[Sayantan]

In PCl5, since chlorine is more electronegative than phosphorus,the d-orbitals of phosphorus contract due to formal positive charge and thereby decreasing its energy to participate in hybridisation as the lower s-orbital electron pair is unpaired and promoted to vacant d.Now for me there arises two questions-
1.Why the d-orbitals contract when there is partial formal positive charge?
2.Why the concept of lowering of energy is being related to hybridisation when hybrid orbitals are in itself equivalent energy orbitals after mixing of different energy orbitals to generate the same number of same energy level orbitals??
Reference of doubt-concise inorganic chemistry-JD LEE Covalent bond chapter-participation of d-orbitals in molecuar bonding
even this link also relates to this: http://pubs.rsc.org/en/Content/ArticleLanding/1956/JR/JR9560004895#!divAbstract