Chapter 10: Covalent Bond Theories Valence Bond Theory Molecular Orbital Theory First a couple of jokes… (You’ll need a laugh this chapter!) If a bear from Yosemite and a bear from Alaska fall into the water, which one dissolves faster? Which is lower energy, hamburger or steak? My name is Bond - Ionic Bond; taken not shared!

Valence Bond Theory – Covalent Bonds form from the __________ of atomic orbitals. Consider a BeF2 molecule: Be F F

Valence Bond Theory F 1s22s22p5 F 1s22s22p5 One unpaired e- How can Be form covalent bonds with F if it doesn’t have unpaired electrons??? Be 1s22s2 No unpaired electrons

Valence Bond Theory Promote one of the 2s e- to a 2p orbital Be 1s22s2 no unpaired electrons Be 1s22s12p1 2 unpaired electrons

Valence Bond Theory F 1s22s22p5 F 1s22s22p5 One unpaired e- Be 1s22s12p1 Predicted overlaps: 2s (Be) – 2p (F) 2p (Be) – 2 p (F) Implies two different kinds of Be – F bonds! 2 unpaired electrons

Valence Bond Theory Both of the Be – F bonds in BeF2 are identical! The solution: Mix the Be 2s orbital with one of the Be 2p orbitals to form two hybrid orbitals atomic orbitals formed by mixing 2 or more atomic orbitals on an atom

Valence Bond Theory F 1s22s22p5 F 1s22s22p5 Be 1s 2sp 2p When Be forms covalent bonds with two F, each sp hybrid orbital on the Be atom overlaps with a p orbital located on a F atom.

Valence Bond Theory The BeF2 molecule is linear. sp hybridization implies that the electron geometry around the central atom is linear.

The sp2 hybrid orbitals in BF3.

The sp3 hybrid orbitals in CH4.

The sp3 hybrid orbitals in NH3.

The sp3 hybrid orbitals in H2O.

The sp3d hybrid orbitals in PCl5.

The sp3d2 hybrid orbitals in SF6.

Show the orbital hybridization of SF4

Multiple Bonds   All of the bonds shown thus far have been single bonds - which result in sigma bonds Sigma bond (s)  A bond where the line of electron density is concentrated symmetrically along the line connecting the two atoms. Results from head to head overlap of orbitals

Pi bond (p)  A bond where the overlapping regions exist above and below the internuclear axis. Pi bonds result from side to side overlap of atomic orbitals. Pi bonds cannot form from hybrid orbitals A double bond consists of one sigma (hybrid) and one pure pi bond. A triple bond consists of one hybrid sigma and 2 pure pi bonds

Example: H2C=CH2

Example: H2C=CH2 Only sigma bond pairs influence molecular geometry. What is the geometry around each central atom?

Multiple Bonding: Formaldehyde: H2CO Acetylene: C2H2 Identify sigma and pi bonds in the following molecules and name the orbital hybridization involved. Formaldehyde: H2CO Acetylene: C2H2

Nitrogen gas Acetonitrile, CH3CN

The paramagnetic properties of O2 Using the valence bond theory, explain the paramagnetism of oxygen gas.

Molecular Orbital Theory – Another theory for the formation of covalent bonds which explains phenomena for which the Valence Bond Theory can’t account . When 2 atomic orbitals overlap, two new molecular orbitals are formed: Bonding molecular orbital Antibonding molecular orbital Electrons are spread or delocalized over the whole molecule instead of remaining in atomic orbitals

Molecular Orbital Theory Bonding molecular orbital Addition of overlapping atomic orbitals results in an area of increased e- density; increased probability e- will be found here. lower energy than atomic orbital Bonding MO

Molecular Orbital Theory Antibonding molecular orbital An area where the probability of finding an electron is reduced Results from subtracting one atomic orbital from the other higher energy than atomic orbital Antibonding MO Bonding MO

Molecular Orbital (MO) Theory ANTBONDING These two new orbitals have different energies.  BONDING The one that is lower in energy is called the bonding orbital, The one higher in energy is called an antibonding orbital.

Molecular Orbital Theory The MO diagram for H2 molecule: Placing e-s in MO’s follows the Aufbau principle, Pauli’s and Hund’s Rule. s*1s 1s 1s H atom s1s H atom bonding electrons H2 molecule

Molecular Orbital Theory The MO diagram for He2 molecule: antibonding electrons s*1s 1s 1s He atom s1s He atom bonding electrons He2 molecule Adding electrons to antibonding molecular orbitals destabilizes the molecule.

Molecular Orbital Theory In MO Theory, the stability of a covalent bond can be related to its bond order: Bond order = (# bonding e- - # antibonding e-) 2 Old def.: # e- pairs / # links formed Single bond: bond order = 1 Double bond: bond order = 2 Triple bond: bond order = 3 Fractional bond orders also exist! (e.g. He2+1)

Energy level diagrams / molecular orbital diagrams

Molecular Orbital Theory The bond order for H2 molecule: Bond Order = (2 - 0) = 1 2 The hydrogen atoms in an H2 molecule are held together by a single bond.

Molecular Orbital Theory The bond order for an He2 molecule: Bond order = (2 - 2) = 0 2 No bond exists between two He atoms.

Example: Li2 Atomic orbitals combine to form molecular orbitals best when the atomic orbitals are of equal energy.

Second-Row Diatomic Molecules Electron Configurations for B2 through Ne2

Relative MO energy levels for Period 2 homonuclear diatomic molecules. without 2s-2p mixing with 2s-2p mixing MO energy levels for O2, F2, and Ne2 MO energy levels for B2, C2, and N2

For B2, C2, and N2 the interaction is so strong that the s2p is pushed higher in energy than p2p orbitals

Molecular Orbital Theory Example: Complete the MO diagram for N2. Calculate the bond order. # valence electrons for each N atom = Total # of valence electrons =

Molecular Orbital Theory s*2p # bonding e- = # antibonding e- = Bond order = p*2p s2p p2p s*2s s2s

Molecular Orbital Theory Example: Complete the MO diagram for the O2 molecule. Determine the bond order. # valence electrons for each O atom = Total # of valence electrons =

Molecular Orbital Theory s*2p # bonding e- = # antibonding e- = Bond order = p*2p p2p s2p s*2s s2s

Isomers 1,1-dichloroethylene 1,2-dichloroethylene cis-1,2-dichloroethylene trans-1,2-dichloroethylene