Is Electron Possession a Misleading Concept for Formal Charges?

[etotheipi]

This isn't a specific question, but more a case of trying to mitigate any potential confusion which might arise in the future. When drawing out curly arrows for mechanisms, and the like, I'm used to thinking about which electrons "belong" to certain atoms (in a book-keeping sense) in order to work out the changes in formal charge. If a nucleophile went to form a bond with another atom, I'd think of something along the lines of "the nucleophile transfers possession of one of its own electrons to the other atom", and this would result in the formal charge of the nucleophile increasing by one, etc.

I was wondering whether this sort of interpretation/internal thought process is common? Of course, we'd need to be slightly careful to clarify exactly what we mean by the electrons an atom "owns", since we might determine this by splitting all of the bonds evenly (i.e. formal charges) or by awarding the electrons to the more electronegative atom (i.e. oxidation numbers).

 

[ZapperZ]

While that picture may "work" for ionic bonding, how do you picture a covalent bonding, for example, where two atoms "share" that same electron?

It may be fine if you recognize this as simply a matter of "bookeeping". It is another story if you extrapolate this to be physically realistic.

Zz.

 

[etotheipi]

While that picture may "work" for ionic bonding, how do you picture a covalent bonding, for example, where two atoms "share" that same electron?

It may be fine if you recognize this as simply a matter of "bookeeping". It is another story if you extrapolate this to be physically realistic.

Zz.

In the case of the nucleophile forming a covalent bond, to be accurate we would, like you say, need to picture the electrons as shared.

However, if we talk in terms of formal charges, before the bond formation both of the electrons in the lone pair contributed to the formal charge of the nucleophile. After the bond formation, one electron now counts toward the formal charge of the recipient atom. In a loose, formal charge, sense, the recipient atom has “gained” possession of it.

Though I think I understand what you are getting at; perhaps it would be better in the long run (and for rigour’s sake) to ditch the notion of ownership!

 

[TeethWhitener]

This approach works for Lewis acid/base-type situations (including nucleophilic addition), but there are plenty of arrow-pushing mechanisms that involve completely nonpolar covalent bonds exclusively (Diels-Alder comes to mind).

Edit: things get even more complicated with organometallic complexes, where for instance the electron-rich pi system of an alkene is a decent Lewis base. Then even nucleophilic addition (e.g., of ethylene to platinum) can’t be treated in terms of formal charges on individual atoms.

 

[etotheipi]

This approach works for Lewis acid/base-type situations (including nucleophilic addition), but there are plenty of arrow-pushing mechanisms that involve completely nonpolar covalent bonds exclusively (Diels-Alder comes to mind).

Edit: things get even more complicated with organometallic complexes, where for instance the electron-rich pi system of an alkene is a decent Lewis base. Then even nucleophilic addition (e.g., of ethylene to platinum) can’t be treated in terms of formal charges on individual atoms.

Thank you, this is really helpful! On the basis of your and ZapperZ's response, I'll try to think of this method then as more of a "trick" that is applicable in some cases rather than a formal (no pun intended) rule...

 

[DrDu]

Maybe you may enjoy to read about Bader's "atoms in molecules" method.