Simple Bonding Theory Review of Lewis Structures and VSEPR Theory

Simple Bonding Simple Bonding TheoryTheory

Review of Lewis Structures Review of Lewis Structures and VSEPR Theoryand VSEPR Theory

ResonanceResonance

Some molecules may have more Some molecules may have more than one valid Lewis structure. than one valid Lewis structure. These structures differ in the These structures differ in the placement of multiple bonds. placement of multiple bonds.

In molecules with resonance, In molecules with resonance, none of the Lewis structures none of the Lewis structures accurately represents the true accurately represents the true bonding in the molecule. bonding in the molecule.

ResonanceResonance

The molecule SOThe molecule SO22 has two has two resonance structures:resonance structures:

[ O=S-O: ] ↔ [ :O-S=O][ O=S-O: ] ↔ [ :O-S=O]

The molecule has two The molecule has two equivalentequivalent bonds between sulfur and oxygen.bonds between sulfur and oxygen.

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ResonanceResonance

The sulfur-oxygen bonds are The sulfur-oxygen bonds are identical- longer than double bonds, identical- longer than double bonds, and shorter than single bonds.and shorter than single bonds.

[ O=S-O: ] ↔ [ :O-S=O][ O=S-O: ] ↔ [ :O-S=O]

The true structure of the The true structure of the molecule is in between the two Lewis molecule is in between the two Lewis structures drawn.structures drawn.

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ResonanceResonance

The Lewis structure and VSEPR The Lewis structure and VSEPR theory correctly predict the shape, theory correctly predict the shape, polarity and bond angles (≈120polarity and bond angles (≈120oo)) of the of the molecule. molecule.

However, this approach cannot However, this approach cannot accurately predict the identical nature of accurately predict the identical nature of the bonds. The sulfur oxygen bonds are the bonds. The sulfur oxygen bonds are equivalent, and somewhat shorter than equivalent, and somewhat shorter than single bonds, and slightly longer than single bonds, and slightly longer than double bonds.double bonds.

Formal ChargesFormal Charges

Formal charge is a way to keep Formal charge is a way to keep track of the electrons in a covalent track of the electrons in a covalent molecule. The formal charges can molecule. The formal charges can also be used to determine if one also be used to determine if one Lewis structure is more valid than Lewis structure is more valid than another.another.


The formal charge on an atom is The formal charge on an atom is a comparison between the number a comparison between the number of valence electrons on each atom of valence electrons on each atom and the number of electrons it has in and the number of electrons it has in the Lewis structure.the Lewis structure.


Consider the ion SCNConsider the ion SCN-1-1. There . There are three valid Lewis structures for are three valid Lewis structures for the ion.the ion.

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:]

Formal charges can be used to Formal charges can be used to determine the major contributor(s) to determine the major contributor(s) to the actual structure of the ion.the actual structure of the ion.

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Divide the bonds in half and Divide the bonds in half and determine the number of electrons determine the number of electrons on each atom.on each atom.

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:] 6e 4e 6e6e 4e 6e 7e 4e 5e 7e 4e 5e 5e 5e

4e 7e 4e 7e

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Compare the number of electrons in Compare the number of electrons in the structure to the number of valence the structure to the number of valence electrons.electrons.

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:] 6e 4e 6e6e 4e 6e 7e 4e 5e 7e 4e 5e 5e 4e 7e 5e 4e 7e

6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e

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The net charge is the formal charge The net charge is the formal charge on each atom.on each atom.

0 0 -10 0 -1 -1 0 0 +1 0 -1 0 0 +1 0 -2 -2

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:] 6e 4e 6e6e 4e 6e 7e 4e 5e 7e 4e 5e 5e 4e 7e 5e 4e 7e 6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e 6e 4e 5e

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The net charge is the formal The net charge is the formal charge on each atom.charge on each atom.

0 0 -10 0 -1 -1 0 0 +1 -1 0 0 +1 0 -2 0 -2

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:] The sum of the formal charges The sum of the formal charges

must equal the charge on the ion.must equal the charge on the ion.

