[mistche20]

[Re2OCl10] 4-

Each Re has an octahedral geometry basically, with 5 Cl- attached to it. It also has a bridging O, which bridges the two Re together.

O also produces pi donation to each Re, so that there is basically a double bond between Re-O

But still... which can't there be a bond between the Re? there are unpaired electrons which can be paired by making this bond!!!! Can there be metal-ligand and metal-metal multiple bonding in the same molecule?

thank you!

[Schrödinger]

Well, looking at this molecule, I would say the bridging O atom sounds/seems more symmetrical, and when there is a symmetrical system available, that's probably more stable than the other non-symmetrical ones...
For example consider what you proposed, the oxygen would have to sit on one Re atom and the other will be forced to carry only Cl atoms. That seems a bit 'indigestible'

[mistche20]

Well, looking at this molecule, I would say the bridging O atom sounds/seems more symmetrical, and when there is a symmetrical system available, that's probably more stable than the other non-symmetrical ones...
For example consider what you proposed, the oxygen would have to sit on one Re atom and the other will be forced to carry only Cl atoms. That seems a bit 'indigestible'

no need to do that! the oxygen will be double bonded to both metals!!! I know it looks a bit strange, since it would look like a triangle with double bonds on each side, but technically should be possible right?

[Schrödinger]

What about oxygen maximum valence? You are violating that if there's a double bond between both the metal atoms and oxygen

[mistche20]

What about oxygen maximum valence? You are violating that if there's a double bond between both the metal atoms and oxygen

Uhm, well, Oxygen has two full orbitals with pi symmetry: py and px, which may interact each with one metal and hence for a double bond with each!

[DrCMS]

Well, looking at this molecule, I would say the bridging O atom sounds/seems more symmetrical, and when there is a symmetrical system available, that's probably more stable than the other non-symmetrical ones...
For example consider what you proposed, the oxygen would have to sit on one Re atom and the other will be forced to carry only Cl atoms. That seems a bit 'indigestible'

no need to do that! the oxygen will be double bonded to both metals!!! I know it looks a bit strange, since it would look like a triangle with double bonds on each side, but technically should be possible right?

Wrong

What about oxygen maximum valence? You are violating that if there's a double bond between both the metal atoms and oxygen

Uhm, well, Oxygen has two full orbitals with pi symmetry: py and px, which may interact each with one metal and hence for a double bond with each!

Wrong


Why do you think a Re-Re bond would be favoured in this molecule?

What would the Re-Re bond length need to be to prevent steric interactions?  Is that within the range of known Re-Re bonds?

[mistche20]

Well, looking at this molecule, I would say the bridging O atom sounds/seems more symmetrical, and when there is a symmetrical system available, that's probably more stable than the other non-symmetrical ones...
For example consider what you proposed, the oxygen would have to sit on one Re atom and the other will be forced to carry only Cl atoms. That seems a bit 'indigestible'

no need to do that! the oxygen will be double bonded to both metals!!! I know it looks a bit strange, since it would look like a triangle with double bonds on each side, but technically should be possible right?

Wrong

What about oxygen maximum valence? You are violating that if there's a double bond between both the metal atoms and oxygen

Uhm, well, Oxygen has two full orbitals with pi symmetry: py and px, which may interact each with one metal and hence for a double bond with each!

Wrong


Why do you think a Re-Re bond would be favoured in this molecule?

What would the Re-Re bond length need to be to prevent steric interactions?  Is that within the range of known Re-Re bonds?

uhm... but technically there are enough orbitals and electrons right? PLEASE SAY YEES!!!

[Schrödinger]

No there aren't enough orbitals the way you want your molecule. O lacks empty d orbitals. So, double bonds with both Re atoms is impossible. So, NO

[mistche20]

No there aren't enough orbitals the way you want your molecule. O lacks empty d orbitals. So, double bonds with both Re atoms is impossible. So, NO

What?? Why? Oxygen doesn't need empty d orbitals. Oxygen has 2 orbitals with pi symmetry, filled, and Re has also orbitals with the same symmetry, but empty!!! In fact, if you look at the structure, it says it has double O-Re bonds. What i'm asking if its is possible to have ALSO re-re bonds

[Yakimikku]

Hello mistche20, I hope I can answer your question and clear up a few things that have been talked about in this thread.

First, let's clarify about what molecule you are talking about:
The [Re₂OCl₁₀]^4- anion. It's structure can be thought of as two octahedral Re⁴⁺ centers bridged by an oxygen. The Re-O-Re is linear. So we have [Cl5Re-O-ReCl5]^4-.

Now, you are asking why this molecule doesn't instead have an extra Re-Re bond, making a triangle consisting of Re, Re, and O. Without considering the rest of the molecule, this is a reasonable idea. There are in fact examples in the literature that have that Re-Re-O triangular motif (although oxidation states may differ, I'm not sure if any of the examples are Re^IVRe^IV). However, I do not believe this motif is possible (or at least not stable) with the particular molecule you are considering. I think DrCMS hinted at the simplest reason for why this would not work--sterics. You have 10 chlorides--how could you arrange all of those about the two Re without having significant steric repulsion between the two ReCl5 pieces? An oxo bridged Re-Re bond should be pretty short. I'm not sure if this is the best answer though. I believe the Re-Re bond will only form if it lowers the energy of the system. That is, as you combine orbitals, you are getting stabilization (electrons filled in bonding orbitals). So I would guess that forming the Re-Re-O triangle doesn't give you more stabilization in terms of MO theory. If you want, you can try building a qualitative MO diagram for your proposed structure to see if there is an MO argument against it.

