Cis-platin formal charge

[capn jazz]

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So you have this molecule, cis-platin. It's a platinum atom bound to two Cls and 2 NH3s. http://en.wikipedia.org/wiki/Cisplatin

What's the formal charge on the platinum? The answer says that the 2 Cls have a charge of -2 total and the two NH3s are neutral so the charge on Pt is +2 to make the molecule neutral.

But... the NH3s are bound to the platinum! So shouldn't they be +NH3-R, with a positive charge due to having 4 bonds?

Help!

 

[capn jazz]

bump! anyone want to explain formal charge to me?

 

[Dr Gerrard]

FC = Number of Valence Electrons - Number of Lone Pair Electrons - (1/2) Number of Covalent Bonding Electrons

Using this formula, the two Cls should have a formal charge of 0 (just like they do in HCl)

The two nitrogens should have a formal charge of +1 each.

The Platinum should have a formal charge of 2- (which makes no sense, because it is a transition metal).

I don't even see why Cl would have a formal charge of -2. Does question ask for oxidation number?

 

[capn jazz]

How do you know how many valence electrons a transition metal element has? The question specifically says formal charge, not oxidation. Hmm. I would think that NH3R has one + charge each, and that Pt needs a -2 to balance it, but apparently not.

 

[Dr Gerrard]

Maybe its because chlorine and ammonia do not form covalent bonds with the metal, just weird complexes.

I think to approach these problems, you just take each individual group and figure out the transition metal's formal charge using that.

The overall charge is 0.

NH3 is neutral by itself.

Cl is usually an anion, having a formal charge of -1.

Then you know that the platinum has to have +2, because there are two chloride ions.

Why it works like this? I have no idea. Probably because the transition metal does not covalently bond with ammonia or chloride, because if it did, then the formal charges would be how we predicted.

 

[capn jazz]

Yea, maybe they're not covalent bonds. Weird. I hate complexing agents!

 

[joshto]

just ran by this questions in a TBR CBT; I got it wrong.

I can't seem to wrap my head around it.

 

[loveoforganic]

This is organometallic chemistry and above the scope of the MCAT.

As far as the answer, the overall complex is uncharged. It has 2 negative ligands and 2 neutral ligands. To compensate for the 2 negative ligands and retain a net neutral complex, the metal must have an oxidation number of +2.

 

[RickyJamesMD]

This is organometallic chemistry and above the scope of the MCAT.

As far as the answer, the overall complex is uncharged. It has 2 negative ligands and 2 neutral ligands. To compensate for the 2 negative ligands and retain a net neutral complex, the metal must have an oxidation number of +2.

Chlorines are negative ligands because they strongly donate electrons. And the nitrogens on ammonia are known as neutral. These are just some of the rules of Organometallic chemistry that one has to memorize.

And I agree with the above poster. Chances are, this question won't be on the test. It wasn't on mine, nor the AAMC practice tests I took.

 

[BerkReviewTeach]

This is organometallic chemistry and above the scope of the MCAT.

You should probably check one of the MCAT threads from either January or March this year to verify if I recall this correctly, but I think some posters mentioned getting a passage on this very topic (coordination chemistry), but with a different metal. I seem to recall reading about this subject being on a couple MCATs last year too. I think test writers are interested in structure and shapes when they ask about coordination complexes.

Also, Grignard reactions, organocuprates, and so forth are fair game on the MCAT, so I would think that organometallic chemistry is fair game at a simplistic first year level.