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There are two rules used to There are two rules used to determine the most likely Lewis determine the most likely Lewis structure(s).structure(s).

1. Atoms try to achieve formal 1. Atoms try to achieve formal charges as close to zero as charges as close to zero as possible.possible.

2. Any negative formal charges 2. Any negative formal charges should reside on the most should reside on the most electronegative atoms.electronegative atoms.


The third Lewis structure is The third Lewis structure is unlikely, due to the high formal charge unlikely, due to the high formal charge on nitrogen.on nitrogen.

0 0 -10 0 -1 -1 0 0 +1 0 -1 0 0 +1 0 -2 -2

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:] Since nitrogen is more Since nitrogen is more

electronegative than sulfur, the first electronegative than sulfur, the first structure should be the major structure should be the major contributor.contributor.

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The actual molecule will be The actual molecule will be somewhere in between the first and somewhere in between the first and second structures.second structures.

0 0 -10 0 -1 -1 0 0 +1 0 -1 0 0 +1 0 -2 -2

[:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:][:S=C=N:] ↔ [:S-C N:] ↔ [:S C-N:]

The sulfur-carbon bond should be The sulfur-carbon bond should be slightly longer than a double bond, and slightly longer than a double bond, and the carbon-nitrogen bond should be the carbon-nitrogen bond should be slightly shorter than a double bond.slightly shorter than a double bond.

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Some Lewis structures violate Some Lewis structures violate the formal charge guidelines (ex. CO the formal charge guidelines (ex. CO and BFand BF33).).

In either example, if the “octet In either example, if the “octet rule” is to be satisfied, a formal rule” is to be satisfied, a formal charge of +1 must be placed on the charge of +1 must be placed on the more electronegative element.more electronegative element.


In the case of BFIn the case of BF33, experimental , experimental evidence suggests there is some multiple evidence suggests there is some multiple bonding between the boron and the bonding between the boron and the fluorine atoms to create an extended fluorine atoms to create an extended ππ system. system.

VSEPR Theory/Molecular VSEPR Theory/Molecular ShapesShapes

Molecules with lone pairs of Molecules with lone pairs of electrons on the central atom are electrons on the central atom are described as “distorted” from ideal described as “distorted” from ideal geometry.geometry.

NHNH33 has a bond angle of 106.6 has a bond angle of 106.6oo

HH22O has a bond angle of 104.5O has a bond angle of 104.5oo

Hybridization of Distorted Hybridization of Distorted MoleculesMolecules

These molecules can be viewed These molecules can be viewed as having different hybridization in as having different hybridization in the bonds and lone pairs. the bonds and lone pairs.

Since lone pairs are more Since lone pairs are more diffuse and “bulky”, they will have diffuse and “bulky”, they will have greater “s” character. greater “s” character.

Since bonds are more compact Since bonds are more compact and directional, they will have and directional, they will have greater “p” character.greater “p” character.

Hybridization of Distorted Hybridization of Distorted MoleculesMolecules

cos cos θθ = = ss == p-1 p-1

Determine the hybridization of the Determine the hybridization of the bonding and non-bonding orbitals on bonding and non-bonding orbitals on nitrogen in ammonia. The bond nitrogen in ammonia. The bond angle is 106.6angle is 106.6oo..

ss--11

pp

““Expanded” CoordinationExpanded” Coordination

Elements below period 2 can use Elements below period 2 can use dd orbitals to make more than 4 bonds. orbitals to make more than 4 bonds.

A A trigonal bipyramidtrigonal bipyramid, with sp, with sp33d d hybridization, is the common structure hybridization, is the common structure for 5 atoms or lone pairs. These for 5 atoms or lone pairs. These molecules are not fully hybridized. molecules are not fully hybridized. The trigonal plane is spThe trigonal plane is sp22 hybridized, hybridized, and the axial positions are a mixture of and the axial positions are a mixture of the pthe pzz and d and dzz2 2 orbitals.orbitals.

