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BENT'S RULE OF HYBRIDISATION:


CONCEPTUAL FACT: The more electronegative atom prefer to stay in that hybrid orbital having less S-character or more p- character and more electropositive atom prefer to that hybrid orbital which have more S-character or less p-character.
EXPLAINATION: The more electropositive atom or group will withdraw the bond pair more from central atom. it is easy when hybrid orbital is having less S-character and more P-character .
 S-orbital is closer to the nucleus so it electronegativity is more than p-orbital.
ILLUSTRATIVE EXAMPLE (1): PCl3F2 (sp3d Hybridization) and TBP (Trigonal bi pyramidal)
  Where angle < F-P-F =180 and <Cl-P-Cl =120 two P-Cl bond is axial while three P-Cl bond is equatorial                                  

(1) APICOPHIOLICITY IN TBP GEOMETRY:
(1) Trigonal bipiramidal geometry more electronegative atom prefers to stay in low electronegative PZdz2 orbital of sp3d hybridization. You can also say that more electronegative atom prefer that hybrid orbital having low s-character or no s-character.
(2) If electronegativity difference between central atom and surrounding atom is large then due to polarity some ionic character is developed and covalent character decreased.
(3)The poor covalence is not only due to electronegativity difference between bonding atom.  it is also generated due to poor overlapping, steric hindrance or mismatch of the overlapping orbitals.
(4) Position of lone pair: Bent rule is very important in predicting position of lone pair. lone pair is attracted by only one nucleus while bond pair is attracted by two nucleuses. Central atom hold lone pair cloud tightly if central atom is having more S-Character.
ILLUSTRATIVE EXAMPLE (2): SF4 (Sp3d) TBP and SF2Cl2 (Sp3d)
ILLUSTRATIVE EXAMPLE (3): XeF2 and XeO3F2
(2) ORBITAL ANALYSIS (CALCULATION OF (% S ) AND ( %P) CHARACTER:  
S% character is equatorial orbital at1 200c :
S% character for orbital at 900 C:



(3) EFFECT OF THE STRAINGTH OF COVALENCY: (Alternate Statement of bent)

RULE: The more electronegative atom not only prefer to stay in that orbital which having less  % S character (more p-character) but it also decreases % S-character and increases % P-character in its attached orbital from the central atom depending on circumstance.
On increasing % s character in hybrid orbital , the bond length  decreases while bond angle increases.
ILLUSTRATIVE EXAMPLE (3): : Explain C-H bond length of CH4 is longer than C-H bond length of Difloromethan (CH2F2) ?

EXCEPTIONS OF BENT’S RULE:
(1): Bent’s rule is applicable in those molecules where central atoms are same and they are also in same Hybridization. For example N-N bond length cannot be compared in N2H4 and N2O4 using Bent rule.
ILLUSTRATIVE EXAMPLE (4): Arrange PF3 ASH3, PH3, NH3, H2Se in decreasing order by Bent’s rule here we used DRAGO’S RUEE”
SOLUTION: NH3> NF3>PF3> PH3>AsH3>H2Se
(2): Bent’s rule violets in those molecules where steric factor’s plays dominating rule.
ILLUSTRATIVE EXAMPLE (5): Compared Bond angle among H2O OF2, OMe2, OCl2  
SOLUTION: 
ILLUSTRATIVE EXAMPLE (6):   IN CH2SF4 


Which of the following option is correct regarding ?

(1) 1800> 120                                                       (2) 1800> >120
(3) 1200> >900                                                         (4) 900 > >00

SOLUTION: S -character present in equatorial so more decrease in Bond angle in equatorial orbital than in axial. Because no S-character in present in axial orbital.
So 1200 > > 90 and 1800 > > 120 

ILLUSTRATIVE EXAMPLE (7):   Compare Dipole moment of following compound pairs

(1) Dipole moment of PCl2F3 is non zero while dipole moment of PCl3F2   is zero why?
(2) Dipole moment of P(CH3)2 (CF)3  is non zero while dipole moment of P(CH3)2(CF3)2 is zero why?

SOLUTION: According to bent rule more electronegative atom or group attached those orbital have minimum S- character there in Trigonal bipiramidal (TBP) Geometry we known that axial orbital hare no S- character so F and -CF3  are attached with axial positions. 
(1) :
(2)

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