Formal Charge

[the prodogy]

Ok, I'm having a hard time understanding how to get formal charge of something. I understand the formula, but I guess I can't seem to get how to draw the figure. I'm going through a book that requires you to find out the number of d-electrons in certain compounds, but to do so, you need to find out the formal charge of the central (transition metal) atom.

Fe(NH3)6^(3+) and FeCl6^3-
I can't seem to get a picture of this in my head and therefore can't findout formal charge. Can someone help?

Also:
Co(H2O)6^3+

Thank in advance

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[Phlame217]

Fe is a transition metal, so it can have up to 6 bonds, where as it does.
Formal charge is (electrons brought) - ((free electrons)+(1/2 * bonded elections))

use that formula and you should be able to figure it out.

 

[the prodogy]

Fe is a transition metal, so it can have up to 6 bonds, where as it does.
Formal charge is (electrons brought) - ((free electrons)+(1/2 * bonded elections))

use that formula and you should be able to figure it out.

Thanks, I understand the formula, but when I ran into the question, I couldn't help but try to get an image of the molecule. What I end up trying to see/draw doesn't make sense. For example, NH3 shouldn't have any extra atoms to bond, since N should be in octet. Same thing goes for Co in Co(H2O)6^3+.
Also, would the "3+" in Fe(NH3)6^3+ do anything when it comes to formal charge?

 

[soby10]

Like the previous post states that is the correct formula for formal charge. But as far as NH3 the formal charge for "N" in NH3 is 0. There are 5 valance electrons (just look at the group # on periodic table) for N, 3 shared w/ H and 1 lone pair (2 e-) which totals 5. So using the formula 5-5=0.
As far as the metals are concerned it looks like a coordination complex. So that is different from formal charges. For example Co(H2O)6^3+ the central metal is bonded to 6 H20's (a bond of six is an octahedral complex, 4 is a tetrahedral complex). So you are looking for the oxidation # for the metal this will allow you to find the d orbitals. So for above example the total charge is 3+ water is neutral in coordination complex thus Co is 3+. ( x+0=3+...x being the unknown metal oxidation state). Ok now you know the oxidation state.
If you want to know the magnetic properties you need the spectrochemical series which list weak to strong field ligands. H20 is a weak field ligand, so it has high spin meaning it cannot hold the d orbital e-'s close. So just fill out the d orbital for octahedral's using hunds rule dxy {|this line represents a unpaired e-} dyz {|} dxz {|} (dx2-y2 , dz2 have no e's ..all 3 e-'s are filled) b/c there are unpaired e-'s Co is paramagnetic...
Thus the d orbitals are filled based on oxidation # of metal not formal charge. Hope this clears it up.

 

[Seraph 84]

For those complex ions I believe you can just look at the oxidation numbers of the transition elements to find the number of d-electrons.

In the first one, NH3 is nuetral, so all 6 ligands will will contribute nothing to the total charge of the complex ion. The total charge is +3, so Fe must be +3. The number of d-electrons is therefore 5.

For FeCl6 3-, you have 6 chloride ligands which each contribute a -1 charge to the complex ion. For a total charge of -6, Fe must be +3.

Cobalt would also be +3 because H2O are neutral ligands. The number of d-electrons would be 6.

I think that's right, can anyone confirm this?

 

[0Complications]

For those complex ions I believe you can just look at the oxidation numbers of the transition elements to find the number of d-electrons.

In the first one, NH3 is nuetral, so all 6 ligands will will contribute nothing to the total charge of the complex ion. The total charge is +3, so Fe must be +3. The number of d-electrons is therefore 5.

For FeCl6 3-, you have 6 chloride ligands which each contribute a -1 charge to the complex ion. For a total charge of -3, Fe must be +3.

Cobalt would also be +3 because H2O are neutral ligands. The number of d-electrons would be 6.

I think that's right, can anyone confirm this?

Yeah, the easiest way to do this is to look at the known values. So for Cl, it is -1, Oxygen is -2 (This is due to how many additional elections these atoms want to take one to get to their noble configuration/full octet of 8), so if you notice Cl is in the 7th group, hence one away from 8 electrons, so it adds 1. Oxygen is in the 6th group, 2 e-'s away from 8, so it adds two.


Now if you have a transition metal bound to a nonmetal, use the oxidation state of the nonmetal (O, Cl, Br, F, etc).

Do it just like Seraph said in the above bolded part.


You take the formal charge which is always given in these types of problems, then you take the known nonmetal's charge and multiply it by the number of of them there are, so if it's O6, there are 6 oxygens which are -2, so the charge is -12 (6 x -2 = -12). H is always +1 one (unless it's bound to a metal, then it's -1). Then you would calculate how many hydrogens and multiply that number by +1 (they are usually bound to nonmetals). Then the transition metal is the difference between the formal charge, the nonmetal charge, and the hydrogen charge.


Now applying these rules: so if you have H20 for instance, your formal charge is 0 because O = -2, and 2xH = +2. For H30+, it has a positive charge, because O = -2, then you have 3xH = +3, so you have -2 + 3 = +1.

Now applying these rules to the examples above:

Fe(NH3)6^(3+)

Fe is your metal that we're calculating, so we'll call it "X."

Now you know the formal charge is +3.

The charge of NH3 is 0 (This is because N= -3 [remember look at the periodic table and how many e-'s it needs for it's octet] and 3xH = +3, so -3 + 3 = 0).

Knowing the formal charge and the charge for ammonia (NH3), we can calculate for Fe:

Fe is the difference between the formal charge and the nonmetal charge.

In this case +3 - 0 = +3 Charge on Fe

So, now you know the charge of Fe is +3 on this salt.

Now for FeCl6^3-:

Formal Charge = -3
Cl = (6xCl = -6)

So, the charge on Fe is the difference between Cl and the Formal charge:

-3 - (-6) = +3


I hope that makes sense.

 

[fizzgig]

i was hoping to clarify a bit on this too.

if you have 2 NH3 and 2 halides attached to a central Metal atom, someone above referred to a 'coordination complex', so does that mean when you do oxidation or formal charges for these compounds you should consider the ligands as though they were NOT bonded?

so looking at NH3-Metal, the formal charge of N is 5-0-8/2=+1 (makes sense, as N has its fourth bond). so both Ns would contribute +2 total.

X-Metal would be 7-6-2/2=0, contributing nothing. this leaves the metal with a MINUS 2 to compensate.

if you look at NH3 and X as separated from the metal, however, NH3 is neutral and the Xs give -2 total, leaving the metal as +2 which is correct.

is the reason for this what i listed (look at ligands as though not attached in metal complexes) or is there something i don't get?

thanks!