[Donaldson Tan]

HClO₄ <-> ClO₄^- + H⁺

HClO4 dissociates according to the above equation to produce chlorate(VI) ion. My lecturer said that the chlorate(VI) ion exhibit electron delocalisation. I, however, am not convinced.that this ion should. VSEPR predicts this ion to be tetrahedral and therefore I cant see how the p and d orbitals overlap in a plane to faciliatate electron delocalisation.

[Mitch]

Anyone who tells you that d-orbitals are involved needs to be beat down, this is a common lie people tell students and it adds nothing to the understanding of the phonomena of hypervalency.

I think he only means that the charge is not localized on one oxygen but spread accross the entire molecule. Also, VSEPR has no way to make sence of Hypervalency only MO theory can handle it properly.

[Donaldson Tan]

the charge is spread over the entire molecule?

not only chlorate(VI) is tetrahedral, but also each Cl-O bond is equivalent to the other, therefore you can't differentiate the the oxygen where hydrogen was originally bonded to from the other oxygen atoms, after the dissociation process.

chlorate(VI) can be described a central chlorine atom doubled bonded to 4 oxygen atoms. this arrangement leads to a -1 formal charge on the Cl atom and 0 formal charge on the O atoms. How can see if the charge should be spread among the entire molecule?

[Mr Amino]

Well I am no inorganic chemist but it would simply make sense that this particular molecule with -1 charge would exhibit electron delocalization.  If, for instance, that charge were localized around a certain oxygen atom, that would be an interesting phenominon indeed.

I dont think this has anything to do with the interaction of molecular orbitals.  I see a central atom surrounded by equivalent atoms with identical electronegativity and similar bonds so it would only make sense that the charge be distributed around the molecule.

Im not sure what your class is studying right now so I can't really comment on what theory your professor is trying to demonstrate with this example.

[Demotivator]

It's not chlorate, it's perchlorate(VII)

Perchlorate has a lewis structure of three double bonds and one single bond (one O formal charge of -1) and 4 resonance structures.

[Mitch]

Domotivator is right.

[Donaldson Tan]

perchlorate(VII) with the -1 formal charge on one of the oxygen atoms. electron delocalisation happens because we are unable to account which oxygen atom where the -1 charge should be located?