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Advanced Lewis structures

I recently came across problems asking us to draw the Lewis structure of N5+, cyclic N5-, P4, P4O6, P4O10, P3O105-, P3O93-.

I really enjoyed these. Can anyone say where I might find some more of this type of problem? (i.e. larger-than-usual and pretty complicated polyatomic inorganic molecules or ions).
(edited 10 years ago)

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B2H6

Helium compounds.

ylides

Al2Cl6..........

Bear in mind that the lewis structures do not adequately describe these systems.
Reply 2
Original post by JMaydom
B2H6

Helium compounds.

ylides

Al2Cl6..........

Bear in mind that the lewis structures do not adequately describe these systems.


I've seen both already :frown: Any sources of bigger molecules or ions?

I might add the likes of S2O62-, S2O82-, S4O62- to that list, if that adds better perspective ...
(edited 10 years ago)
Original post by Big-Daddy
I've seen both already :frown: Any sources of bigger molecules or ions?

I might add the likes of S2O62-, S2O82-, S4O62- to that list, if that adds better perspective ...


OK, how about intercalated LiC60 :tongue:

periodate, perchlorate, SF6, Benzene!!!!, azo compounds, P4, borazine, benzyne.....Please do bear in mind that this is all a bit pointless. If you want a challenge (which I guess is the whole point of this), move onto MO theory. Lewis structures are pointless frankly.
Reply 4
Original post by JMaydom
OK, how about intercalated LiC60 :tongue:

periodate, perchlorate, SF6, Benzene!!!!, azo compounds, P4, borazine, benzyne.....Please do bear in mind that this is all a bit pointless. If you want a challenge (which I guess is the whole point of this), move onto MO theory. Lewis structures are pointless frankly.


Thanks for the suggestions :smile:

I have started MO theory. The issue is that I'm often asked to draw the structure of a molecule or ion and the molecular orbital diagram is not what they're looking for! Also, it's quite fun :tongue:
Original post by Big-Daddy
Thanks for the suggestions :smile:

I have started MO theory. The issue is that I'm often asked to draw the structure of a molecule or ion and the molecular orbital diagram is not what they're looking for! Also, it's quite fun :tongue:


Do you get what I'm saying? Lewis structures are pretty much pointless, and the examples you will be given to work out will be fairly simple, probably because the examiners realise that they are very much an outdated view of bonding. i.e. nearly 100 years out of date.

I intentionally chose some examples that either are near impossible to draw lewis structures for, or give imply a structure which is totally different to the real structure.

You always seem to want a challenge when you post threads, so why not try something which is a huge part of undergraduate chemistry, and start MO theory.
Reply 6
Original post by JMaydom
Do you get what I'm saying? Lewis structures are pretty much pointless, and the examples you will be given to work out will be fairly simple, probably because the examiners realise that they are very much an outdated view of bonding. i.e. nearly 100 years out of date.


MO theory is its own field. However when it comes to drawing structures - if there is any need to draw structures (which, I agree, there isn't really, at least for inorganic) - MO theory does not replace the work that constitutes the majority part of difficulty in drawing Lewis structures (finding out which atoms have bonds between them in the first place). As I say, I have started MO theory.

The issue is "the examples you will be given to work out will be fairly simple" doesn't really seem to be true, given we've been asked to work out boranes with styx numbers, and the list of complicated ions I listed in my OP. If you call those fairly simple structures I would absolutely love to see where you get more of them (or harder ones!) from; but they can still be written, as far as I know, in Lewis form, without the need to apply MO theory (boranes not counting).

No time soon am I likely to reach the level with MO theory where I would start asking around for abnormally challenging questions. The exam I'm training for requires only a reasonable knowledge of MO theory, not a high-level or detailed one, whereas it asks questions like how to draw the (Lewis) structure of P3O93- which MO theory doesn't. On the basis that this exam doesn't include higher-level MO theory in its syllabus I am happy to put off mastering MO theory until I actually go to university.
(edited 10 years ago)
Reply 7
Things like structural isomerism in inorganic molecules would be up your street OP. Lends itself nicely to exotic NMR.


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Reply 8
Original post by Big-Daddy
On the basis that this exam doesn't include higher-level MO theory in its syllabus I am happy to put off mastering MO theory until I actually go to university.


If you don't actually want to learn the theory which actually explains these systems... why bother with all this intellectual masturbation?

