random gen.chem question

[amar21]

this type of question was on my MCAT last year.. and I didnt know how to do it... it just popped into my head...

say they give you a huge molecule with a formal charge of -2... say, for example the central element was Fe and it had a ton of different things braching off if it... like a hemoglobin molecule expanded to confuse the hell out of you

and then it asks, whats the formal charge on the central Fe

would you ONLY take into account what is directly attached to the Fe? I hope that makes sense

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[Quantum Chem]

Yeah, the charge on Fe would only depend on the ligands attached to it and the formal charge of the overall complex. If you had the complex it would be easier to tell what the charge is.

 

[amar21]

you said AND the overall charge?

say we found that the charge on Fe was +1, and the overall charge charge of the entire molecule was -2.

That still means that the charge on Fe is just +1, right?

i dont have an actual diagram to post, as im just hypothetically making a question up in my head haha

 

[sleepy425]

if you add up the charge of everything in a molecule, you'll get the overall charge.

it is NOT determined simply by the charge of the atoms directly attached to the metal. if you have a complex that has a net charge of +2, that means that the sum of all the localized formal charges (including that of the metal) is +2.

so let's take your example amar21:

you said it was Fe(+1) and that the overall charge on the complex is -2. So that means that the sum of the charges on everything else in the complex except for the Fe is -3.

ok, go to http://en.wikipedia.org/wiki/Chlorophyll

scroll down till you see the 5 structures drawn. the overall charge on all of those complexes is 0. you see that the Mg has a +2 charge and the rest of the molecule has a -2 charge (two negatively charged nitrogens).

Now, look at chlorophyll c1 and chlorophyll c2. If I gave you those structures except I drew the carboxyl groups as deprotonated. In that case, the overall charge on the complex will be -1. The negatively charged nitrogens are still -1 charged, and the carboxyl group now carries a negative charge. So the net charge on everything in the complex BUT the metal is -3. So that means the metal is still +2
(right, charge on metal (+2) + charge on the rest of the complex (-3)= total charge (-1)).

I hope that helps, lemme know if you have questions

 

[Quantum Chem]

If Fe was found to be +1 then the overall charge of the complex has already been taken into account. But if you are finding the formal charge of Co on say [Co(OH)2]-2 then the charge on Co would be 0 since OH is minus 1 with two in the complex for -2, and the overall is minus 2.

 

[amar21]

Got it now guys! Thanks!

 

[ej37]

If Fe was found to be +1 then the overall charge of the complex has already been taken into account. But if you are finding the formal charge of Co on say [Co(OH)2]-2 then the charge on Co would be +4 since OH is minus 1 with two in the complex for -2, and the overall is minus 2.

Overall charge on Co is zero here... Co(0) d7

 

[sleepy425]

Overall charge on Co is zero here... Co(0) d7

yep, it's definitely 0, not +4

 

[amar21]

yep, it's definitely 0, not +4

good call. It is 0. I didn't even try it.

 

[Quantum Chem]

Yeah, sorry about putting +4. Looks like I got distracted watching tv and typing, and the tv won my attention.