Daniel L. Reger Scott R. Goode David W. Ball Chapter 10 Molecular Structure and Bonding Theories

Valence-Shell Electron-Pair Repulsion Model (VSEPR) predicts shape from Lewis Structures. VSEPR Rule 1: A molecule has a shape that minimizes electrostatic repulsions between valence-shell electron pairs. Minimum repulsion results when the electron pairs are as far apart as possible. VSEPR

Steric number = (number of lone pairs on central atom) + (number of atoms bonded to central atom) The steric number is determined from the Lewis structure. Steric number determines the bonded-atom lone-pair arrangement, the shape that maximizes the distances between the valence-shell electron pairs. Steric Number

Geometric Arrangements

In the Lewis structure of BeCl 2, beryllium has two bonded atoms and no lone pairs, steric number = 2. A linear geometry places the two pairs of electrons on the central beryllium atom as far apart as possible. Steric Number = 2

The Lewis structure of HCN (H-C  N:) shows that the carbon atom is bonded to two atoms and has no lone pairs, steric number = 2. The bonded-atom lone-pair arrangement is linear. The number of bonded atoms, not the number of bonds, determines the steric number. Molecules with Multiple Bonds

The Lewis structure of BF 3 shows the boron atom has a steric number = 3; the bonded-atom lone-pair arrangement is trigonal planar. Steric Number = 3

The Lewis structure of CH 4 shows the carbon atom has a steric number = 4; the bonded-atom lone- pair arrangement is tetrahedral. Steric Number = 4

The phosphorus atom in PF 5 has a steric number = 5; the bonded- atom lone-pair arrangement is trigonal bipyramidal. Steric Number = 5

The sulfur atom in SF 6 has a steric number = 6; the bonded-atom lone-pair arrangement is octahedral. Steric Number = 6

The Lewis structure of H 2 O is Steric number = 4, 2 bonded atoms and 2 lone pairs. The bonded-atom lone-pair arrangement is tetrahedral. Central Atoms with Lone Pairs O HH

Molecular shape is the arrangement of the atoms in a species. The bonded-atom lone- pair arrangement of H 2 O is tetrahedral (top); the molecular shape is bent or V-shaped (bottom). Molecular Shape of H 2 O

What is the electron pair geometry and molecular shape of NH 3 ? Molecular Shape of NH 3

The measured bond angle in H 2 O (104.5 o ) is smaller than the predicted angle (109.5 o ) Explanation: repulsions vary lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair Electron Pair Repulsions

The favored structure for a trigonal bipyramid minimizes 90 o lone pair interactions – the one on the right. Two structures are possible: Location of Lone Pair in SF 4

Lone pairs always occupy the equatorial positions in a trigonal bipyramid so that lone pair-lone pair repulsions are oriented at 120 o. Lone Pairs in Trigonal Bipyramids

The structure on right has no 90 o lone pair-lone pair interactions and is favored. Location of Lone Pairs in XeF 4

What is the steric number, the bonded- atom lone-pair arrangement, and the molecular shape of ClF 3 ? Test Your Skill

The geometry of each central atom is determined separately. The CH 3 carbon in CH 3 CN has tetrahedral geometry and the other carbon has linear geometry. Multiple Central Atoms

What are the bonded-atom lone-pair arrangements and the shapes about each central atom in NH 2 SH? Draw the Lewis structure. The bonded-atom lone-pair arrangements of both are tetrahedral, the nitrogen shape is trigonal pyramidal and sulfur is “V” shaped. Shapes of Molecules N S H H H

Ethylene, C 2 H 4, could be planar (left) or nonplanar (right). The VSEPR model does not predict which is preferred. Overall Shape of C 2 H 4

The bond dipoles in CO 2 cancel because the linear shape orients the equal magnitude bond dipoles in exactly opposite directions. Polarity of Molecules

The bond dipoles do not cancel in COSe; they are oriented in the same direction and are of unequal length. They do not cancel in OF 2 because the V-shape of the molecule does not orient them in opposite directions. Polarity of Molecules

The bond dipoles in BCl 3 and CCl 4 cancel because of the regular shape and equal magnitude. Polarity of Molecules

The bond dipoles in BCl 2 F and CHCl 3 do not cancel because they are not of the same magnitude. Polarity of Molecules

Are the following molecules polar or nonpolar: H 2 S, SiF 4, CH 2 Cl 2 ? Test Your Skill

Valence bond theory describes bonds as being formed by overlap of partially filled valence orbitals. Valence Bond Theory

Identify the orbitals that form the bond in HCl. Test Your Skill

The observed bond angles of o in NH 3 are not consistent with the angles of 90 o expected if the bonds formed from N 2p orbitals. Bonding in NH 3

Hybrid orbitals are orbitals obtained by mixing two or more atomic orbitals on the same central atom. Appropriate hybrid orbitals formed by mixing one s and xp atomic orbitals make bonds at either 180 o (x = 1), 120 o (x = 2), or o (x = 3). Hybrid Orbitals

Analogy for Hybrid Orbitals

sp Hybrid Orbitals

Shape of Hybrid Orbitals For clarity, hybrid orbitals are pictured as elongated with the small lobe omitted.

