Stereoisomerism and optical isomerism in complex ions

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abitpissy
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Could someone please explain to me when there would be isomerism in a tetrahedral, square planar and octahedron. In my book it says it depends on the number and type of ligands (monodentate and bidentate). I have tried to understand this, it would be helpful if someone could just outline what causes optical and stereo isomerism when taking into account the no. and type of ligand please.
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charco
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(Original post by Questioness)
Could someone please explain to me when there would be isomerism in a tetrahedral, square planar and octahedron. In my book it says it depends on the number and type of ligands (monodentate and bidentate). I have tried to understand this, it would be helpful if someone could just outline what causes optical and stereo isomerism when taking into account the no. and type of ligand please.
Optical isomerism arises when two non-superimposible mirror images are possible.
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shengoc
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(Original post by Questioness)
Could someone please explain to me when there would be isomerism in a tetrahedral, square planar and octahedron. In my book it says it depends on the number and type of ligands (monodentate and bidentate). I have tried to understand this, it would be helpful if someone could just outline what causes optical and stereo isomerism when taking into account the no. and type of ligand please.
Optical isomers are as explained already. For octahedral complexes, look for 3 x bidentate ligands. ie 3 x en or 3 x oxalate anion ligand

Stereoisomers can refer to BOTH optical and/or geometric isomers. With d8 configuration, like Ni(+2), Pd(+2) and Pt(+2) complexes, which are square planar, then you could get cis-trans possibility if you have the right set of ligands.
(common case study cited includes cis-platin - a historically active drug against cancer vs trans-platin which is not effective).
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MexicanKeith
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(Original post by Questioness)
Could someone please explain to me when there would be isomerism in a tetrahedral, square planar and octahedron. In my book it says it depends on the number and type of ligands (monodentate and bidentate). I have tried to understand this, it would be helpful if someone could just outline what causes optical and stereo isomerism when taking into account the no. and type of ligand please.
Just to add to what's already been said, for octahedral metal centres here are a few examples of isomerism and compounds that exhibit certain types of isomerism:

  • Any octahedral centre with 3 bidentate ligands exhibits optical isomerism (e.g Co(acac)3)


  • linkage isomerism can be exhibited by metal centres which have ambident ligands (e.g [Co(NH3)5 NO2]2+) (a couple of other ambident ligands include CN- and SCN-)


  • Any octahedral centre with 2 of one ligand and 4 of another exhibits cis and trans isomerism (e.g [Co(en)2F2])


  • Any octahedral compound with 3 of one ligand and 3 of another ligand exhibits fac and mer isomerism (e.g [CoCl3(NH3)3])


  • A more complicated example is when the metal centre has 3 pairs of monodentate ligands (e.g [Pt(NH3)2(py)2(cl)2] 2+). In this case 5 geometric isomers exist, where different ligand are cis and trans to one another. One of these geometric isomers is also optically active (when all pairs of ligands are cis)


Wikipedia is quite good on some of these different types of isomerism in octahedral compounds:
https://en.wikipedia.org/wiki/Octahe...dral_complexes

Square planar compounds can exhibit cis/trans and linkage isomerism (linkage isomerism is simply a feature of a given ligand rather than of the metal centre per se).

Tetrahedral complexes don't exhibit much isomerism (linkage is again possible)

Theoretically comformational isomers could exist (ie a compound might have tetrahedral and square planar isomers) but in reality compounds show a significant bias, favouring one form or another (eg Platinum(2+) with 4 ligands is always square planar for reasons to do with 5d8 electron stabilisation). Nickel(2+) is more interesting, some compounds are tetrahedral (eg with Cl- ligands) and others are square planar (eg with CN- ligands) so perhaps with careful ligand choice a compound of nickel(2+) might exhibit an equilibrium between the forms which isn't completely one sided? I can only speculate on that!

Hope that's useful, overall, octahedral compounds show lots of isomerism arising from different ligand combinations!
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