, and the bromide ion would still be on the outside as a counter ion.

So this here would represent a linkage isomer right? So these are examples of a coordination isomer and a linkage isomer based on the original coordination complex given.

Geometric Isomers

With geometric isomers, we're going to say this is where a ligand has different spatial orientation around the metal. Now this occurs in complexes of formula M2X2Y2 and M2X2Y4. Here we're going to say CIS equals the ligand pair on the same side and trans equals ligand pair on opposite sides.

So if we take a look here at M2X2Y2 here, we could say that X could represent the ammonia and then Y could represent the chlorine. If we want to make this CIS, that means same side. We'd have to put a chlorine here so that they're both on the same side and we put an ammonia here so they're on the same side. Trans. Here they'd have to be opposite, so there would have to be on different sides. So the chlorine would be here. And now they're kind of opposite of each other, opposite sides. And the ammonia would be here again opposite sides.

With M2X2Y4. Here, let's say we're using as our X2 chloride ions. Again, for them to be CIS, they'd have to be on the same side. So I'd put a CL here. Actually, let me just make this yeah, we'll put a CL here and AC all here. They're both on the same side, and to make them trans to each other, they'd have to be on opposite sides. So a seal would go here and a seal would go here. Now they're on opposite sides of each other.

So again, geometric isomers, We're talking about different spatial orientation around the metal, and this happens when the formula is M2X2Y2 or M2X2Y4. So keep that in mind when looking at different types of geometric isomers.

Isomerism in Coordination Complexes Example

Identify the following pairs of complexes as coordination, linkage or geometric isomers. For the first one, look for where the difference is. The difference is here and here. In the first one we have no two where the nitrogen is connecting directly to the cobalt metal cation, but then next to it it's the oxygen within the nitrite ion that's connecting to the metal cation. How is this possible? Through resonance? So we'd say the first one is a linkage pair or a pair of linkage isomers.

For the next one, look to see where is the change. Where is the difference here? Well, in both they're connected to Nichols, connected to two waters and it's connected to to ammonia. But as we can see in the first one, the waters are on the same side, the ammonias are on the same side, but the iceberg next to it, they're opposite of each other now. So this would be a geometric pair of isomers.

So next let's see. Next we have what? What's the difference? Well, the chlorines here are next to each other, making them cysts, and now they're opposites of each other, making them trans to each other. Also remember in this first example, if we look the formulas are what MX₂Y₂ and these two are MX₂Y₄. Those tell us that these are geometric isomers as well.

Then finally the last one. What's the difference between these two? Well it looks like we have a branch here and a CL here and it looks like they swaed places. We substituted out the counter ion and the basically anionic ligand here. This is an example of coordination isomers, so that's what we can say about each of the following pairs given to us within this example problem.

Which of the following complexes cannot have geometric isomers?

i) [PtCl₂(NH₃)₂]

ii) K₄[Fe(CN)₄(OH)₂]

iii) [Ag(NH₃)₂]Cl

iv) [Ni(H₂O)₂(NH₃)₂]Br₂

• A. i, ii, and iv
• B. ii and iv
• C. iii only
• D. i and iii
• E. iv only

The complex [Fe(NH₃)₅OCN]²⁺ has two isomers. Draw their structures.

• A.
• B.
• C.
• D.

How many isomers are possible for [Cu(H₂O)₂(NH₃)₂]SO₄? Draw their structures.

• A. 2
• B. 2
• C. 2
• D. 3