Question

The strength of chemical bonding is as follow

Metallic > covalent > ionic > H+ > dipole2 > Van Der Waals

So, shouldn't the boiling point of compounds with covalent bond higher than that of ionic??
But why does the boiling point of AlF3 which is an ionic compound higher than PF3, which is a covalent compound??

Answer 1

Your list is a little shaky. For one thing, vdW attraction isn't saturable, so it grows without limit as the size of the molecules increases. For very large molecules, e.g. the polymers that make up most of living tissue, the vdW attraction between two molecules can be considerably larger than any single bonding interaction.

However, the most important fact you're missing here is that boiling doesn't generally destroy covalent chemical bonds, for small molecules. When you boil water, you don't tear apart the water molecules -- each goes into the vapor phase as a whole. That means to boil water you need only overcome the relatively weak vdW forces between the water molecules. The strength of the covalent bonds between the H and O doesn't matter.

It's certainly true when you boil a metal that you must destroy covalent bonds, because the metal is bonded throughout. The same thing would happen if you tried to boil a solid held together throughout by covalent bonds, like diamond or silica, and indeed these have boiling points quite as high as ionic or metallic solids.

AlF3 is not exactly an ionic compound, which should not be too surprising, given the peculiar character of Group 3A metals. It forms an extended lattice with strong bonds throughout, which gives it a high melting and boiling point. On the other hand, PF3 is a classic covalent compound, and solids made of it are held together nly by weak dipole-dipole and London forces.

Answer 2

I should add: this means when you boil AlF3, you do need to disrupt Al-F bonds, because you are forming AlF3 units from an extended array of Al and F. On the other hand, when you boil PF3, you are merely disrupting the intermolecular attractions between whole PF3 molecules, which you then send into the gas phase.