Stereoisomerism in complex ions
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Some complex ions can show either optical or geometric isomerism.
This occurs in planar complexes like the Pt(NH3)2Cl2 we've just looked at. There are two completely different ways in which the ammonias and chloride ions could arrange themselves around the central platinum ion:
The two structures drawn are isomers because there is no way that you can just twist one to turn it into the other. The complexes are both locked into their current forms.
The terms cis and trans are used in the same way as they are in organic chemistry. Trans implies "opposite" - notice that the ammine ligands are arranged opposite each other in that version, and so are the chloro ligands. Cis means "on the same side" - in this instance, that just means that the ammine and chloro ligands are next door to each other.
You recognize optical isomers because they have no plane of symmetry. In the organic case, it is fairly easy to recognize the possibility of this by looking for a carbon atom with four different things attached to it. It isn't qute so easy with the complex ions - either to draw or to visualize! The examples you are most likely to need occur in octahedral complexes which contain bidentate ligands - ions like [Ni(NH2CH2CH2NH2)3]2+ or [Cr(C2O4)3]3-.
The diagram below shows a simplified view of one of these ions. Essentially, they all have the same shape - all that differs is the nature of the "headphones". The charges are left off the ion, because obviously they will vary from case to case. The shape shown applies to any ion of this kind.
If your visual imagination will cope, you may be able to see that this ion has no plane of symmetry. If you find this difficult to visualize, the only solution is to make the ion out of a lump of plasticene (or a bit of clay or dough) and three bits of cardboard cut to shape.
A substance with no plane of symmetry is going to have optical isomers - one of which is the mirror image of the other. One of the isomers will rotate the plane of polarization of plane polarized light clockwise; the other rotates it counter-clockwise. In this case, the two isomers are:
If you have a really impressive visual imagination, you may be able to see that there is no way of rotating the second isomer in space so that it looks exactly the same as the first one.
Contributors and Attributions
Jim Clark (Chemguide.co.uk)