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In which temperature range does WF_6 melt?

  1. Jul 11, 2017 #1
    • Thread moved from the technical forums, so no Homework Template is shown
    Hi Everyone

    I'm studying material Engineering and i'm currently preparing chemistry for the summer exams.

    Now, there is an old exam question which I don't know how to solve:

    "In which temperature range does ##[W^{+VI}F_{6}^{-I}]## melt?"

    My solution:

    Well, the 18-Electron rule is not fulfilled. There is no crystal field stabilization energy since there are no d-electrons and there are no Pi-backbonds. But:
    The electronegativity difference is larger then 1.5. Wolfram is a hard acid and Fluor a hard base. The lattice-energy is very high because both compounds are in a high oxidation state and have a small atom radius (especially Fluor).

    Based on this i would say ##[W^{+VI}F_{6}^{-I}]## has a melting-point over 1000 degrees Celsius.

    I then looked it up on Wikipedia and it says that ##[W^{+VI}F_{6}^{-I}]## is a gas!?

    How can one know this?
    And is my train of thought correct? Because our professor ask this kind of question every time... He asked it once with ##OsO_{4}##, ##GeCl_{4}##,....

    Thanks a lot for your help!
     
  2. jcsd
  3. Jul 11, 2017 #2

    TeethWhitener

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    What is the geometry of WF6 and what does that tell you about its interactions?
     
  4. Jul 11, 2017 #3
    It's an octahedral crystal field. But I don't know what that tells me about its interactions :(
     
  5. Jul 11, 2017 #4

    TeethWhitener

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    Right. So we know that WF6 is neutral and that it's an octahedron with W in the middle and F's completely surrounding the tungsten at each of the six vertices. With this geometry in mind, what do you think will be the dominant interactions when you bring two of these WF6 units close together?
     
  6. Jul 11, 2017 #5
    I guess there will be coulomb interactions between the non-bonding orbitals of the fluorine?
     
  7. Jul 11, 2017 #6

    TeethWhitener

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    The important thing to think of is: will there be significant interaction between the W of one WF6 unit and the F's of another WF6 unit? If yes, then this ionic interaction will dominate the lattice energy (by at least an order of magnitude). If no, then the lattice energy will consist chiefly of the weak dispersion interactions between the fluorines.
     
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