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The infulence of ligands in complexes

  1. Feb 5, 2017 #1
    Hello! I analise metal chemical shifts, its formula is MR4, where M is transition metal, like Palladium,
    and there is some trend:

    I > CN > Br > NH3 > NO2 > Cl > H2O,

    the same trend is observed for similar complex of formula: Pd(R)3Cl, Pd(NH3)2X2, Pd(R)Cl3
    So, the metal signal shielding increase in order showed below... I noticed, the less electronegative and bigger halogen ion as ligand, the chemical shift of metal, like palladium, is more shielded. It is probably caused by backdonation of electrons from ligand towards metal. What about the other ligands? For example, NH3 is "less electronegative" ligand than H2O, so lone pair on nitrogen may donate electrons? bond metal-OH2 is more ionic probably, and electrons are shiftes slightly more to oxygen than metal... What about CN, NO2? What can be the effect of this trend? which properties of those ligands may influence chemical shift? it is usually influenced by donation/withdrawing of electrons and so on... polarizability?
  2. jcsd
  3. Feb 5, 2017 #2


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    I apologize, I'm having a little bit of trouble understanding your English. Just to clarify: the series you give (I, CN, Br, etc) goes from a PdNMR signal that is more shielded (further upfield) to less shielded (further downfield)? You might want to look at the nephelauxetic effect. It's basically a lowering of the Racah B parameter due to partial covalency of the coordinate bond between a ligand and a metal. The series for the nephelauxetic effect is pretty close to the one you've given.
  4. Feb 6, 2017 #3
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