TBR question involving formal charge

[Meredith92]

Hi, I got this answer right, but I'm having some difficulty understanding the formal charge distribution with Cr(NH3)6 3+ and Cu(H2O) 6+ in this question:

Which of the following statements is NOT true?
A) Cr has a higher formal charge in Cr(CN)6 than Cr(NH3)6 3+
B) Cu(H2O)6+ has ten d-electrons
C)Fe(NH3)g 3+ has the same shape as FeCL6 3-
D) Scandium is more likely to have 4+ formal charge than titanium

Answer: The cyano ligand carries a -1 charge, so chromium must have a +^ charge for Cr(CN)6 to be neutral. the amino ligand is neutral so chromium must have a +3 charge in order for Cr(NH3)6 to have an overall positive three charge. The charge of chromium is in fact greater in Cr(CN)6 than Cr(NH3)6 so A is valid. The water ligand is neutral, so copper must have a +1 charge in order for Cu(H2O)6 to have an overall +1 charge. Answer continues...

Maybe I'm not understanding the structures but why is NH3 neutral? If it is bonding with Cr wouldnt it have a +1 charge on it? Also, why is water neutral if it is also in a bond with Cu?

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[Postictal Raiden]

Hi, I got this answer right, but I'm having some difficulty understanding the formal charge distribution with Cr(NH3)6 3+ and Cu(H2O) 6+ in this question:

Which of the following statements is NOT true?
A) Cr has a higher formal charge in Cr(CN)6 than Cr(NH3)6 3+
B) Cu(H2O)6+ has ten d-electrons
C)Fe(NH3)g 3+ has the same shape as FeCL6 3-
D) Scandium is more likely to have 4+ formal charge than titanium

Answer: The cyano ligand carries a -1 charge, so chromium must have a +^ charge for Cr(CN)6 to be neutral. the amino ligand is neutral so chromium must have a +3 charge in order for Cr(NH3)6 to have an overall positive three charge. The charge of chromium is in fact greater in Cr(CN)6 than Cr(NH3)6 so A is valid. The water ligand is neutral, so copper must have a +1 charge in order for Cu(H2O)6 to have an overall +1 charge. Answer continues...

Maybe I'm not understanding the structures but why is NH3 neutral? If it is bonding with Cr wouldnt it have a +1 charge on it? Also, why is water neutral if it is also in a bond with Cu?

I only know how to answer the bolded part. To determine the formal charge of any atom use the following equation:

FC = # of valence electron - (# of unbound electrons + 1/2 # of bound electrons)

For H, the formal charge is 1 - (0 + 1) = zero. For N, the formal charge is 5 - (2 + 3) = zero. Therefore, the formal charge of the entire molecule is zero.

 

[Meredith92]

But it is in a bond with Cr?

I spent some time thinking about it and I think what was confusing me about this is that it is a coordination compound... and I had been thinking of it as if it were a typical covalent bond.

 

[Trayshawn]

To make a long story short, this is simply a VERY bad question.
Why? Formal charges are almost NEVER used when metals are involved. Rather, oxidation states are.
Both systems (FC and OS) are simply bookkeeping methods to track electrons. One assumes a full covalent bond, the other assumes a full ionic bond. Since metals usually have low electronegativities and give up electrons, you use the ionic bond approximation (even in transition metal complexes as you have here).

Thats where the confusion is coming from. The question SHOULD be replacing "formal charge" with "oxidation state." In this case, you simply consider the charge of each ligand BEFORE binding. And give the metal whatever charge (again, all of these charges are OXIDATION STATES) it needs to get the overall charge. Ammonia is neutral, so it has no charge. But chloride is an anion with a -1 charge, so you count it as -1.

You are essentially pretending that all the electrons in the coordinate covalent bonds the ligands make with the metal are ON THE LIGAND ONLY since the ligands usually have higher electronegativities and, in the oxidation state method, the more electronegative atom gets all the electrons in a bond (as if it were an ionic bond)

 

[Meredith92]

I'm confused as to when we should use this sort of method... Is it only with transition metals?

 

[Meredith92]

( and thanks for your explanation!)

 

[Trayshawn]

yeah, transition metal complexes and anything with metals in general use oxidation states.

 

[gallons]

in which chapter of gen chem book would this specifically be covered? I did not learn this specific topic in my gen chem classes.

 

[aldol16]

Look at oxidation states. As people have already said a long long time ago, this is a terrible question. For coordination complexes, we always work with oxidation states and not with formal charges. Whoever wrote this obviously had no idea how transition-metal chemistry is actually performed in the lab.