5 Coordinate Complexes5 Coordinate Complexes

In molecules with trigonal In molecules with trigonal planar geometry, not all bond angles planar geometry, not all bond angles and positions are equivalent. and positions are equivalent. If the If the molecule contains lone pairs of molecule contains lone pairs of electrons, they will occupy the electrons, they will occupy the trigonal plane rather than the axial trigonal plane rather than the axial sites. This will minimize repulsion sites. This will minimize repulsion within the molecule.within the molecule.

5 Coordinate Complexes5 Coordinate Complexes

The lone pair of electrons in the The lone pair of electrons in the trigonal plane will force the fluorine trigonal plane will force the fluorine atoms closer together.atoms closer together.

FF

S S ::FF

F

F101.6o

187o


Molecules with 6 atoms or lone pairs Molecules with 6 atoms or lone pairs typically take an octahedral shape, with typically take an octahedral shape, with spsp33dd22 hybridization. Some complexes hybridization. Some complexes with coordination number 6 may be with coordination number 6 may be distorted prisms.distorted prisms.


Molecules Molecules with coordination with coordination number of 7 may number of 7 may have a pentagonal have a pentagonal bipyramidal shape, bipyramidal shape, and those with a and those with a coordination coordination number of 8 may number of 8 may be a square be a square antiprism.antiprism.

Multiple Bonds and Multiple Bonds and DistortionDistortion

Multiple bonds have an Multiple bonds have an influence on structure similar to that influence on structure similar to that of lone pairs. The diffuse nature of of lone pairs. The diffuse nature of ππ bonds causes repulsion with lone bonds causes repulsion with lone pairs and bonding pairs of electrons.pairs and bonding pairs of electrons.

ElectronegativityElectronegativityElectronegativity is a measure of

an atoms ability to attract electrons from an atom to which it is bonded.

Mulliken Mulliken ElectronegativitesElectronegativitesMulliken defined electronegativity Mulliken defined electronegativity

as:as:

½ (electron affinity + ionization energy)½ (electron affinity + ionization energy)

This approach yields very high This approach yields very high electronegativity values for He and Ne, electronegativity values for He and Ne, even though they do not form even though they do not form compounds.compounds.

Electronegativity ValuesElectronegativity Values

Electronegativity and Bond Electronegativity and Bond AnglesAngles

Electronegative atoms bonded to a Electronegative atoms bonded to a less electronegative central atom tend to less electronegative central atom tend to draw electron density away from the draw electron density away from the central atom, thus lowering repulsion.central atom, thus lowering repulsion.

PFPF33 PClPCl33 PBrPBr33

97.897.8oo 100.3100.3oo 101101oo

Hybridization in Period 3 Hybridization in Period 3 and Belowand Below

For central atoms in period 3 or For central atoms in period 3 or below, hybridization is less common.below, hybridization is less common.

AsHAsH33 PHPH33 PFPF33 PClPCl33 PBrPBr33

91.891.8oo 93.893.8oo 97.897.8oo 100.3100.3oo 101101oo

Steric Number 5Steric Number 5

In molecules such as PClIn molecules such as PCl55, the , the five bonds are not equal in length.five bonds are not equal in length.


Any lone pairs preferentially Any lone pairs preferentially occupy the equatorial positions so as occupy the equatorial positions so as to minimize electron pair repulsion.to minimize electron pair repulsion.


In five-coordinate compounds, In five-coordinate compounds, the more electronegative (less bulky) the more electronegative (less bulky) atoms occupy the axial positions.atoms occupy the axial positions.


Methyl groups, considered to be less Methyl groups, considered to be less electronegative than fluorine atoms, electronegative than fluorine atoms, occupy the equatorial sites to minimize occupy the equatorial sites to minimize repulsion between the bonding electron repulsion between the bonding electron pairs.pairs.

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Simple Bonding Theory Review of Lewis Structures and VSEPR Theory