Chapter 9 Bonding and Molecular Structure: Orbital Hybridization and Molecular Orbitals

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Chemical Bonding Chapter 9 Advanced Theories of Chemical Bonding Chapter 9 Atomic Orbitals Molecules

Two Theories of Bonding MOLECULAR ORBITAL THEORY — Robert Mullikan (1896-1986) valence electrons are delocalized valence electrons are in orbitals (called molecular orbitals) spread over entire molecule.

Two Theories of Bonding VALENCE BOND THEORY — Linus Pauling valence electrons are localized between atoms (or are lone pairs). half-filled atomic orbitals overlap to form bonds. See Figures 9.1 and 9.2. Linus Pauling, 1901-1994

Sigma Bond Formation by Orbital Overlap Two s orbitals overlap

Sigma Bond Formation Two s orbitals overlap Two p orbitals overlap

Using VB Theory Bonding in BF3 planar triangle angle = 120o

Bonding in BF3 How to account for 3 bonds 120o apart using a spherical s orbital and p orbitals that are 90o apart? Pauling said to modify VB approach with ORBITAL HYBRIDIZATION — mix available orbitals to form a new set of orbitals — HYBRID ORBITALS — that will give the maximum overlap in the correct geometry.

Bonding in BF3 rearrange electrons hydridize orbs. unused p orbital three sp 2 hybrid orbitals 2p 2s See Figure 9.8

Bonding in BF3 The three hybrid orbitals are made from 1 s orbital and 2 p orbitals  3 sp2 hybrids. Now we have 3, half-filled HYBRID orbitals that can be used to form B-F sigma bonds.

Bonding in BF3 An orbital from each F overlaps one of the sp2 hybrids to form a B-F  bond.

BF3, Planar Trigonal

Bonding in CH4 How do we account for 4 C—H sigma bonds 109o apart? Need to use 4 atomic orbitals — s, px, py, and pz — to form 4 new hybrid orbitals pointing in the correct direction.

Bonding in a Tetrahedron Formation of Hybrid Atomic Orbitals 4 C atom orbitals hybridize to form four equivalent sp3 hybrid atomic orbitals. PLAY MOVIE

Bonding in a Tetrahedron — Formation of Hybrid Atomic Orbitals 4 C atom orbitals hybridize to form four equivalent sp3 hybrid atomic orbitals. PLAY MOVIE

Bonding in CH4, See Figure 9.6

Bonding in Glycine

Bonding in Glycine

Bonding in Glycine

Bonding in Glycine

Bonding in Glycine

Orbital Hybridization See Figure 9.5 BONDS SHAPE HYBRID REMAIN 2 linear sp 2 p’s 3 trigonal sp2 1 p planar 4 tetrahedral sp3 none

Multiple Bonds Consider ethylene, C2H4

Sigma Bonds in C2H4 PLAY MOVIE

π Bonding in C2H4 The unused p orbital on each C atom contains an electron and this p orbital overlaps the p orbital on the neighboring atom to form the π bond. (See Fig. 9.8)

π Bonding in C2H4 The unused p orbital on each C atom contains an electron and this p orbital overlaps the p orbital on the neighboring atom to form the π bond. (See Fig. 9.10) PLAY MOVIE

Multiple Bonding in C2H4 PLAY MOVIE

s and π Bonding in C2H4 See Figure 9.10

s and π Bonding in CH2O See Figure 9.11

s and π Bonding in C2H2 See Figure 9.12

s and π Bonding in C2H2 See Figure 9.12

Consequences of Multiple Bonding There is restricted rotation around C=C bond. See Figure 9.13

Consequences of Multiple Bonding Restricted rotation around C=C bond. PLAY MOVIE

Double Bonds and Vision PLAY MOVIE See Screen 9.13, Molecular Orbitals and Vision

Molecular Orbital Theory Valence electrons are delocalized Valence electrons are in orbitals (called molecular orbitals) spread over entire molecule.

The Paramagnetism of O2 PLAY MOVIE

Molecular Orbital Theory Bonding and antibonding sigma MO’s are formed from 1s orbitals on adjacent orbitals. PLAY MOVIE PLAY MOVIE

Molecular Orbital Theory 1. No. of MO’s = no. of atomic orbitals used. 2. Bonding MO is lower in energy than atomic orbitals. Antibonding MO is higher. 3. Electrons assigned to MO’s of higher and higher energy. See Figure 9.16

Dihelium Molecule Bond order = 1/2 [# e- in bonding MOs - # e- in antibonding MOs]

Sigma Bonding from p Orbitals

π Bonding from p Orbitals Sideways overlap of atomic 2p orbitals that lie in the same direction in space give π bonding and antibonding MOs.

s & π Bonding from p Orbitals

s & π Bonding from p Orbitals

Note change in order of energy of sigma and pi MOs on going from N2 to O2.