[dcosden]

Looking at the periodic table, Boron has 3 valence electrons while Lithium has 1. To me, they should be able to bond together and form a molecule containing 1 boron atom and 3 lithium atoms that would have each valence shell stable.

Looking online, I didn't find any of such compounds, which leads me to believe that they don't form. Is that true? If so, could anyone explain fundamentally why that is the case? I'm sure that I must be missing something.

I'm assuming it's not as simple as plugging any elements of the periodic table together and if their valence electrons match, they'd form a molecule. If someone could provide some insight/resources for me to learn more, it would be greatly appreciated.

[Corribus]

Both boron and lithium are most stable with positive formal charges. To create a molecule containing boron and lithium, one of them would have to take on a negative charge, which would be a pretty unfavorable situation.

(About the closest you'll probably get is lithium borate.)

[dcosden]

Could you explain why they would prefer a positive formal charge? If you include only 3 Lithium atoms and 1 Boron atoms in the molecule with nothing else, then each would have a formal charge of 0, with a full valence shell for each. To me, that seems preferable to having a positive formal charge, no?

You wouldn't need to add any other elements either. 3 of Lithium and 1 of Boron would fill each's valence shell (2p for Boron, and 2s for each Lithium), resulting in a formal charge of 0 for all and should (to me) result in a stable, non-reactive molecule: BLi3.

[Corribus]

General chemistry treatment of the problem:

Are you aware of the octet rule?

Basically, lithium (having a single valence electron) would prefer to lose 1 electron to reach a fully filled shell, and boron (having 3 valence electrons) would prefer to lose 3 electrons to reach a fully filled shell. This leaves both atoms with positive formal charges (for boron: +5 nuclear charge, -2 electronic charge). If boron and lithium bonded in this way, the molecule would not be neutral, which is not favorable.

You might consider the hydride analogue of boron - BH₃. Here boron is in a +3 formal oxidation state (satisfying full octet), and hydrogen is in a -1 formal oxidation state. This is unusual for hydrogen but it does satisfy the "octet rule" because the first shell only has 2 electrons. BH₃ isn't a very stable molecule, though, and nor are most hydrides, because reaction products are usually more stable where hydrogen can obtain a +1 or even 0 (for covalent sharing) formal oxidation state.

Lithium can't form a "hydride analog" and be happy gaining a single electron, because the 2nd shell requires a full 8 electrons to satisfy the octet rule. That means, in bonding, lithium either needs to lose 1 electron or gain 7. (Likewise, boron either needs to lose 3 or gain 5). Li gaining 7 and boron gaining 5 are both not very favorable situations.

This is why neither the complex Li₅B or BLi₃ are observed under normal conditions.

[dcosden]

Alright I understand. Thanks for the explanation and for taking the time to write it. I had mistakenly thought about boron as having 5 valence electrons instead of 3, thinking it would prefer to take electrons rather than give. But your explanation made me realize that was not the case.

[Enthalpy]

https://www2.ph.ed.ac.uk/~aherman2/Andreas_Hermann,_Edinburgh/Research_files/JAmChemSoc_134_18606_2012.pdf
http://jes.ecsdl.org/content/126/5/866.abstract
https://link.springer.com/article/10.1007/BF02403669
first page of googling: lithium boron

Some papers state "alloy". While boron isn't formally a metal, it's not far from it, and for instance silicon makes "alloys" with other metals. How much the name is correct or not, I have no opinion.

[Corribus]

@Enthalpy. From the context, and the fact that this is the high school chemistry forum, it was implied we were talking about isolated molecules. If the OP wants to extend the discussion to quasi-infinite solids, it's a whole different ball game of course.

[dcosden]

Basically, to provide some more context, I am working on building an educational platform that could be used to teach chemistry. Something like an interactive tool where you create your own vacuum, insert the elements you want, and have them auto-interact together accurately (as much as possible). The overall logic, and hence the reason for my question, is that I am seeing that elements interact together based on a set of rules which can be programmable.

