Formal Charge

[CATSandKILOS]

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In Fe(NH3)6 ^3+ Fe carries a formal charge of +3. Yes, this is fairly easy, but I am missing something. How does it have a formal charge of +3?

 

[justadream]

@Kate2
What is "it"? Are you talking about how do you know that the overall charge is +3? I think that would have to be specified.

 

[CATSandKILOS]

@Kate2
What is "it"? Are you talking about how do you know that the overall charge is +3? I think that would have to be specified.

How does Fe have a formal charge of +3 ?

 

[justadream]

@Kate6

The overall charge of the overall thing is +3. NH3 itself has no charge.

So having six NH3s still provides a charge of o.

At the end of the day, you still need an overall +3 charge. The only thing remaining is the Fe. Thus, the Fe must have a +3 charge.

 

[kelminak]

I think @justadream did a good job but I'll try to explain slightly differently. I solve formal charge by thinking about what atom I'm trying to solve the formal charge for (x) and then say (x) + (charge of surrounding groups) = (charge of entire molecule). You know that the charge of the molecule is +3 and the charge on each NH3 is 0 (since it's neutral), so that leaves us with the formula X + 0 = 3. Solving for X gives you the +3 formal charge on the Fe atom. I might have overexplained it but I think you might have been making a simple problem more complicated in your head! 🙂

 

[kraskadva]

In Fe(NH3)6 ^3+ Fe carries a formal charge of +3. Yes, this is fairly easy, but I am missing something. How does it have a formal charge of +3?

Are you asking...
...how to calculate a formal charge?
...how is it that a Fe can have a charge at all?
...how Fe can have a charge of +3 (vs +1 or +2)?
...or something else? like why does it have a +3 charge with 6 (NH3) groups around it...

because
...how to calculate a formal charge? has been answered above
...how is it that a Fe can have a charge at all? it's a metal, they like to do that.
...how Fe can have a charge of +3 (vs +1 or +2)? Fe typically carries +2 or +3 when it's charged
...or something else? ...?
like why does it have a +3 charge with 6 (NH3) groups around it... The Fe^3+ would exist anyway. In solution with NH3 (which has a lone pair), Fe (which is electron deficient) will attract NH3 groups that hang out around it. There aren't bonds per se between Fe and the NH3 groups, just charge coordination. Static cling on the atomic level.

 

[Teleologist]

like why does it have a +3 charge with 6 (NH3) groups around it... There aren't bonds per se between Fe and the NH3 groups, just charge coordination. Static cling on the atomic level.

What the hell? Those are coordinate covalent bonds between the iron and ammonia.

What the hell is charge coordination also? Ammonia doesn't have an overall formal charge.

 

[kraskadva]

What the hell? Those are coordinate covalent bonds between the iron and ammonia.

What the hell is charge coordination also? Ammonia doesn't have an overall formal charge.

Not exactly (to the bolded)....It's a coordination complex. So it acts like they're bonded, and they kinda are, but not in the classic, covalent bond way that you're thinking. Because metals are weird.
Under normal formal charge rules, if they were covalently bonded each NH3 group would carry a +1 charge, and even the the Fe (if it started out +3) would have to carry a -3 charge to give a overall formal charge of +3, which isn't the case here.
The NH3 keeps a 0 formal charge, the Fe keeps its +3 formal charge, and they just really, really like each other. So they act like they're bonded, but they technically aren't.

As an analogy...it's like a common law marriage- they share everything, you're not likely to separate them, you treat them like they're married- but they never filled out the actual paperwork.

And both species don't have to be charged in charge coordination. The charge on the Fe coordinates the lone pairs on the uncharged NH3 groups.

 

[Teleologist]

I was referring to "static cling." Ionic bonding may be described as "static cling"

The phenomenon of static electricity requires a separation of positive and negative charges. When two materials are in contact, electrons may move from one material to the other, which leaves an excess of positive charge on one material, and an equal negative charge on the other.

Covalent bonding isn't exactly "static cling." The distinction between coordinate covalent and covalent bonding is also facile.