Covalent Bonding: Orbitals Chapter 9

Atomic Orbital (Valence Bond) Approach Advantages of Lewis Dot structures 1. 2.

Predict geometries Predict polarities of molecules

Disadvantages 1. 2.

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No information about energies of electrons No information about orbitals used in bonding.

Valence bond approach is helpful for these. (Basic idea in valence bond)

Atomic Orbital (Valence Bond) Approach „ „

A covalent bond is formed from a pair of electrons with opposite spins in overlapped atomic orbitals. Bond formed from half-filled valence orbitals.

Atom 1H ____ 1s Atom 9F ___ ___ 1s

___ ___ ___

2s

2p

H ____ H Molecule H 2 1s Molecule HF 9F ___ ___ 1s

2s

___ ___ ___ H 2p

Atomic Orbital (Valence Bond) Approach – He, Ne Filled orbitals do not form bonds He ___ 1s

Ne ___ ___ ___ ___ ___ 1s

2s

2p

Atomic Orbital (Valence Bond) Approach – Be, B, C Problems with Valence Bond Approach: Be ___ ___ ___ ___ ___ (no bonds?) 1s

B ___ 1s

2s

2p

___ ___ ___ ___ (1 bond?) 2s

2p

C ___ ___ ___ ___ ___ ( 2 bonds?) 1s

2s

2p

Atomic Orbital (Valence Bond) Approach – C – sp3 „

To explain bonding for atoms like these it is assumed electrons are promoted and hybrid orbitals are formed.

C ___ ___ ___ ___ ___ 1s

2s

2p

___bonds form ___ hybrid

Atomic Orbital (Valence Bond) Approach – CH4

Atomic Orbital (Valence Bond) Approach – B – sp2 „

To explain bonding for atoms like these it is assumed electrons are promoted and hybrid orbitals are formed.

B ___ ___ ___ ___ ___ 1s

2s

2p

___bonds form ___ hybrid

Atomic Orbital (Valence Bond) Approach – sp2, p

Atomic Orbital (Valence Bond) Approach – Be - sp „

To explain bonding for atoms like these it is assumed electrons are promoted and hybrid orbitals are formed. Be ___ ___ ___ ___ ___ ___bonds form ___ hybrid 1s

2s

2p

Atomic Orbital (Valence Bond) Approach – Be, B, C Summary „

SUMMARY

hybridized orbitals

Be ___ ___ ___ ___ ___ 2s

1s

2p

B ___ ___ ___ ___ ___ 1s

2s

2p

C ___ ___ ___ ___ ___ 1s

2s

2p

un-hybridized orbitals?

Atomic Orbital (Valence Bond) Approach „

Hybrid orbitals have new properties that are different from the orbitals used to form them. „ „

Shape energy

Atomic Orbital (Valence Bond) Approach – table 1 „

The geometries are the same as predicted from electron repulsion theory.

# electron groups 2 3 4 5 6

Hybridization

Geometry

angle

Atomic Orbital (Valence Bond) Approach - table 2a # electron groups 3

# non-bonded Hybridization pairs 0

Geometry

sp2

Planar triangular

sp3

Tetrahedral

1 4

0 1 2

angle

Atomic Orbital (Valence Bond) Approach – table 2b # electron groups 5

# non-bonded Hybridization pairs 0

Geometry

dsp3

Trigonal bipyramidal

d2sp3

octahedron

1 2 3 6

0 1 2

angle

Atomic Orbital (Valence Bond) Approach – XeF4

Atomic Orbital (Valence Bond) Approach – sigma & pi Two types of Covalent Bonds: 1. Sigma bonds - form from hybrid orbitals. They have the e- density symmetrical with bond axis. Pi bonds - form from unhybridized p-orbitals They have the e- density parallel but outside bond axis.

2.

Atomic Orbital (Valence Bond) Approach –sigma & pi

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Single bonds have __ sigma bond(s) __ pi bond(s) Double bonds have __ sigma bond(s) __ pi bond(s) Triple bonds have __ sigma bond(s) __ pi bond(s)

Atomic Orbital (Valence Bond) Approach – CO2

Double bonds in CO2

Atomic Orbital (Valence Bond) Approach - ethene

Valence Bond Approach – N2

Atomic Orbital (Valence Bond) Approach – summary hydbridization HYBRIDIZATION IN MULTIPLE BONDS: The extra electron pairs in multiple bonds (1 extra pair in double, 2 extra pairs in triple) are NOT hybridized. TO DETERMINE AMOUNT OF HYBRIDIZATION OF ATOM: Hybridize enough orbitals to contain: „ „ „

all unshared electron pairs electron pairs to form single bonds one and only one pair in multiple bonds

Molecular Orbitals Results of Valence Bond approach to bonding & Lewis Dot Structures Weakness: Inability to predict the correct magnetic properties, O2 & B2 „ Need for resonance to handle special problems „ Gives no direct information on bond energies „

Reason - Assumed electrons stayed in atomic orbitals of the individual atoms.