The bonds in BeCl 2 arise from the overlap of two sp hybrid orbitals on the beryllium atom with the 3p orbitals on the two chlorine atoms. Bonding in BeCl 2

sp 2 Hybrid Orbitals

The bonds in BF 3 arise from the overlap of three sp 2 hybrid orbitals on the boron atom with 2p orbitals on the three fluorine atoms. Bonding in BF 3

sp 3 Hybrid Orbitals

The bonds in CH 4 arise from the overlap of four sp 3 hybrid orbitals on the carbon atom with 1s orbitals on the four hydrogen atoms. Bonding in CH 4

Hybrid orbitals can hold lone pairs as well as make bonds. Lone Pairs and Hybrid Orbitals

Hybrid orbitals of central atoms with steric numbers of 5 or 6 involve d orbitals. Hybridization with d Orbitals

Hybrid Orbitals

Test Your Skill Identify the hybrid orbitals on the central atoms in SiH 4 and HCN.

Sigma bonds (  ): the shared pair of electrons is symmetric about the line joining the two nuclei of the bonded atoms. Types of Bonds: Sigma Bonds

The C-C sigma bond in C 2 H 4 arises from overlap of sp 2 hybrid orbitals and the four C-H sigma bonds from overlap sp 2 hybrid orbitals on C with 1s orbitals on H. The second C-C bond forms from sideways overlap of p orbitals. Bonding in C 2 H 4

Pi bonds (  ) places electron density above and below the line joining the bonded atoms – they form by sideways overlap of p orbitals. Types of Bonds: Pi Bonds

The double bond in C 2 H 4 is one sigma bond and one pi bond – each bond is of similar strength. Bonding in C 2 H 4

C 2 H 4 is planar (A) because pi overlap is at a maximum. Rotation of one end by 90 o (B) reduces pi overlap to zero. Proof of Pi Bonds: Shape of C 2 H 4

The triple bond in C 2 H 2 is one sigma bond and two pi bonds between the sp hybridized carbon atoms. Triple Bonds

Each carbon atom in benzene, C 6 H 6, forms three sigma bonds with sp 2 hybrid orbitals. Sigma Bonds in Benzene

The remaining p orbital on each carbon atom (top) overlap to form three pi bonds. Pi Bonds in Benzene

Test Your Skill Describe the bonds made by the carbon atom in HCN.

Molecular orbital theory is a model that combines atomic orbitals to form new molecular orbitals that are shared over the entire molecule. A bonding molecular orbital concentrates electron density between atoms in a molecule. An antibonding molecular orbital reduces electron density between atoms in a molecule. Molecular Orbital Theory

Addition of the 1s orbitals of two H atoms forms a sigma bonding molecular orbital and subtraction forms a sigma antibonding molecular orbital, indicated with a * symbol. Hydrogen Molecule

Bonding molecular orbitals are more stable and antibonding molecular orbitals are less stable than the atomic orbital that are combined. Molecular Orbital Diagram: H 2

Bond order = 1/2 [number of electrons in bonding orbital - number of electrons in antibonding orbitals] Bond order in H 2 = 1/2 [2 - 0] = 1 Bond Order

Bond order in He 2 = 1/2 [2 - 2] = 0; the molecule does not form. Molecular Orbital Diagram: He 2

Sigma Molecular Orbitals from p

Pi Molecular Orbitals from p

MO Diagram Second-Period Diatomics

The electron configuration is (  2s ) 2 (  * 2s ) 2 (  2p ) 4 (  2p ) 2. The bond order in N 2 is three and there are no unpaired electrons. Lewis theory (:N  N:) predicts the same result. Molecular Orbital Diagram: N 2

The electron configuration is (  2s ) 2 (  * 2s ) 2. Bond order in Be 2 is zero and the molecule does not exist. Molecular Orbital Diagram: Be 2

Draw the molecular orbital diagram of O 2. What is the electron configuration, the bond order and how many unpaired electrons are present? Molecular Orbital Diagram for O 2

Draw the molecular orbital diagram of B 2. What is the electron configuration, the bond order and number of unpaired electrons? Test Your Skill