[Lars Fred riksson]

According to standard tables of hybridisation, the central atom tetragonal arrangements should always be regarded to have sp³ hybridised orbitals (disregarding Bent's rule).

So, the sulphate ion SO₄^2-, should therefore have S with sp³ orbitals. But, in the VB theory framework SO₄^2- is usually drawn with two double bonds to O, implying participation of two d orbitals. But then, shouldn't S have sp³d² hybridised orbitals for this to work? But sp³d² is used for octahedral arrangements as in SF₆ which is clearly not the case for the sulphate ion.

I know this participation of d orbitals have been theoretically firmly disproved, but still, VB theory (with its hybridisation schemes) and classic Lewis structures should be consistent, shouldn't they?

[AWK]

The two methods you are considering are very approximate and will use different descriptions.
As you learn about the more accurate quantum methods (although only less approximate, as there are no exact methods for more complex chemical molecules), you will learn that some approximation reproduces well the geometry of the molecules, other dipole moments, and still other energy, and you have to to deal with it (or to create an even better calculation method in the future).
If you are so interested, compare the calculation results for sulfuric acid and sulfate ion. If you don't understand any of this, don't worry yet.
http://www.colby.edu/chemistry/webmo/sulfate.html
http://www.colby.edu/chemistry/webmo/H2SO4.html

[Lars Fred riksson]

Thanks a lot! Very interesting webpage with a lot of molecules and polyatomic ions with results seemingly strongly supporting Bent's rule.

Hybridisatation is not hard to comprehend (at least when I figured out that you don't have the letter Å on your keyboard).

MO:s, though, are best avoided for all us chemists not specialising in computional chemistry (except of course for diatomics).