Covalent structures and bonding
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Can you tell me the shapes of NH2- and NH2+ ions?
Corrie says
If you are familiar with the shapes of molecules like H2O, NH3 and BF3 and how they are explained in terms of electron pair repulsion (VSEPR) theory, then you should be able to predict shapes for these two ions fairly easily. I'll give you a few hints.
NH2- can be obtained from NH3 by loss of a proton (H+). So NH2- will have the same number of electron pairs around N as NH3 has. The only difference is that now two are lone (non-bonding) pairs and two are bonding pairs. Of which common molecule does this electron arrangement remind you? Water, hopefully! So the NH2- ion will have much the same shape as a water molecule. Can you remember what the H-O-H angle in H2O is - and the reason why?
NH2+ can be obtained from NH3 by the loss of H-, which has two electrons. So left around the N are only three pairs of electrons, one lone pair and two bonding pairs. Assuming for the moment all the pairs are the same, their arrangement should be trigonal planar - as in the case of BF3. But as you know, lone pairs repel more than bonding pairs. So what effect will this have on the H-N-H angle? Will it be slightly bigger or smaller than 120°? And the overall shape - remember there are only two bonds.
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updated: 20 November 2006
