Why is the backbonding of BF3 more than PF3?
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Answer:
FIRST ONE: FROM THE VIEW POINT OF .THE NF3 IS SP3 HYBRIDISED WHERE AS BF3 IS SP2 HYBRIDISED .SO THE DIFERENCE IN B/W THE BOND ANGLE.
NOW THE SECOND PART: WHY IT IS SO? THE ANSWER IS BACK BONDING IN BF3 WHICH HELP IT TO MAKE A PLANNER STRUCTURE ATTENDING A BOND ANGLE OF 120 DEGREE.THAT TYPE OF BACK BONDING IS IMPOSSIBLE IN THE CASE OF NF3 MOLECULE BECAUSE NITROGEN HAS NOT GOT ANY VACANT ORBITAL LIKE BORON.AND ITS OCTET IS ALREADY BEEN FILLED UP IN NF3 SO DUE TO LP-BP INTERACTION IT IS SLIGHTLY TILTED HAVING A BOND ANGLE 102°(SP3 HYBRIDISATION=104.5°).BUT IN CASE OF BF3 THE CENTRAL ATOM BORON ATTEND A SEXTET CONFIGURATION OR SUFFERING FROM OCTET OR HAVING ELECTRON DEFICIENCY(HAVING 6 ELECTRON NOT 8 ELECTRON).SO THERE IS A 2P-2P BACK BOND PRESENT FROM FILLED 2P ORBITAL OF FLUORINE TO THE VACANT 2P ORBITAL OF BORON.AND THE SINGLE B-F BOND ATTAINS DOUBLE BOND CHARACTER AT A TIME FORMING A RESONENCE HYBRID STRUCTURE WHICH SPREAD ACROSS THE 3 SINGLE B-F BOND.ATTAINING ALMOST A TP( TRIANGULAR PLANNER OR SP2) GEOMETRY.HENCE THE DIFERRENCE IN THEIR BOND ANGLE.