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Homework Help: Dodecahedral metal coordination?

  1. Oct 20, 2005 #1
    Is it true that there should be two distances from a dodecahedrally coordinated cation to the two types of coordinating atoms?
    When I calculate what the octahedron should look like assuming that all anion-anion distances are equal I will get two different cation-anion distances:
    if all anion-anion distances equal 2R, the cation-anion distances will be 3.5R for four of the anions and only 1.8R (!) for the other four anions.

    Does this make sense?
    Or am I wrong in assuming that all the anion-anion distances should be equal?

    What holds for capped trigonal prisms? There probably there will be a larger cation-anion distance for the capping anions, right? (Otherwise the anions would come too close! Or?)
  2. jcsd
  3. Oct 24, 2005 #2
    OK, I just calculated for a triply capped trigonal prism that there is just one radius ratio: 0.73

    For a dodecahedron I calculate 0.83 as the minimum radius ratio (for cation-anion interactions) but this leaves four out of eight coordinating atoms at almost twice the distance (as described above)...
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