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It is a phemer on in which e-pair(s) of lower sub energy level undergo delocalisation in to vacant d-orbital of same shell.Such type of phenomenon reduces availibility of vacant-d orbital to accept back-bond as well as availibility of lone pairs for donation also decreases.Hence,extert of back bonding in those molecules where such type of phenomenon occurs.
In the presence of d-orbital resonance hyp. of donor atom remains almost same for eg.
If e- pair of s-orbital undergo delocalisation in to vacant p-orbital of same shell then such phenomenon is called p-orbital resonance.
CONSEQUENCES OF BACK-BONDING
1.Due to back-bonding B.L.
2.Due to B.B. bond angle either ^or remains same but does not provide all subsitituent excepting B.B. must be same.
3.If central atom of a molecule eccepots B.B.then its lewis acidic character and its hyb. does not change due to B.B.
4.If lone pair of central atom participates in back bonding then Lewis basic character and hyb. changes when d-orbital resonance does not occur at substituents.
However if d-orbital resonance is presence at subtituents then hyp. of central atom remains same.
5.Generally,steraic factors extent of B.B.
6.B.B. reduces bond polarity in general.However,In CCL*3 B.B. bond polarity.
BRIDGE BONDS
Most of e-deficient molecules undergo stabalization either by participating in B.B. or in dimmerisation provide specific condition must be satisfied in respective case.
BF*3 does not dimmerise due to most effective 2pn-2pn B.B. while Bcl*3,BBr*3,BI*3,BME*3 does not dimmeaise due to steric fector.
ANALYSIS (IMPORTANT POINTS)
=Total number of bonds =6 (3c-2e bond=2), (2c-2e bond=4)
=B.E. of terminal B-H-B bond (441 kg/mol)
B.E of terminal B.H bond (381 kg/mol)
=Max number of atom can be lie in molecule=6
=Max number of 2c-2e that can lie in plane in B*2H*6=4
=Max number of 2c-2e bonds that can lie in a plane of B*2H*6 molecule that contains bridge bonds=0
=At any instant octet of boron atom are not completed.Hence,B*2H*6 molecule is hypovalent and it acts as Lewis Acid.
=When B*2H*6 reacts with Lewis base it undergoes two type of cleavages.
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