Inorganic I - 8 Coordinate Complexes

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Discussion Overview

The discussion centers around the existence and characteristics of 8-coordinate complexes in coordination chemistry, specifically focusing on their point groups and structural representations. Participants explore various geometric configurations such as square antiprisms and dodecahedra, and their implications for molecular symmetry.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the existence of an 8-coordinate complex point group and suggests that such complexes, while rare, do exist, citing the XeF8 2- anion as an example.
  • Another participant identifies two possible geometric forms for 8-coordinate complexes: the square antiprism and the dodecahedron.
  • A participant describes the staggered arrangement of squares in a square antiprism and expresses confusion regarding the symmetry of 8-coordinate complexes compared to 6-coordinate complexes, noting both share the Oh point group.
  • Another participant clarifies that cubic and octahedral arrangements have the same symmetry due to their dual relationship, and mentions the symmetry group I_h for icosahedra and dodecahedra.

Areas of Agreement / Disagreement

Participants express differing views on the characteristics and classifications of 8-coordinate complexes, with no consensus reached on the specifics of their point groups or structural representations.

Contextual Notes

There are unresolved aspects regarding the definitions and properties of 8-coordinate complexes, particularly in relation to their symmetry and geometric configurations.

Nezva
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Main question: What is the name of the 8 coordinate complex pointgroup? Or does it even exist?

I've been exposed to octahedrons and icosohedrons, however, the 8 coordinate high symmetry complexes appear to have been skipped. I'm aware that these complexes would be rare but I think that they do exist. I was able to find this image of a XeF8 2- anion:
http://ce.sysu.edu.cn/echemi/inocbx/ic3/Xe/images/XeF8_2-.jpg

Another visual representation is the f-orbital (xyz)... if the lobes were treated as not having spins (all the same "ligand").
http://en.wikipedia.org/wiki/File:F4M2.png
 
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For a coordination number of 8, either a square antiprism (like the XeF82- and IF8- anions) or a dodecahedron (ZrO8) can be formed.
 
In square antiprism the squares to not overlay each other, they are staggered 45 degrees. But thank you for showing me antiprisms, very neat structures to try to do molecular symmetry on.
A dodecahedron has too many coordinates from what I see but also an interesting structure.

The pointgroup is Oh, which confused me because there is a 6 coordinate version of the Oh pointgroup. For anyone having the problem I had... The 6 coordinate Oh complex has 3 sigma-h along 5 atoms (4 ligands and center), while the 8 coordinate has 3 sigma-h in between bonds (only intersecting the center atom). Visually, I had a lot of trouble believing they were both the same pointgroup.
 
So you don't seem to be interested in all 8 fold coordinate complexes but in those with a cubic arrangement of the ligands.
The cubic and the octahedral coordination have the same symmetry because they are dual to each other: an octahedron can be inscribed into a cube so that the corners of the octahedron coincide with the centers of the faces of the cube. This works also the other way round. The same relation holds for an icosahedron and a dodecahedron which have both symmetry group I_h. The tetrahedron is dual to itself.