Question

What atomic or hybrid orbitals make up the sigma bond between As and F in hexafluoroarsenate ion, AsF6- ?
orbital on AS:?
orbital on F:?

Answer 1

The six orbitals on the central atom, As, are sp^3d^2 hybrids. That is the kind of hybridization you have in an octahedral structure, "XY6". Each of the hybrid orbitals on the As atom is forming a sigma bond with the half filled p orbital on an F atom.

Answer 2

As is [Ar]3d10 4s2 4p3.
F is [He] 2s2 2p5

So As has 5 electrons
F has 7 and 7x6 = 42
Add 1 electron for the negative charge on the ion 1
42+5+1 = 48.
So this has 6 regions of high electron density (from the 6 F atoms), probably this is d2sp3 hybridization and octahedral structure.