The valence electron configuration for antimony (Sb) is:
Sb = 5s²5p³5d⁰
In SbCl₅²⁻, antimony has a -2 charge i.e. it has 2 additional electrons
Sb²⁻ = 5s²5p⁵5d⁰
Following a two electron transition from p→d orbital we have:
Sb²⁻ = 5s²5p³5d²
There is a total of 5 unpaired electrons (3 in the p and 2 in the d) which can form five bonds with the 5 Cl atoms.
Thus the hybridisation of Sb in SbCl₅²⁻ is sp³d²
I *think* through weathering and erosion. Hope this helped