Molecular Orbitals Another approach - Linear combination of atomic orbitals to give molecular orbital L.C.A.O. = M.O. BASIC ASSUMPTION OF MOLECULAR ORBITAL APPROACH „ Orbitals are properties of the molecule not the atoms.

Molecular Orbital Model – Main Ideas MAIN IDEAS OF MOLECULAR ORBITAL MODEL: 1. Same number of molecular orbitals as the # atomic orbitals that were combined 2. Molecular orbitals can hold two e- with opposite spins 3. square of molecular orbital function indicates e- probability 4. Important properties of orbitals: size, shape, and energy (See fig. 9.26 for shape and size) 5. Molecular orbital configurations can be written much like e- configurations for atoms

Molecular Orbital Model – H2 MAIN IDEAS OF MOLECULAR ORBITAL MODEL: 1. Same number of molecular orbitals as the # atomic orbitals that were combined 2. Molecular orbitals can hold two e- with opposite spins Ex. Hydrogen _____ σ1s* _____1s 1sA _____ B _____ σ1s

Molecular Orbital Model – H2 Ex. Hydrogen _____ 1sA _____

σ1s*

_____ 1sB

_____ σ1s

The orbitals described above are both sigma (σ) molecular orbitals bonding molecular orbital - (σ1s) antibonding molecular orbital - (σ1s*) „

Molecular Orbital Model – H2 MAIN IDEAS OF MOLECULAR ORBITAL MODEL: 5. Molecular orbital configurations can be written much like e- configurations for atoms Ex. Hydrogen _____ 1sA _____

σ1s*

_____ σ1s

ex. H2: __________

_____ 1sB

Molecular Orbital Model – H2Example: Predict the molecular orbital configuration in H2- using the diagram below _____ σ1s* _____ 1sB 1sA _____ _____ σ1s

ex. H2-: a) Is this ion stable (does it have lower energy that its separated parts)? ___ b) How do you expect this bond strength to compare to H2? ___

Molecular Orbital Model – Bond Order Bond Order - the difference between the number of bonding e- and the number of antibonding edivided by two Bond Order = (# bonding e- ) - (# antibonding e-) 2 „

Calculate the Bond order for H2 and H2-

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Bond order is an indication of _______________ The larger the bond order the _____________ the bond

Molecular Orbital Model – Bond Order He2 Ex. Predict the bond order and stability of He2

Molecular Orbital Model – Bond Order – Li2 and Be2 Bonding in Homonuclear Diatomic Molecules: Homonuclear diatomic molecule – Ex. Predict the Bond Order and stability of Li2 and Be2

___ 2sA ___

σ2s*

___ ___ 2sB

2sA ___

σ2s*

___

___

σ2s

σ2s

___ 2sB

Molecular Orbital Model – Li2 NOTE: In order to participate in molecular orbitals, atomic orbitals must overlap.

Molecular Orbital Model – π vs. σ Pi (π) molecular orbitals - (see Fig. 9.33 for shapes of p and s bonding p-orbitals) How would you expect the p orbitals to compare in energy to the s orbitals? π σ

Molecular Orbital Model – expected MO E Expected MO E diagram:

Molecular Orbital Model – B2

Molecular Orbital Model – B2

Molecular Orbital Model Expected MO E diagram: What is the molecular orbital configuration for

____ σ2p* ____ ____ π2p* π2p* ____ ____ π2p π2p ____ σ2p ____ σ2s* ____ σ2s

a) F2?

____ σ2p* ____ ____ π2p* π2p* ____ ____ π2p π2p ____ σ2p ____ σ2s* ____ σ2s

b) B2?

Molecular Orbital Model Paramagnetism Paramagnetic -

Diamagnetic –

Molecular Orbital Model Paramagnetism „

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The expected E level diagram needs to be modified slightly to account for the magnetic properties of B2. This change results from p-s mixing, thus the π2p and σ2p orbitals are reversed Because the importance of p-s mixing becomes less important across the period, the π2p and σ2p orbitals revert to the order expected in absence of p-s mixing for O2 and F2.

Molecular Orbital Model Paramagnetism This change results from p-s mixing, thus the π2p and σ2p orbitals are reversed Because the importance of p-s mixing becomes less important across the period, the π2p and σ2p orbitals revert to the order expected in absence of p-s mixing for O2 and F2.

Bonding in Heteronuclear Diatomic Molecules heteronuclear diatomic molecule „

When two atoms are near each other in the periodic table, we can use the MO diagram for homonuclear molecules.

Ex. NO (like N2)

CN-

Bonding in Heteronuclear Diatomic Molecules „

When the two atoms are different, a new diagram must be used. Ex. HF

Bonding in Heteronuclear Diatomic Molecules „

Because 2p is lower in energy than the hydrogen 1s orbital, the electrons prefer to be closer to the fluorine atom.

Combining Localized Electron & Molecular Orbital Models „

In molecules that require resonance, the s bond is localized while the p bonding is delocalized. Ex. NO3- and C6H6

Combining Localized Electron & Molecular Orbital Models -

Combining Localized Electron & Molecular Orbital Models -