Difference between co-ordinate covalent bonds and ionic bonds?

[Hammad Shahid]

Right now we are learning about the Lewis definition of acids and bases.
I don't get the clear difference between co-ordinate covalent and ionic bonds. For example, AlCl3 is listed as a co-ordinate covalent bond because experiments show that it behaves like a covalent molecule. Why can't it be Al(3+) + 3 Cl(-) ?

 

[chemisttree]

Right now we are learning about the Lewis definition of acids and bases.
I don't get the clear difference between co-ordinate covalent and ionic bonds. For example, AlCl3 is listed as a co-ordinate covalent bond because experiments show that it behaves like a covalent molecule. Why can't it be Al(3+) + 3 Cl(-) ?

It could I guess except it really isn’t. Purely ionic compounds have very high boiling points but AlCl₃ sublimes at less than 200C. I used to purify it by sublimation before using it in moisture sensitive reactions. A schlenk to another schlenk via a u-tube under static vacuum as I recall. You can’t do that with an ionic compound!
So there must be something going on that causes this compound to significantly share its electrons rather than transfer them. The difference in electronegativity is about 1.5 so its classed as a polar covalent bond by that (handwaving) rule. There are other rules we use to predict the ionic/covalent character in a bond like Fajan’s rule, for example. That rule states that a metal/nonmetal bond is ionic if the cation is large, has a small charge and if the anion is small. None of those are true for AlCl₃. Of course these rules don’t really explain the why as much as the what but they help to predict what is observed in nature.

Try drawing the lewis dot structure for AlCl₃ and see what you get. You’ll find that the Al in this compound is a couple of electrons shy of a full octet. Thus, it wants to share a couple of extra electrons which are supplied by nearby Cl in a coordinate covalent fashion.

 

[Dr_Nate]

In science, we use models to describe the essence of a particular situation. Many models have a limited range of validity. That's sort of what's going on here when you want to say something is ionic or covalent. As was mentioned by @Chemistree there is a concept of a continuum between covalent and ionic.

Not that being said, AlCl₃ dissolves into its ions in aqueous solution. I don't think many chemists would call that covalently bonded.

Coordinate covalent bonds are used to get around the rule that "each atom contributes an electron to a covalent bond" in Lewis structures. In ammonium, for example, there should be a coordinate covalent bond and three covalent bonds, but in fact all 4 bonds are the same.

 

[TeethWhitener]

Not that being said, AlCl3 dissolves into its ions in aqueous solution. I don't think many chemists would call that covalently bonded.

No, AlCl₃ reacts with water. The difference is that if you evaporate the water, you don’t get back AlCl₃.

 

[TeethWhitener]

The coordinate covalent bonding that AlCl₃ undergoes can be most clearly seen in its dimer, which has a structure analogous to diborane, featuring 3-center-2-electron bonds. In the case of AlCl₃, however, it’s more accurate to view this as one of the lone pairs on a Cl atom acting as a Lewis base and donating to the empty p_z orbital centered on Al in another AlCl₃ monomer (with concomitant rehybridization). This Lewis acid-Lewis base interaction is generally described as “coordinate covalent” (or less frequently now, “dative”).

 

[chemisttree]

The coordinate covalent bonding that AlCl₃ undergoes can be most clearly seen in its dimer, which has a structure analogous to diborane, featuring 3-center-2-electron bonds.

I was kinda hoping the OP would actually try the Lewis structure thing and ponder the extra Cl.

 

[Dr_Nate]

No, AlCl₃ reacts with water. The difference is that if you evaporate the water, you don’t get back AlCl₃.

Yes, you are correct.