Aamc 4, PS# 6

[starfar]

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The ability of NH3 to form coordination compounds with transition metal ions can best be accounted for by the fact that NH3:

A. Acts as an electron pair donor
B. is capable of H bonding
C. Is a weak base in aqueous solution
D. Contains N with a -3 oxidation number

answer: A


Im confused about this one.

NH3 does act as an electron donor because its in a covalent bond and the passage basically gives that answer away. But NH3 is ALSO a weak base in solution which again makes it an electron donor and CO3+ its Lewis Acid electron acceptor.

So... With that logic, it looks to me as if 2 answers, A and C are correct. How do you ascertain which is "best"???

 

[ZeaL6]

I see your point, and though NH3 isn't listed as a weak base, it has a pka of 36 and will deprotonate most hydrogens at will. Also, being a weak base has to do with how well it can deprotonate another atom. It is forming a coordination compound, which would show its properties as a nucleophile, not a base. That's my spin on it anyway.

 

[ToldYouSo]

I see your point, and though NH3 isn't listed as a weak base, it has a pka of 36 and will deprotonate most hydrogens at will. Also, being a weak base has to do with how well it can deprotonate another atom. It is forming a coordination compound, which would show its properties as a nucleophile, not a base. That's my spin on it anyway.

Ammonia is a weak base and you can't deprotonate a hydrogen. It is acting as a lewis base.

OP whenever transition metals come up always think of electron pair donating/accepting. The basically of ammonia is a general fact that doesn't directly explain what the question is asking.

 

[QrtrLifeCrisis]

Ammonia is a weak base and you can't deprotonate a hydrogen. It is acting as a lewis base.

OP whenever transition metals come up always think of electron pair donating/accepting. The basically of ammonia is a general fact that doesn't directly explain what the question is asking.

This is correct. Whenever you see coordinate covalent bonds mentioned, as was mentioned in reactions with transition metals (pretty colored compounds), think of the Lewis base definition, not the Bronsted or Arrhenius definitions of a base. The MCAT is all about the best answer, not just the random correct information answer. You'll run into that a lot in the verbal section.