chemical bonding theories

8/16/2019 Chemical Bonding Theories

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Bonding III

Valence Bond Theory

and

Molecular Orbital Theory


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Valence Bond Theory

• Extends the Lewis concept of electron pairbonds by introducing the notion of orbitaloverlap and hybridization.

• ybridi!ation is necessary to account formolecular geometry predicted by V"E#$

• ybrid ato%ic orbitals are constructed by

%a&ing linear combinations 'su%s anddifferences( between hydrogen)li&eorbitals in the *alence shell of each ato%


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ybridi!ation

• Exa%ple+ Be,• Linear Molecule- eui*alent Be) bonds• "y%%etry /dapted Linear 0o%bination '"/L0(

of hydrogen)li&e ato%ic orbitals in the *alenceshell of Be• Basis set consists of+ ,s- ,px- ,py- ,p!• Only ,s and ,px are %ixed1 ,py and ,p! retain

their original for%.• 2ideal3 hybrids 4 s- 4 p character• Energetic 2cost3 of hybridi!ation 'promotion

energy( is offset by for%ation of stronger bonds


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sp hybridi!ation


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Bonding 5escription

• Valence electrons of each ato% aredistributed a%ong the a*ailable orbitals.

• In this case- the two *alence electrons ofBe are placed in the sp hybrid orbitals1 there%aining p orbitals are *acant.

• Each ato% has one electron in the 6s

orbital.• Orbitals of each ato% o*erlap to for%

electron pair bonds.


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"ig%a and #i Bonding


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sp7 hybridi!ation


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VBT description of ,O

• Oxygen has sp7 hybridi!ation

• Ideal tetrahedral angle is 689.:o

• Obser*ed bond angle in ,O is ; 68:o

• This indicates that hybrids in*ol*ed in O)

bonding orbitals ha*e %ore p)character

• Lone)pair orbitals on oxygen ha*e %ore s)character


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Molecular Orbital Theory

•


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Molecular Orbitals of ,

• Basis set of orbitals consists of only the 6sorbital on each )ato%

• Linear co%binations of these orbitals

produce two molecular orbitals

• > co%bination gi*es bonding orbital σ'6s( 'constructi*e interference between

wa*efunctions• ) co%bination gi*es antibonding orbital σ?'6s( 'destructi*e interference(


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Molecular Orbitals of ,


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Molecular Orbitals of Li,

@OTE+ Both bonding and antibonding orbitals arising fro% the 6s

orbitals are filled A no net bonding results fro% core electrons


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p)Orbital o*erlap %odes


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o%onuclear 5iato%ic with p)Orbitals


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Effect of s)p interaction


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MO diagra% for O, and


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Molecular Orbital diagra%sfor ,nd row diato%ics


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eteronuclear 5iato%ics

"trength of orbital interactions depends upon+

• "y%%etry %atch• "patial o*erlap• Energy o*erlap


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MO diagra% for <


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Bonding in ,O


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#olyato%ic Molecules

• =roup orbital approach

• "y%%etry)/dapted Linear 0o%binations

• Exa%ples+ ,O


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0haracter Table for 0,*


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MOCs of ,O '/6 sy%%etry(

ψ '6a6( D 's( > 'φ6( ψ ',a6( D 'p!( ) 'φ6( Ψ'7a6( D 'p!( > 'φ6(

O',s( and O',p!( /OCs %ix with 'φ6( L=O to for% 7 MOCs+

"trongly Bonding ea&ly Bonding "trongly /ntibonding


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MOCs of ,O 'B, sy%%etry(

ψ '6b,( D 'py( > 'φ,(

Bonding

ψ ',b,( D 'py( ) 'φ,(

/ntibonding

O',py( /O %ixes with 'φ,( L=O to for% , MOCs+


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Molecular Orbitals of ater


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Bonding in "


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MO diagra% for "

chemical bonding theories

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