BONDING THEORIES (VB)____________________________Chapter 11.1-11.2, Carey

The Covalent Chemical Bond (9.3) FIG I ‒ Potential Energy Change to Form H2

What is a chemical bond?

Why do chemical bonds occur?

Descriptions of bonding: Valence Bond Theory (Chapter 11.1 – 11.2, Carey)

molecule - collection of atoms with localized bonds between pairs of atoms bonds formed by overlap of atomic orbitals on adjacent atoms (constructive interference of wave

functions) orbitals refined by hybridization

Molecular Orbital Theory (Chapter 11.3, Carey, Morrison and Boyd) molecule - collection of nuclei valence electrons occupy molecular orbitals that extend over entire molecule molecular orbitals formed from a combination of atomic orbitals

11.1 - Valence Bond (VB)Theory and Orbital Hybridization 1. Lewis structures (based on valence electrons)

octet rule bonding, lone pairs single, double, triple bonds formal charges: (FC = GN ‒ # lone electrons ‒ 1/2 # bonding electrons) resonance structures valence shell expansion (only ≥ 3rd period when more than four atoms are attached to a cen-

tral atom or a formal charge separation exists) electron deficient atoms: Be - 4; B, Al - 6

2. Valence Shell Electron Pair Repulsion (VSEPR) Theory repulsions: BP/BP < BP/LP < LP/LP number of electron pairs (steric number) => electronic geometry molecular geometry (shape) => bond angles, distortions electronegativity => bond polarity molecular polarity => dipole moment

3. Bonding (based on atomic orbitals of valence electrons) overlap of atomic orbitals on adjacent atoms when necessary, hybridize orbitals

- 2 - Valence Bond Theory: Orbital Overlap as a Chemical Bond (11.1)

obtain orbitals from atomic electron configuration of valence electrons bond = overlap (constructive interference) of atomic orbitals on adjacent atoms two electrons of opposite spin can occupy the overlapped orbital "singly" connected bond is a sigma (σ) bond ‒ it freely rotates without breaking the bond

FIG II ‒ VB Description of H2 VB Description of HF

H: ls

FIG III ‒ VB Description of F2 F: 1s22s22px

22py22pz

VB Description of H2O

EX 2. How many valence electrons does H2O have? Sketch the VB description for H2O. What are the bond angles? O: 1s22s22px

22py2pz

Something is wrong! We do not predict the correct bond angles!

building blocks

sigma, σ, bond

overlap of two atomic orbitals

building blocks

EX 1. Sketch the valence bond description for the H ‒ F bond in HF. F: 1s22s22p x

22p y22p z Put

each valence electron in its proper orbital.

sigma, σ, bond building blocks

building blocks

- 3 - Hybridization and the Valence Bond Theory: Hybridization of Atomic Orbitals ‒ Refinement (Correction) of Simple Valence Bond Approach (11.1)

steric number (SN) = number of bonded atoms + number of lone pairs

FIG IV ‒ sp Hybridization ‒ BeH2 (SN Be = 2 => linear => sp hybridization) Be: ls22s2

FIG V - sp2 Hybridization ‒ BF3 (SN B = 3 => trigonal planar => sp2 hybridization)

B: ls22s22p

left bond

right bond

building blocks

EX 4. Draw Lewis structure; use VSEPR. What is SN for B (= # "equivalent" B orbitals needed = # B AOs to be mixed)? Which B AO’s are they? How many valence e- does B have? How many B e-′s go in each hybridized orbital?

building blocks

building blocks

building blocks denoted as an orbital diagram

building blocks

EX 3. Draw Lewis structure; use VSEPR. What is SN for Be (= # "equivalent" Be orbitals needed = # Be AOs to be mixed)? Which Be AO’s are they? How many valence e-′s does Be have? How many Be e-′s go in each mixed (hybridized) orbital?

building blocks – denoted as an orbital diagram

- 4 - FIG VI ‒ sp3 Hybridization ‒ CH4 (SN C = 4 => tetrahedral => sp3 hybridization)

C: ls22s22px2py

FIG VII – sp3 Hybridization (1s22s2 CH4 2px2py ; NH3 2px2py2pz ; H2O 2px22py2pz) steric number of C, N, and O all 4 => sp3 hybridization

EX 6. Draw Lewis structure; use VSEPR C N O SN # orbitals needed # AO to be mixed which AO’s # valence e- # e- in each hybrid

EX 5. Draw Lewis structure; use VSEPR. What is SN for C (= # "equivalent" C orbitals needed = # C AOs to be mixed)? Which C AO’s are they? How many valence e- does C have? How many C e-′s go in each hybridized orbital?

