question on kaplan organic test

[deeentist79]

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B(CH3)3 + NH3--------> ?
I dont know why the answer has a Negative sign on Boron and a Positive on NH3...can some one explain.

 

[qnzbabi91]

When the boron molecule behaves as a lewis acid, it snatches up NH3's electrons and forms a coordinate covalent bond. Since boron is an exception which perfers a sixtet (made than up lol) he becomes positive. Boron has only 3 valance electrons, so it if you use assign a formal charge to the molecule it is 3- (bonding pairs/2) and you get 3- (8/2)=-1. For nitrogen, the atom perfers 3 bonds and a lone pair. Initially, it had a formal charge of zero. Thus, when it makes a new bond, its formal charge becomes 5- (8/2) or plus 1 since nitrogen cannot expand its octet.

 

[deeentist79]

Thanks.
one of the answer choices had simply a negative charge on Boron and and a NH2 attached to it...i want to know why that was not one of the answer choices?