Which sulphur compound did you have in mind particularly?

EDIT: Now that you've changed the question I don't need to ask my question. I'll leave it there for the record.

I'm no chemist, but at school we learnt that sulphate enjoys some of the benefits of covalent bonding and some of ionic bonding. If we forget P orbitals and just think in terms of shells; we have 4 oxygens with 6 outer electrons, each wanting 2 electrons for a full shell; and a sulphur with 6 outer electrons. The sulphur shares 6 electrons with 3 of the oxygens, but still needs 2 electrons to fill its shell.... as does the remaining oxygen. Get a couple of electrons from a passing metal and the single sulphur can fall into a tetrahedral arrangement with 4 oxygens. Voila, you have SO4-- and all outer shells are full.

I know this is very simplistic but I'm not enough of an expert to explain it in terms of P orbitals. Hope that helps.