Also, seeing as you asked for complicated stuff. The solid and solution state structures of organolithiums, amidolithiums and imidolithiums should occupy you for a while.
(edited 10 years ago)
Original post by illusionz
If you don't actually want to learn the theory which actually explains these systems... why bother with all this intellectual masturbation?

Also, seeing as you asked for complicated stuff. The solid and solution state structures of organolithiums, amidolithiums and imidolithiums should occupy you for a while.


Thats why I suggested intercalated KC60, also to say 'master' MO theory is a bit, well... either ignorant of how hard it can be, or very overconfident.
(edited 10 years ago)
Reply 10
Original post by illusionz
If you don't actually want to learn the theory which actually explains these systems... why bother with all this intellectual masturbation?


Because I'm asked for it! I'd love to learn MO theory to a higher level of detail than I need right now but I don't have time - my syllabus prescribes what I need, and I will complete it before moving on to topics it doesn't cover. ("My syllabus" referring to the International Chemistry Olympiad syllabus, not the A-level :tongue: )

Original post by illusionz
Also, seeing as you asked for complicated stuff. The solid and solution state structures of organolithiums, amidolithiums and imidolithiums should occupy you for a while.


I was asking for complicated inorganic molecules really :tongue: i.e. you give the molecular formula, I work out the Lewis structure. Rather than you give the group, I work out the chemical properties.
Reply 11
Original post by JMaydom
Thats why I suggested intercalated KC60, also to say 'master' MO theory is a bit, well... either ignorant of how hard it can be, or very overconfident.


Neither ... I said I will avoid attempting to master it until uni. Would it be more or less arrogant to say I will attempt to master it now? :rolleyes:
Original post by Big-Daddy
Neither ... I said I will avoid attempting to master it until uni. Would it be more or less arrogant to say I will attempt to master it now? :rolleyes:




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Its that you expect to master it which is arrogant. I have had 3 years of teaching on MO theory and would never claim to be anywhere near mastering it.
Reply 13
Original post by JMaydom
Posted from TSR Mobile

Its that you expect to master it which is arrogant. I have had 3 years of teaching on MO theory and would never claim to be anywhere near mastering it.


I will be disappointed if never in my whole life do I manage to master it. That said, I have only just started learning and have no clue about more difficult cases. Perhaps no-one in the world has truly mastered it (a bit like QM ...)

All this is beside my main point, though, which is that I can only afford to give a little bit of time to MO theory at this stage. "The rest" (if you will tolerate that) will have to wait until later.

I'm getting the impression that something like what I'm asking for is quite uncommon because people don't tend to face such complicated cases when at A-level, and when at undergraduate level the theory is focused on just drawing the molecular orbital diagrams rather than more complicated Lewis structures. That noted, it's unsurprising that you can't think of any complicated Lewis structures.

However, if anyone does have some suggestions, I would much appreciate them.
Original post by Big-Daddy
I will be disappointed if never in my whole life do I manage to master it. That said, I have only just started learning and have no clue about more difficult cases. Perhaps no-one in the world has truly mastered it (a bit like QM ...)

All this is beside my main point, though, which is that I can only afford to give a little bit of time to MO theory at this stage. "The rest" (if you will tolerate that) will have to wait until later.

I'm getting the impression that something like what I'm asking for is quite uncommon because people don't tend to face such complicated cases when at A-level, and when at undergraduate level the theory is focused on just drawing the molecular orbital diagrams rather than more complicated Lewis structures. That noted, it's unsurprising that you can't think of any complicated Lewis structures.

However, if anyone does have some suggestions, I would much appreciate them.


Well with that post you have just shown how little of MO theory you actually know, it is MO theory. It is all very well drawing MO diagrams, which any undergrad can do, but the theory behind actual MO calculations are some serious QM. Good luck trying to understand diracs equations.

We have given you very difficult Lewis structures, to the point where they are ridiculously difficult. I would have no problem working out the Lewis representation of any of the ions you have written as it is simply a case of following some very general rules.
There is no point in me doing this however, as it is not informative of the actual bonding scheme. For example, the basic treatment of the orbitals involved in the bonding scheme of PbO can explain why you get structural distortions in the solid state. Try explaining that with your darling Lewis schemes.
You do also get that the Lewis schemes are what we draw out when we draw skeletal formula? Adding dots and crosses to the diagram is not challenging. So actually to say you don't do any complex examples at uni is wrong.

