Advanced Theories of Chemical Bonding Chapter 8 Atomic Orbitals Molecules

Theories of Bonding VALENCE BOND THEORY — Linus PaulingVALENCE BOND THEORY — Linus Pauling valence electrons are localized between atoms (or are lone pairs).valence electrons are localized between atoms (or are lone pairs). half-filled atomic orbitals overlap to form bonds.half-filled atomic orbitals overlap to form bonds.

Sigma Bond Formation by Orbital Overlap Two s orbitals overlap

Sigma Bond Formation Two s orbitals overlap Two p orbitals overlap

Sigma Bonds The bond that arises from the overlap of two orbitals, one from each of two atoms as in H 2, is called a sigma (σ) bond. The electron density of a σ bond is greatest along the axis of the bond. These overlaps can exist between any orbital … s,p,or d

Sigma Bond Formation Summary: Orbitals overlap to form a bond between two atoms. Two electrons, of opposite spin, can be accommodated in the overlapping orbitals. Usually one electron is supplied by each of the two bonded atoms. Because of orbital overlap, the bonding electrons have a higher probability of being found within a region of space influenced by both nuclei. Both electrons are simultaneously attracted to both nuclei.

MOLECULAR GEOMETRY

VSEPR VSEPR V alence S hell E lectron P air R epulsion theory.V alence S hell E lectron P air R epulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs.Most important factor in determining geometry is relative repulsion between electron pairs. Molecule adopts the shape that minimizes the electron pair repulsions. MOLECULAR GEOMETRY E:\Media\Movies\09M15AN2.mov

Electron Pair Geometries Figure 9.12

Structure Determination by VSEPR Ammonia, NH 3 1. Draw electron dot structure 2. Count BP’s and LP’s = 4 H H H N 3. The 4 electron pairs are at the corners of a tetrahedron.

Structure Determination by VSEPR Ammonia, NH 3 There are 4 electron pairs at the corners of a tetrahedron. The ELECTRON PAIR GEOMETRY is tetrahedral.

Ammonia, NH 3 The electron pair geometry is tetrahedral. Structure Determination by VSEPR The MOLECULAR GEOMETRY — the positions of the atoms — is PYRAMIDAL.

Structure Determination by VSEPR Water, H 2 O 1. Draw electron dot structure The electron pair geometry is TETRAHEDRAL. 2. Count BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron.

Structure Determination by VSEPR Water, H 2 O The electron pair geometry is TETRAHEDRAL The molecular geometry is BENT.

Geometries for Four Electron Pairs Figure 9.13

Structure Determination by VSEPR Formaldehyde, CH 2 O 1. Draw electron dot structure The electron pair geometry is PLANAR TRIGONAL with 120 o bond angles. CHH O C HH O 2. Count BP’s and LP’s at C 3. There are 3 electron “lumps” around C at the corners of a planar triangle.

Structure Determination by VSEPR Formaldehyde, CH 2 O The electron pair geometry is PLANAR TRIGONAL The molecular geometry is also planar trigonal.

H-C-H = 109 o C-O-H = 109 o In both cases the atom is surrounded by 4 electron pairs. Structure Determination by VSEPR H H H—C—O—H 109˚ Methanol, CH 3 OH Define H-C-H and C-O-H bond angles

Structure Determination by VSEPR Acetonitrile, CH 3 CN H H H—C—CN 180˚ 109˚ H-C-H = 109 o C-C-N = 180 o H-C-H = 109 o C-C-N = 180 o One C is surrounded by 4 electron “lumps” and the other by 2 “lumps” Define unique bond angles

Phenylalanine, an amino acid

PhenylalaninePhenylalanine

Structures with Central Atoms with More Than or Less Than 4 Electron Pairs Often occurs with Group 3A elements and with those of 3rd period and higher.

