Since phosphorus have 5 unpaired electrons in the excited state, it can form 5bonds with the chlorine atoms, this is due to expanded octet in phosphorus.
Expanded octet also known as hyper valency is a concept where valence shell electron count generally exceeds eight electrons. In methane (CH4), carbon has a full octet that is eight valence electrons. In phosphate ion (PO43-), each oxygen has a full octet (eight valence electrons), whereas phosphorus has an expanded octet that is it has ten valence electrons.
P has a 3s23p33d0 electron configuration so it can form 3 bonds with the 3 unpaired electrons. But it can also excite one of the 3s2 electrons to an empty 3d orbital and now has a 3s13p13p13p13d1 electron configuration which can now bond with 5 Cl atoms that is it has expanded octet.
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