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boron nitrogen coordinate covalent bond

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foxi

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In a boron-nitrogen coordinate covalent bond, why is boron the acid and nitrogen the base? I figured it would be opposite..
 

aldol16

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Why do you think it would be the opposite? Nitrogen has a lone pair in a compound so it usually acts as a Lewis base. Boron in compounds almost always has an empty orbital so it usually acts as a Lewis acid.
 

foxi

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I was simply basing it off of electron affinity... Since nitrogen has a higher affinity, it is more willing to accept an electron, and therefore is more acidic. It also follows the general periodic trend in which elements (within a period) are more acidic than their neighbors to the left.
 

foxi

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Why do you think it would be the opposite? Nitrogen has a lone pair in a compound so it usually acts as a Lewis base. Boron in compounds almost always has an empty orbital so it usually acts as a Lewis acid.
>I was simply basing it off of electron affinity... Since nitrogen has a higher affinity, it is more willing to accept an electron, and therefore is more acidic. It also follows the general periodic trend in which elements (within a period) are more acidic than their neighbors to the left.
 

aldol16

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>I was simply basing it off of electron affinity... Since nitrogen has a higher affinity, it is more willing to accept an electron, and therefore is more acidic. It also follows the general periodic trend in which elements (within a period) are more acidic than their neighbors to the left.

Those electronegativity trends are good for a general picture, but you must always consider the local electronics - what other things are around the boron, for instance. Second, how the electrons in a bond are polarized has nothing to do with which one acts as the Lewis acid/base. What you're saying is that in a B-N bond, the electrons should be polarized towards the nitrogen because it wants the electrons more. That's fine. But that doesn't mean that boron donated those electrons to nitrogen to begin with. No - in such complexes, boron doesn't have electrons to give. Furthermore, in coordination chemistry, the ligand (nitrogen in this case) acts as a Lewis base and donates electrons into the coordination sphere of the "metal" - in this case, boron.

Now, you're correct that if you were to look at the B-N bond, it would be weak and polarized towards the nitrogen. But you're confusing this concept with how the bond was made in the first place.
 
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