I have some other comments. I do not believe these are Re=O bonds--I think they are formally Re-O single bonds. There are definitely many Re=O bonds in the literature. You can examine this by looking at Re-O bond distances. I might offer though that with a quick literature search, I did see a Re=O-Re motif. In addition, I don't think your triangle with a double bond with each Re-O is possible. I believe bond order has to do with filling bonding orbitals and keeping anti bonding orbitals free of electrons. That doesn't mean there aren't Re-O pi interactions and I'm sure there is. Combining orbitals doesn't mean they are occupied.

A comment to Schrodinger: You mentioned oxygen maximum valence. This concept is best left with organic chemistry as it fails very quickly with inorganic compounds. I've seen examples (and have synthesized) compounds with oxygen atoms with up to 6 bonds.

[mistche20]

Hello mistche20, I hope I can answer your question and clear up a few things that have been talked about in this thread.

First, let's clarify about what molecule you are talking about:
The [Re₂OCl₁₀]^4- anion. It's structure can be thought of as two octahedral Re⁴⁺ centers bridged by an oxygen. The Re-O-Re is linear. So we have [Cl5Re-O-ReCl5]^4-.

Now, you are asking why this molecule doesn't instead have an extra Re-Re bond, making a triangle consisting of Re, Re, and O. Without considering the rest of the molecule, this is a reasonable idea. There are in fact examples in the literature that have that Re-Re-O triangular motif (although oxidation states may differ, I'm not sure if any of the examples are Re^IVRe^IV). However, I do not believe this motif is possible (or at least not stable) with the particular molecule you are considering. I think DrCMS hinted at the simplest reason for why this would not work--sterics. You have 10 chlorides--how could you arrange all of those about the two Re without having significant steric repulsion between the two ReCl5 pieces? An oxo bridged Re-Re bond should be pretty short. I'm not sure if this is the best answer though. I believe the Re-Re bond will only form if it lowers the energy of the system. That is, as you combine orbitals, you are getting stabilization (electrons filled in bonding orbitals). So I would guess that forming the Re-Re-O triangle doesn't give you more stabilization in terms of MO theory. If you want, you can try building a qualitative MO diagram for your proposed structure to see if there is an MO argument against it.

I have some other comments. I do not believe these are Re=O bonds--I think they are formally Re-O single bonds. There are definitely many Re=O bonds in the literature. You can examine this by looking at Re-O bond distances. I might offer though that with a quick literature search, I did see a Re=O-Re motif. In addition, I don't think your triangle with a double bond with each Re-O is possible. I believe bond order has to do with filling bonding orbitals and keeping anti bonding orbitals free of electrons. That doesn't mean there aren't Re-O pi interactions and I'm sure there is. Combining orbitals doesn't mean they are occupied.

A comment to Schrodinger: You mentioned oxygen maximum valence. This concept is best left with organic chemistry as it fails very quickly with inorganic compounds. I've seen examples (and have synthesized) compounds with oxygen atoms with up to 6 bonds.

Hello, first, thanks so much for your reply!

Second, I have tried doing the Mo, and the thing is THERE ARE ENOGUH BONDING ORBITALS!! But yes, I'm beginning to think that sterics wouldn't work here.

You've said that "stabilizing the system consists on putting electrons into bonding orbitals". I was just reading some papers and I read "electron donating groups incrase the energy of the molecule". I'm confussed!!!!!

[mistche20]

ACtually ,about what I just ask, it literally says:
(talking about pi donation to complexes)
"electron donors such as PMe3 raise the energy of metal centred non bonding electron pairs" ...

why?
thank youuuuu!!!

[Yakimikku]

Hi again,

Yes, it might just be sterics. My idea for an MO exercise would be to make an MO diagram for the known complex and for the Re-Re bonded version and try to qualitatively connect and compare the two systems by variance of the Re-O-Re bond angle. Even if there are enough bonding orbitals, there may be poor orbital overlap in the Re-O-Re triangle structure which makes it less favorable. Also, the Re in the latter would be 7-coordinate which would also have an effect on the electronic structure. From a quick literature search I do see 7-coordinate Re complexes to be known, thought it did not seem like there were many examples and so 7-coordinate Re might just be not favorable for some reason.

"electron donors such as PMe3 raise the energy of metal centred non bonding electron pairs" ...

Yes, I believe this to be correct, but I think more needs to be said. Consider an idealized octahedral coordination environment. d_z2 and d_x2-y2 interact with the sigma orbitals of the ligands producing bonding and antibonding orbitals. Since we focus our attention on the frontier orbitals, we don't worry about the bonding orbitals (ie, all the metal MO diagrams you see focus on the five "antibonding" side of the MOs). Thus, degenerate antibonding orbitals arise from d_z2 and d_x2-y2 (the e_g set). In a complex with only sigma-donor ligands, the remaining d-orbitals on the metal (d_xz d_xy d_yz) have no net interaction with the sigma orbitals and thus are non-bonding. Here you get the t_2g non-bonding orbitals. Now--when you change the ligands to pi-donors. The ligand pi orbitals can now interact with the d_xz d_xy d_yz orbitals. This forms a set of bonding orbitals (which again, we don't generally focus on) and a set of antibonding orbitals (a t_2g set). Since antibonding orbitals are destabilized relative to non-bonding orbitals, the pi-donation raised the energy of the t_2g set (previously non-bonding).

So, the statement I quoted above is only true in a comparison between assuming the ligands are sigma-donors to the ligands being pi-donors. It's just a qualitative thought experiment.