Based on my understanding when asking the question, should you have a vacuum with only 5 lithium atoms and 1 boron atom, they'd form 5 covalent bonds (ignoring that I had mistaken that number to be 3 initially) and reach a stable molecule.

I'm still not sure what is the best way to approach this. Because from what I understand now, that molecule could theoretically exist under certain conditions, but as Corribus said, it is not the optimal structure for lithium and boron.

So now I am slightly at a loss of direction. If the purpose is to teach how chemical bonds/molecules form, what would be the best approach to this? Any advice/ideas?

edit: I just realized that Li5B wouldn't really work because they would all be covalent bonds. Consequently, each Li wouldn't be stable with only one covalent bond since it does not satisfy the octet rule, unless it was bonded to something with a higher electronegativity which would lead to it being stripped of its electron. Am I correct?

[Borek]

you create your own vacuum, insert the elements you want, and have them auto-interact together accurately (as much as possible). The overall logic, and hence the reason for my question, is that I am seeing that elements interact together based on a set of rules which can be programmable.

Beware, this is a can of worms. What kind of effect do you expect when mixing hydrogen and oxygen? Reaction and water production? Unrealistic, at room temperature they will stay unchanged practically for ever. No reaction? While correct, it is rather boring (and probably misses the point).

[dcosden]

Fair enough. But say I implement a temperature slider in the vacuum, as well as dimensions of the vacuum that will affect the pressure/energy/reactions. Although more complicated, it's an option.

Another option is to simply do it in a way that you put H and O in a simplified vacuum and you'd see all the possible molecules/bonds that could form (H2, O2, H2O, H2O2, etc..) regardless of temperature and you'd be able to explore each molecule and see its properties like what elements it's made of, what type of bonds there are, bond angles, temperature at which it forms, etc...

[Enthalpy]

This approach has the same kind of difficulty as molecule optimisation software has: it lets "synthesize" molecules that are unrealistic in real life.

Many molecules would just hold together, especially if made from individual atoms rather than from molecular elements, but in real life they snap apart because other combinations are much more favourable and there is a possible path for the transformation.

Software that lets "create" meaningless molecules would be counter-productive for education in my opinion, and I don't see any means, simple or complicated, to sort out what molecules are reasonable or not.

Even if basing on oxidation state: most elements have many possible oxidation states, but not every compound that satisfies some oxidation state of every atom is possible.

About the octet rule: Li₂ exists as a gas.
https://en.wikipedia.org/wiki/Dilithium
Xenon fluorides can be isolated and stored
https://en.wikipedia.org/wiki/Xenon_fluoride
With boranes, the idea of valence gets uneasy for hydrogen
https://en.wikipedia.org/wiki/Boranes
And so on.

Would oversimplified rules still be useful?

[Borek]

I don't see any means, simple or complicated, to sort out what molecules are reasonable or not.

Quite simple: table with all possible outcomes.

(I am not saying it is is trivial to prepare, it is trivial conceptually and trivial to implement).

[dcosden]

About the octet rule: Li₂ exists as a gas.
https://en.wikipedia.org/wiki/Dilithium

Interesting about dilithium. Didn't know of its existence before you mentioned it. I unfortunately don't yet know enough about chemistry to fully understand how that molecule can exist and be stable (is it?). What would happen if you introduce oxygen in a vacuum full of dilithium? Will it react? If not, then there must be some kind of logic as to why it doesn't, no?

[Corribus]

Would oversimplified rules still be useful?

At Chemical Forums - as in a classroom - I usually try to tailor my response to a question based on the forum (or class) in which the question is asked. This is the high school forum, implying students very new to the rules of chemistry. Yes, the octet rule is a gross simplification and a billion and one exceptions exist to it. But machine-gunning new students with all the exceptions there are to the rules they are about to learn doesn't serve them well at all.

[Enthalpy]

Yes.