Symmetry Elements of XeF4: Meaning of 2C2 & σh

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SUMMARY

XeF4 exhibits D4h symmetry, characterized by specific symmetry elements including 2C2 and σh. The notation 2C2 indicates the presence of two C2 axes that are perpendicular to the principal C4 rotation axis. The σh denotes a horizontal plane of symmetry that is also perpendicular to the principal axis. Understanding these symmetry operations is crucial for analyzing molecular geometry and electronic structure in coordination complexes.

PREREQUISITES
  • Understanding of molecular symmetry and point groups
  • Familiarity with D4h symmetry and its implications
  • Basic knowledge of molecular orbital (MO) theory
  • Concept of ligand atomic orbitals and their symmetry properties
NEXT STEPS
  • Research the implications of D4h symmetry in molecular geometry
  • Study the characteristics of C2 and C4 rotation axes in molecular structures
  • Learn about the shapes and properties of ligand atomic orbitals in MO theory
  • Explore the relationship between symmetry operations and electronic configurations in coordination complexes
USEFUL FOR

Chemistry students, molecular symmetry enthusiasts, and researchers in coordination chemistry will benefit from this discussion, particularly those focusing on molecular geometry and electronic structure analysis.

ampakine
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In my lecture notes the symmetry elements for XeF4 are listed as:
[PLAIN]http://img690.imageshack.us/img690/7216/syms.png
but nowhere does it explain what 2C2 or σh means. What are these symmetry operations?
 
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ampakine said:
In my lecture notes the symmetry elements for XeF4 are listed as:
[PLAIN]http://img690.imageshack.us/img690/7216/syms.png
but nowhere does it explain what 2C2 or σh means. What are these symmetry operations?

Well, XeF4 has D4h symmetry, which is pretty high, so there are a lot of elements. The 2C2 notation means that there are 2 C2 axes perpendicular to the principal axis .. there are actually two sets of these, which is why there is a primed and double-primed notation as well. The σh notation just means that there is a plane of symmetry perpendicular to the principal axis.
 
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Ah right, thanks a lot. Is there a sure shot way to know which is the principle rotation axis? For example in XeF4 why is the C4 the principle rotation axis? Why not the C2 axis perpendicular to it or why not the C2 rotation axis that goes from one F to the other F diagonally opposite it?

Also I didn't want to start a new thread for this so I'll ask it here. In MO theory for octahedral coordination complexes there are 6 ligand atomic orbitals which split into 3 sets of degenerate orbitals: a1g, t1u and eg. I keep seeing them referred to as symmetries. Does this mean the orbitals in these sets have a distinct shape or something? Are these point groups? I found this diagram:
[PLAIN]http://img852.imageshack.us/img852/6572/symm.png
but I'm not too sure what to make of it. I can see how each of the ligand orbitals corresponds to a 4s, 4p or an eg d orbital and I'm guessing these are the orbitals that they merge with to form MOs but I'm pretty confused about the shapes of these ligand orbitals. Does the t1u orbital have 6 lobes or something?
 
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