building blocks denoted as orbital diagrams

building blocks (orbital diagram)

building blocks

- 5 - FIG VIII – sp3d Hybridization – PCl5

SN P = 5 => trigonal bipyramidal => sp3d hybridization

FIG IX – sp3d 2 Hybridization – SF6

SN S = 6 => octahedral => sp3d 2 hybridization

steric number hybridization geometry 2 sp linear 3 sp2 trigonal planar 4 sp3 tetrahedral 5 sp3d trigonal bipyramidal 6 sp3d 2 octahedral

11.2 – Modes of Orbital Overlap and the Types of Covalent Bonds

bond = overlap of orbitals on adjacent atoms when necessary, hybridize orbitals procedure:

draw Lewis structures with all resonance structures use VSEPR to determine geometries assign bonds based upon electron configuration of each atom if unable (or incorrect) => hybridize valence atomic orbitals, usually

SN = 2 => sp SN = 5 => sp3d SN = 3 => sp2 SN = 6 => sp3d 2 SN = 4 => sp3

put electrons in hybrid orbitals following Hund's rule overlap (constructive interference) atomic and/or hybrid orbitals to form bonds

orbital diagram

orbital diagram

alternate way to draw

- 6 - Carbon – Carbon Single Bonds

ethane – C2H6

FIG X – Carbon – Carbon Single Bonds

Valence Bond Treatment of Multiple Bonds more than one bond between atoms often one is a pi (π) bond π bond is "doubly" connected and broken

upon rotation

FIG XI – Pi Bond Formed from Two p Orbitals

EX 7. What is the hybridization of each C atom? What orbitals are overlapped to form the C – C bond? Is this a sigma bond? Why? Is there free rotation about this bond or is it hindered? What orbitals are overlapped to form the C – H bond? Is this a sigma bond?

ball-and-stick model space-filling model

solid/dashed wedge model

~uniform e- distribution over σ bonds

overlap of two unhybridized p orbitals

- 7 -

FIG XII – An sp2 Hybridized Carbon

Double Bonds

ethylene, C2H4

formaldehyde, CH2O – don’t forget that the lone pairs need to be in orbitals!

EX 12. C has 4 valence electrons and O has 6. Draw the Lewis structure for CH2O and put each electron in its proper orbital. Describe the bonds and tell which orbitals are overlapped to form them

FIG XIII – Bonding in Formaldehyde, CH2O

EX 11. What is the SN of each C atom? What is the hybridization? How many carbon valence electrons are there? In what orbital is each electron? How many σ bond are there? Sketch the signma bonds and be sure to indicate which orbitals are being overlapped. Are all electrons and bonds accounted for? If not, which bonds are remaining? Sketch them.

(only a half of the orbitals in the sigma bond are shown)

(unhybridized)

C has 4 valence e-. sp2 hybridized C has 3 hybrid orbitals in a trigonal planar arrangement, each containing 1 e-, and an unhybridized p orbital with 1 e-.

- 8 -

FIG XIV – An sp Hybridized Carbon

Triple Bonds

acetylene, C2H2

nitrogen, N2

carbon monoxide, CO

EX 14. Describe the sigma and pi bonding in N2 and CO.

(1 unhybridized p orbital on each C

C has 4 valence e-. sp hybridized C has 2 hybrid orbitals in a linear arrange-ment, each containing 1 e-, and 2 unhybridized p orbitals each with 1 e-.

EX 13. What is the SN of each C atom? What is the hybridization? How many carbon valence electrons are there? In what orbital is each electron? How many σ bonds are there? Sketch the signma bonds and be sure to indicate which orbitals are being overlapped. Are all electrons and bonds accounted for? If not, which bonds are remaining? Sketch them.

(2 unhybridized p orbitals)

sp hybridized N σ bond from overlap of sp hybridized orbitals

bonds and orbitals containing all valence electrons

(unhybridized p orbital on N)

2 π bonds from p orbital overlap (1 unhybridized p orbital on each N)

- 9 -

FIG XII – An sp2 Hybridized Oxygen

Adjacent Double Bonds

carbon dioxide, CO2

(1 unhybridized p orbital on left O) (2 unhybridized

p orbitals on C)

(1 unhybridized p orbital on right O)

EX 15. Determine the hybridization of each atom, overlap orbitals to make bonds, and fill orbitals with elec-trons. How many sigma and pi bonds are there? What is the orientation of the pi bonds? Why?

(unhybridized)

FIG XVI – Basic Shapes of Organic Molecules