Test 2 - Ch 4 Hybridization and MO Theory

Simple molecules

Overlap of atomic orbitals - look at unpaired e-

Example: H2 - 1s1 (Simple molecules)

Example: Cl2 - [Ne]3s2 3p5 (Simple molecules)

Filled 3s orbital so use unpaired e- in 3p orbital

Example: HCl, H: 1s1, Cl: 3p1 (Simple molecules)

Types of Orbitals: sp

Geometry: linear

Types of Orbitals: sp2

Geometry: Trigonal Planar

Types of orbital: sp3

Geometry: tetrahedral

Predicting hybridization

• all about the steric # (# of e- groups)

• Also count lone pairs(Ex:NH3 - sp3)

Steric # 2 - sp

Linear

Steric # 3 - sp2

Trigonal planar

Steric # 4 - sp3

Tetrahedral

Steric # 5 - sp3d

Trigonal bipyramidal

Steric # 6 - sp3d2

Octahedral

Sigma (σ) bonds

• Bonds formed by end-to-end overlap of atomic hybrid orbitals

• e- density greatest in between the nuclei: right along the bond axis

• Orbitals involved - atomic and/or hybrid orbitals (hybrid orbitals only form σ bonds

Pi (π) bonds

• side-to-side overlap of p orbitals, results in e- density above and below the bond axis

• Orbitals involved - unhybridized p orbitals ONLY

Single bond

σ bond

Double bond

σ + 1π bond

Triple bond

σ + 2π bond

Example of pi/sigma bond

Diamagnetic

Weakly repelled by magnetic field

Paramagnetic

Attracted to magnetic field. Must have unpaired e-

Tenets of Molecular Orbital (MO) Theory

• Mixing of atomic orbitals: atomic wave fxn’s mathematically combined (linear combination add/subtract) to form delocalized molecular orbitals

• Energy considerations: only valence orbitals which are close in energy can mix

• VALENCE ORBITALS

Bonding Orbitals (types of orbitals)

• Wave fxn: AO1 + AO2 (additive combination - in phase)

• e- density: greater along the bond (between nuclei)

• Energy: lower than the AO’s

Antibonding orbitals (types of orbitals)

Wave fxn: AO1-AO2 (subtractive combination - out of phase)

e- density: greater outside the internuclear region

Energy: higher than the AO’s

Shapes: bonding orbital (MO’s for 1s orbital)

Shapes antibonding orbital (MO’s for 1s orbital)

Orbital diagram structure

Parameters than can be determined from MO Diagram

• Bond order = 1/2(# bonding e - # antibonding e)

• Bond types: sigma vs pi

• Magnetic properties:

  Diamagnetic: all e- are paired

  Paramagnetic: 1+ unpaired e-

H2+ vs H2 (orbital diagram)

H2- and He2 (orbital diagram)

Sigma p MO’s (what about p orbitals?)

Pi p MO’s (what about p orbitals?)

Pz (what about p orbitals?)

Same thing happens w/ Pz, giving 2 degenerate Pi p and 2 degenerate Pi*p orbitals

O2 (orbital diagram)

F2 (orbital diagram)

Ne2 (orbital diagram)

B2 (orbital diagram)

C2 (orbital diagram)

N2 (orbital diagram)

Fractional bond orders: N2+

Shortcut: ODD # OF e- IS ALWAYS PARAMAGNETIC AND ALWAYS FRACTIONAL BOND ORDER

Benzene C6H6 (Delocalization and MO Theory)

All the C=C bonds are equal (delocalized)

All C atoms are sp2 hybridized

Delocalized and alternating pi bonds that go around whole ring

The sigma bonds in benzene can be modeled w/ valence bond theory (Delocalization and MO Theory)

Sigma bonds are formed by sp2-sp2 (C-C) or sp2-1s (C-H) overlap

The pi bonds in benzene are modeled as delocalized MO’s

Delocalized pi MO’s, formed by overlap of 2Pz orbitals