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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 "