Boron Compounds Consider boron trifluoride, BF 3 Geometry described as planar trigonal The B atom is surrounded by only 3 electron pairs. Bond angles are 120 o

5 electron pairs Compounds with 5 or 6 Pairs Around the Central Atom

Number of valence electrons = 34Number of valence electrons = 34 Central atom = S Central atom = S Dot structureDot structure Sulfur Tetrafluoride, SF 4 Electron pair geometry --> trigonal bipyramid (because there are 5 pairs around the S) Electron pair geometry --> trigonal bipyramid (because there are 5 pairs around the S)

Lone pair is in the equator because it requires more room. Sulfur Tetrafluoride, SF 4

Molecular Geometries for Five Electron Pairs Figure 9.14

6 electron pairs Compounds with 5 or 6 Pairs Around the Central Atom

Molecular Geometries for Six Electron Pairs Figure 9.14

Using VB Theory Bonding in BF 3 planar triangle angle = 120 o

Bonding in BF 3 How to account for 3 bonds 120 o apart using a spherical s orbital and p orbitals that are 90 o apart?How to account for 3 bonds 120 o apart using a spherical s orbital and p orbitals that are 90 o apart? Pauling said to modify VB approach with ORBITAL HYBRIDIZATIONPauling said to modify VB approach with ORBITAL HYBRIDIZATION — mix available orbitals to form a new set of orbitals — HYBRID ORBITALS — that will give the maximum overlap in the correct geometry. (See Screen 10.6)— mix available orbitals to form a new set of orbitals — HYBRID ORBITALS — that will give the maximum overlap in the correct geometry. (See Screen 10.6)

Bonding in BF 3 See Figure 10.9 and Screen 10.6 rearrange electronshydridize orbs. unused p orbital three sp 2 hybrid orbitals 2p 2s

The three hybrid orbitals are made from 1 s orbital and 2 p orbitals  3 sp 2 hybrids. The three hybrid orbitals are made from 1 s orbital and 2 p orbitals  3 sp 2 hybrids. Bonding in BF 3 Now we have 3, half-filled HYBRID orbitals that can be used to form B-F sigma bonds.Now we have 3, half-filled HYBRID orbitals that can be used to form B-F sigma bonds.

An orbital from each F overlaps one of the sp 2 hybrids to form a B-F  bond. Bonding in BF 3

Bonding in CH 4 How do we account for 4 C—H sigma bonds 109 o apart? Need to use 4 atomic orbitals — s, p x, p y, and p z — to form 4 new hybrid orbitals pointing in the correct direction.

4 C atom orbitals hybridize to form four equivalent sp 3 hybrid atomic orbitals. Bonding in a Tetrahedron — Formation of Hybrid Atomic Orbitals

4 C atom orbitals hybridize to form four equivalent sp 3 hybrid atomic orbitals.

Bonding in CH 4 Figure 10.6

Orbital Hybridization Figure 10.5 BONDSSHAPEHYBRID REMAIN 2linear sp2 p’s 3trigonal sp 2 1 p planar 4tetrahedral sp 3 none

Bonding in Glycine

Multiple Bonds Consider ethylene, C 2 H 4

Sigma Bonds in C 2 H 4

π Bonding in C 2 H 4 The unused p orbital on each C atom contains an electron and this p orbital overlaps the p orbital on the neighboring atom to form the π bond. (See Fig. 10.9)

π Bonding in C 2 H 4 The unused p orbital on each C atom contains an electron and this p orbital overlaps the p orbital on the neighboring atom to form the π bond. (See Fig. 10.9)

Multiple Bonding in C 2 H 4

 and π Bonding in C 2 H 4 Figure 10.11

 and π Bonding in CH 2 O Figure 10.12

 and π Bonding in C 2 H 2 Figure 10.13

Consequences of Multiple Bonding Figure There is restricted rotation around C=C bond.

Consequences of Multiple Bonding See Butene.Map in ENER_MAP in CAChe models. Restricted rotation around C=C bond.

Double Bonds and Vision See Screen 10.13, Molecular Orbitals and Vision See also Chapter Focus 10, page 380