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Reply 15
Original post by JMaydom
Well with that post you have just shown how little of MO theory you actually know, it is MO theory. It is all very well drawing MO diagrams, which any undergrad can do, but the theory behind actual MO calculations are some serious QM. Good luck trying to understand diracs equations.

We have given you very difficult Lewis structures, to the point where they are ridiculously difficult. I would have no problem working out the Lewis representation of any of the ions you have written as it is simply a case of following some very general rules.
There is no point in me doing this however, as it is not informative of the actual bonding scheme. For example, the basic treatment of the orbitals involved in the bonding scheme of PbO can explain why you get structural distortions in the solid state. Try explaining that with your darling Lewis schemes.


Haha did you really expect me to know more than "any undergrad"? :tongue: You probably already have a ballpark idea of how much I know of MO theory just given that I'm still an A-level student. I'm not doubting that it's a vast and complicated field. My post only meant to say that I will not be grappling with anything more advanced than my immediate necessities as far as MO theory is concerned.

Most important thing, though, is:

Could you outline these very general rules for me, or say where I might find them? I can understand that having a proper bonding understanding is more important but I'd at least like to start with a complete foundation. Clearly my understanding of Lewis representation is less than complete if there still exist principles I don't know and which I should be using (my attempts were more trial and error; I know the basics of how to draw Lewis structures but not how to predict how many bonds there will be between two atoms, etc., except for a few basics).

I'm guessing it's things like this: if X (halogen) has 1 bond, O 2 bonds, N 3 bonds or C 4 bonds these will be neutral; if X, O or N have 1 bond less than these, they will be negative in formal charge, whereas if C has 1 bond less it will be positive; if X, O or N have 1 bond more, they will be positive, whereas if C has 1 bond more it will still typically be positive. (Assuming there are no free radicals.) Do not expect to find a pi bond between Period 3 elements. Try to avoid atoms with the same formal charges being bonded. Try to delocalize the formal charges over as many atoms as possible. Try to ensure that, in a molecule with several different elements, a less electronegative one never bears a more negative formal charge than a more electronegative one. If there is a radical, then, unless superceded by the rules above, I can expect to place the lone electron on the less electronegative atom.

However, even with all these rules running through my head, drawing the structure of P3O93- isn't easy ... I would probably need more guidelines to avoid drawing the wrong structure. Any more principles I can use?

As I say, I understand the superiority of MO theory. But drawing Lewis structures isn't something I can avoid or go past for the time being, so please help me get them completely straight. It's less looking for a challenge and more making sure I can handle cases like these that I want to deal with this now.
Original post by Big-Daddy

Could you outline these very general rules for me, or say where I might find them? I can understand that having a proper bonding understanding is more important but I'd at least like to start with a complete foundation. Clearly my understanding of Lewis representation is less than complete if there still exist principles I don't know and which I should be using (my attempts were more trial and error; I know the basics of how to draw Lewis structures but not how to predict how many bonds there will be between two atoms, etc., except for a few basics).

I'm guessing it's things like this: if X (halogen) has 1 bond, O 2 bonds, N 3 bonds or C 4 bonds these will be neutral; if X, O or N have 1 bond less than these, they will be negative in formal charge, whereas if C has 1 bond less it will be positive; if X, O or N have 1 bond more, they will be positive, whereas if C has 1 bond more it will still typically be positive. (Assuming there are no free radicals.) Do not expect to find a pi bond between Period 3 elements. Try to avoid atoms with the same formal charges being bonded. Try to delocalize the formal charges over as many atoms as possible. Try to ensure that, in a molecule with several different elements, a less electronegative one never bears a more negative formal charge than a more electronegative one. If there is a radical, then, unless superceded by the rules above, I can expect to place the lone electron on the less electronegative atom.

However, even with all these rules running through my head, drawing the structure of P3O93- isn't easy ... I would probably need more guidelines to avoid drawing the wrong structure. Any more principles I can use?

As I say, I understand the superiority of MO theory. But drawing Lewis structures isn't something I can avoid or go past for the time being, so please help me get them completely straight. It's less looking for a challenge and more making sure I can handle cases like these that I want to deal with this now.


