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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.

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    $\begingroup$ You thought wrong. Oxygen is more electronegative. $\endgroup$
    – Ivan Neretin
    Apr 24, 2016 at 22:00
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    $\begingroup$ 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. $\endgroup$
    – permeakra
    Apr 25, 2016 at 4:35

4 Answers 4

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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:

enter image description here

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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  • $\begingroup$ 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. $\endgroup$
    – Tan Yong Boon
    May 14, 2018 at 23:15
  • $\begingroup$ 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 ... $\endgroup$
    – Ian Bush
    May 15, 2018 at 10:35
  • $\begingroup$ Sir, are these 3 centre 4 electron bonds stronger than the terminal bonds? (Couldn't find on the internet) $\endgroup$
    – Satya
    May 28, 2021 at 18:56
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$\ce{Al2Cl6}$ is covalent as the electronegativity difference between Al and Cl is 1.5 which is less than 1.7.
$\ce{Al2O3}$ is ionic as the electronegativity difference between Al and O is 2.0 which is greater than 1.7.

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$\ce{Al2O3}$ is ionic due to relative size of oxygen and aluminium and polarizing power of Al, (since we know that aluminium has a charge of +3, provides three electrons) in case of $\ce{Al2Cl6}$ & $\ce{AlCl3}$ , it seems to be covalent due to similarities like banana bonding & bigger radius of Cl (in compression to oxygen). Banana bonding is not possible in $\ce{Al2Cl6}$ due to size of Cl atom . Radius of Cl even gets bigger when it forms an anion by receiving the electron from aluminium , The aluminium's cation being smaller than its original atom has a high polarizing power which attracts and distorts the electron cloud of the chloride ion (has a high polarizability) and forms a covalent bond in case of $\ce{Al2Cl6}$ & $\ce{AlCl3}$ ($\ce{AlCl3}$ form ionic for very shot time) . So is not in the case of $\ce{Al2O3}$, as the oxide ion's atomic radius is not large enough for the aluminium cation to distort it thus it stays in ionic bonding.

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Firstly let's take $\ce{AlCl3}$, since we know that aluminium has a charge of +3, provides chlorine with three electrons (1 to each chlorine). The chlorine being in the second period has a larger atomic radius (in comparison to oxygen). This even gets bigger when it forms an anion by receiving the electron from aluminium. The aluminium's cation being smaller than its original atom has a high polarizing power which attracts and distorts the electron cloud of the chloride ion (has a high polarizability) and forms a covalent bond.

So is not in the case of $\ce{Al2O3}$, as the oxide ion's atomic radius is not large enough for the aluminium cation to distort it thus it stays in ionic bonding.

In short, $\ce{AlCl3}$ does form an ionic bond at first but it's a really small transition state for like nanoseconds, so it quickly forms a covalent bond by the polarizing process.

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