Valence bond theoryValence bond theory

Electrons are not simply dotsElectrons are not simply dots

And bonds are not sticksAnd bonds are not sticks

Learning objectivesLearning objectives

Describe principles of valence bond theoryDescribe principles of valence bond theory Predict hybridization of orbitals based on Predict hybridization of orbitals based on

Lewis dot structures and electronic Lewis dot structures and electronic geometrygeometry

Describe difference between Describe difference between sigmasigma and and pipi bondingbonding

Taking it to the next level: Taking it to the next level: acknowledging orbitalsacknowledging orbitals

VSEPR is quite successful in predicting VSEPR is quite successful in predicting molecular shapes based on the simplistic molecular shapes based on the simplistic Lewis dot approachLewis dot approach

But our understanding of the atom has the But our understanding of the atom has the electrons occupying atomic orbitalselectrons occupying atomic orbitals

How do we reconcile the observed shapes How do we reconcile the observed shapes of molecules with the atomic orbital picture of molecules with the atomic orbital picture of atomsof atoms

Valence bond theoryValence bond theory

Valence bond theory is the simplest Valence bond theory is the simplest approach to an orbital picture of covalent approach to an orbital picture of covalent bondsbonds

Each covalent bond is formed by an overlap Each covalent bond is formed by an overlap of atomic orbitals from each atom of atomic orbitals from each atom

The individual orbital identity is retainedThe individual orbital identity is retained The bond strength is proportional to the The bond strength is proportional to the

amount of orbital overlap amount of orbital overlap

Overlap of two 1s orbitals in HOverlap of two 1s orbitals in H22

Overlap of two 2p orbitals directed along the bond Overlap of two 2p orbitals directed along the bond axis (sigma bond)axis (sigma bond)

Overlap of p and s orbitalsOverlap of p and s orbitals

Problems with tetrahedral bondsProblems with tetrahedral bonds

In CHIn CH44 the bonds are all equivalent and at the bonds are all equivalent and at

angles of 109.5°angles of 109.5° The 2p orbitals in C are at 90° - far from The 2p orbitals in C are at 90° - far from

optimum for overlapoptimum for overlap The ground state configuration is 2sThe ground state configuration is 2s222p2p22

Reconcile these facts with the known Reconcile these facts with the known structurestructure

HybridizationHybridization

The wave mechanics permits mixing of the The wave mechanics permits mixing of the atomic orbital set to produce “hybrid” orbitalsatomic orbital set to produce “hybrid” orbitals

Hybridization alters the shape and energy of Hybridization alters the shape and energy of the originalthe original

In the case of C, the differences between In the case of C, the differences between the 2s and 2p are smoothed out and a the 2s and 2p are smoothed out and a homogeneous collection of four sphomogeneous collection of four sp33 hybrid hybrid orbitals is producedorbitals is produced

spsp3 3 hybridizationhybridization

Formally, one of the 2s Formally, one of the 2s electrons is promoted electrons is promoted to the empty 2p orbital to the empty 2p orbital (an energy cost, which (an energy cost, which is repaid on bond is repaid on bond formation)formation)

The four basis orbitals The four basis orbitals are then “hybridized” to are then “hybridized” to yield the set of four spyield the set of four sp33

Tetrahedral directions and spTetrahedral directions and sp33 hybridshybrids

Valence bond picture of CHValence bond picture of CH44

Each C spEach C sp33 hybrid contains one electron hybrid contains one electron Each H 1s contains one electronEach H 1s contains one electron

Lone pairs occupy spLone pairs occupy sp33 hybrid orbitals hybrid orbitals

Valence bond picture of the tetrahedral electronic Valence bond picture of the tetrahedral electronic geometry provides same results for the molecules geometry provides same results for the molecules with lone pairswith lone pairs

Notes on hybridizationNotes on hybridization

The total number of orbitals is unchangedThe total number of orbitals is unchanged Four atomic orbitals (s + 3 x p) give four hybrid Four atomic orbitals (s + 3 x p) give four hybrid

orbitals (4 x sporbitals (4 x sp33))

The electron capacity remains unchangedThe electron capacity remains unchanged There is one hybridization scheme for each There is one hybridization scheme for each

of the five electronic geometriesof the five electronic geometries The same hybridization scheme is always The same hybridization scheme is always

used for a given electronic geometryused for a given electronic geometry

sp hybridization for linear geometrysp hybridization for linear geometry

One s and one p orbitalOne s and one p orbital

spsp22 hybridization for trigonal planar hybridization for trigonal planar

One s and two p One s and two p orbitalsorbitals

Sigma and pi bondingSigma and pi bonding

The hybridized orbitals describe the The hybridized orbitals describe the electronic geometry: bonds along the electronic geometry: bonds along the internuclear axes (sigma bonds)internuclear axes (sigma bonds)

The “unused” p orbitals overlap in a parallel The “unused” p orbitals overlap in a parallel arrangement above and below the arrangement above and below the internuclear axis (pi bonds)internuclear axis (pi bonds)

Comparison of pi and sigma bondingComparison of pi and sigma bonding

Pi bonding accounts for bond Pi bonding accounts for bond multiplicitymultiplicity

Two unused p orbitals in sp hybrid (linear Two unused p orbitals in sp hybrid (linear geometry)geometry) Two pi bondsTwo pi bonds NN≡N triple bond (one sigma, two pi)≡N triple bond (one sigma, two pi)

One unused p orbital in spOne unused p orbital in sp22 hybrid (trigonal hybrid (trigonal planar geometryplanar geometry One pi bondOne pi bond C=C double bond (one sigma, one pi)C=C double bond (one sigma, one pi)

Valence bond picture of ethylene Valence bond picture of ethylene HH22C=CHC=CH22

Sigma bonds between C and H (blue/red) and C Sigma bonds between C and H (blue/red) and C and C (blue)and C (blue) Six electrons around CSix electrons around C

Pi bond between C and C (green)Pi bond between C and C (green) Two electrons around CTwo electrons around C

Valence bond picture of acetylene Valence bond picture of acetylene HCHC≡CH≡CH

Sigma bonds between C and H (red and blue) and Sigma bonds between C and H (red and blue) and C and C (blue)C and C (blue) 4 electrons around C4 electrons around C

Two pi bonds between C and C (green)Two pi bonds between C and C (green) 4 electrons around C4 electrons around C

Beyond coordination number 4Beyond coordination number 4

Invoke empty d orbitals Invoke empty d orbitals (impossible for second row (impossible for second row elements)elements) One d orbital for trigonal One d orbital for trigonal

bipyramidalbipyramidal Two d orbitals for octahedralTwo d orbitals for octahedral

Number of orbitals in Number of orbitals in hybrid always equals hybrid always equals number of charge cloudsnumber of charge clouds

Trigonal bipyramid –Trigonal bipyramid – sp sp33dd

Octahedral –Octahedral –spsp33dd22

Shortcomings of valence bondShortcomings of valence bond

The orbitals still maintain atomic identityThe orbitals still maintain atomic identity Bonds are limited to two atomsBonds are limited to two atoms Cannot accommodate the concept of Cannot accommodate the concept of

delocalized electrons – bonds covering delocalized electrons – bonds covering more than two atomsmore than two atoms

Problems with magnetic and spectroscopic Problems with magnetic and spectroscopic propertiesproperties