The problem with lewis structures is that you could draw more than one perfectly reasonable structure for many molecular formulae.

In your example, I would expect something similar to ATP with 2 bridging O atoms and the rest arranged such that each P is pentavalent and there is a net 3- charge. However another perfectly plausible lewis structure would be a 6 membered ring of alternating P and O atoms, with each P having a P=O and a P-O exo to the ring. You would need more advanced methods (MO theory/computers) to know which is more likely.

There is a nice set of rules which let you determine the structure of borane clusters (Wade's rules), and it can be extended to transition metal/p block clusters - so I would advise looking at those instead.
(edited 10 years ago)
Original post by Big-Daddy
Haha did you really expect me to know more than "any undergrad"? :tongue: You probably already have a ballpark idea of how much I know of MO theory just given that I'm still an A-level student. I'm not doubting that it's a vast and complicated field. My post only meant to say that I will not be grappling with anything more advanced than my immediate necessities as far as MO theory is concerned.

Most important thing, though, is:

Could you outline these very general rules for me, or say where I might find them? I can understand that having a proper bonding understanding is more important but I'd at least like to start with a complete foundation. Clearly my understanding of Lewis representation is less than complete if there still exist principles I don't know and which I should be using (my attempts were more trial and error; I know the basics of how to draw Lewis structures but not how to predict how many bonds there will be between two atoms, etc., except for a few basics).

I'm guessing it's things like this: if X (halogen) has 1 bond, O 2 bonds, N 3 bonds or C 4 bonds these will be neutral; if X, O or N have 1 bond less than these, they will be negative in formal charge, whereas if C has 1 bond less it will be positive; if X, O or N have 1 bond more, they will be positive, whereas if C has 1 bond more it will still typically be positive. (Assuming there are no free radicals.) Do not expect to find a pi bond between Period 3 elements. Try to avoid atoms with the same formal charges being bonded. Try to delocalize the formal charges over as many atoms as possible. Try to ensure that, in a molecule with several different elements, a less electronegative one never bears a more negative formal charge than a more electronegative one. If there is a radical, then, unless superceded by the rules above, I can expect to place the lone electron on the less electronegative atom.

However, even with all these rules running through my head, drawing the structure of P3O93- isn't easy ... I would probably need more guidelines to avoid drawing the wrong structure. Any more principles I can use?

As I say, I understand the superiority of MO theory. But drawing Lewis structures isn't something I can avoid or go past for the time being, so please help me get them completely straight. It's less looking for a challenge and more making sure I can handle cases like these that I want to deal with this now.


Yes those rules you have written seem pretty much correct. The main thing is experience that comes with time and a lot of reading.
We only suggest you mine on because you clearly look for challenges and usually want to know the real answer to a problem. You're very keen on your chemistry olympiad being similar to 1st tear undergrad do why not try some proper mo theory. I'm more that happy to help with any queries you have about it.
Reply 18
Original post by illusionz
The problem with lewis structures is that you could draw more than one perfectly reasonable structure for many molecular formulae.

In your example, I would expect something similar to ATP with 2 bridging O atoms and the rest arranged such that each P is pentavalent and there is a net 3- charge. However another perfectly plausible lewis structure would be a 6 membered ring of alternating P and O atoms, with each P having a P=O and a P-O exo to the ring. You would need more advanced methods (MO theory/computers) to know which is more likely.

There is a nice set of rules which let you determine the structure of borane clusters (Wade's rules), and it can be extended to transition metal/p block clusters - so I would advise looking at those instead.


Thank you. I will have a look at Wade's rules.

The structure I reached for P3O93- (I did manage to draw it) sounds identical to your second. I essentially intended it to be a chain but in the end had to make it cyclical to fit the requirements.

I'm not sure I get what structure you mean for the first one. What you suggest sounds pretty similar to my structure for P3O105- ...
(edited 10 years ago)
Reply 19
Original post by JMaydom
Yes those rules you have written seem pretty much correct. The main thing is experience that comes with time and a lot of reading.
We only suggest you mine on because you clearly look for challenges and usually want to know the real answer to a problem. You're very keen on your chemistry olympiad being similar to 1st tear undergrad do why not try some proper mo theory. I'm more that happy to help with any queries you have about it.


OK, I will try. Chances are I will go as far as I can with the orbital diagrams before looking at anything else though, unless this is a particularly bad idea?

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