How is Al2Cl6 covalent and Al2O3 ionic?

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You are right that the difference in electronegativity is responsible for the ionic bonds in aluminum and oxygen, and normally that would be the case with chlorine too, but $\ce{Al2Cl6}$ is a special molecule.

The structure of $\ce{Al2Cl6}$ can be viewed as two aluminum atoms covalently bonded to four chlorine atoms each. Two of the chlorine atoms bridge between the two aluminum atoms. The following is a possible representation:

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A special type of bonding called 3 center 4 electron bonding is present in the structure. These bonds result from a combination of a filled p orbital and two half filled p orbitals. This causes there to be a filled bonding and a filled non-bonding orbital. The bond orders between each bridging chlorine and aluminum atom are 0.5; the structure of 3 center 4 electron bonds makes it so that the bonding orbital is delocalized over both the bonds. $\ce{Al2Cl6}$ has two of these bonds and they are responsible for the covalent nature of the molecule.

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Updated on August 01, 2022

Comments

  • user289661
    user289661 over 1 year

    I encountered a question where the type of bonding for various oxides are prompted. Two of which are $\ce{Al2Cl6}$ and $\ce{Al2O3}$.

    I thought that chlorine atoms are more electronegative than oxygen atoms, so if one of them is ionic and the other is covalent, surely chlorine would be ionic, because it may be able to "tear off" the electrons from aluminium atoms.

    This is evidently not the case according to the answer key of this question. I would like to know an explanation for the ionic/covalent properties of these bonds.

    • Ivan Neretin
      Ivan Neretin over 7 years
      You thought wrong. Oxygen is more electronegative.
    • permeakra
      permeakra over 7 years
      short answer: both have comparable ionic character, but chloride ions are large in comparison with oxygen, so ionic lattice with high coordination numbers cannot form.
  • Tan Yong Boon
    Tan Yong Boon over 5 years
    The 3c-4e bond demands a linear geometry about the central atom. Because the bond arises from the head-on combinations of 3 p orbitals. Thus, I can't understand how it is the case here.
  • Ian Bush
    Ian Bush over 5 years
    Note this is the structure of AlCl3 in the liquid and low temperature gas phases. In the solid it is a layered structure with octahedrally coordinated aluminium, see e.g. cs.mcgill.ca/~rwest/wikispeedia/wpcd/wp/a/… . In Al203 aluminium is also octahedrally coordinated. Maybe they aren't so different ...
  • Satya
    Satya over 2 years
    Sir, are these 3 centre 4 electron bonds stronger than the terminal bonds? (Couldn't find on the internet)