Effect of ligands on metal NMR shift

  • Thread starter KarolinaPL
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Hi

transition metal complexes, effect of ligands on metal NMR shift... complexes of formula M(amine)2X2, they write, that the difference between cis and trans isomers chemical shift, for dipiridine isomers (amine=piridine) is greater for diiodo than for dichloro complexes... and say if ligands amine and X are far from each other in the trans influence series, this difference chemical shift cis-trans is bigger... but why? wow it influence metal chemical shift? trans influenceof iodine is much larger than chlorine, so the Pt-N bond trans to iodine will be longer than when its trans to chlorine. but how it affects shielding of nmr signal? i would say its just withdrawal of electrons, why order in trans series affects that?
 

TeethWhitener

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Your questions are a bit confusing as written, but I think you're wondering about how differences arise in the NMR chemical shifts of various isomers of square planar platinum complexes.

For example, we observe that there is a difference in the 195Pt chemical shift between cis-diammineplatinum dichloride and trans-diammineplatinum dichloride. Furthermore, we observe that this isomer-based chemical shift difference increases when we replace the chlorides with iodides, and that it increases still further if we replace the chlorides with ligands further up the trans influence series.

Why this happens is tied in with why the trans influence exists in the first place. Good trans influencers are good sigma donors, meaning they push electron density into the metal--specifically the dsp2 hybrid orbitals on the metal center which participate in sigma bonding. Now, cis ligands share the d and s portions of those hybrid orbitals, but trans ligands share d, s, and one p portion of those hybrid orbitals. So trans ligands are automatically competing more for sigma bonding electron density than cis ligands. Good trans influencers "outcompete" bad ones, which is the origin of the trans influence series.

What does this have to do with NMR? The chemical shift on the metal center is related to how well-shielded the nucleus is from the external magnetic field. The more shielded, the more downfield the chemical shift. And in general, the higher the electron density around the nucleus, the better shielded it is. A good trans influencer will push a lot of electron density onto the metal, increasing its shielding. If two good trans influencers are cis to each other, they don't compete as efficiently and can push proportionately more electron density onto the metal center (higher shielding), whereas if they are trans to each other, they compete more efficiently, and comparatively less electron density ends up on the metal center (lower shielding). The better the trans influencer, the bigger this effect.

Also, when you have a complex such as MX2Y2, if X and Y are both good trans influencers, then the cis-trans chemical shift difference will be relatively small (since all four ligands are competing to push electron density onto the metal center). But if (for example) X is a good trans influencer and Y is a bad trans influencer, then the cis-trans chemical shift difference will be larger, since only two of the ligands (in this case, the two X ligands) are effectively in competition.
 

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