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Chapter 11 Theories of Covalent Bonding Valence Bond Theory Central Themes: A covalent bond forms when orbitals of two atoms overlap and the overlap region is occupied by two electrons. The greater the overlap the stronger the bond. The stronger the bond the more stable the bond. Orbitals must become oriented so as to obtain the greatest overlap possible. Types of orbital overlap: sigma (end-to-end) pi (side-by-side) Note: sigma side-by-side overlap is not permitted. Let’s Consider CH4 How can carbon form four bonds? What is the shape of methane? What are the bond angles? Hybrid Orbital Theory Theory is that atomic orbitals mix (hybridize) to form the necessary number of orbitals needed for bonding. The number of hybrid orbitals formed equals the number of atomic orbitals mixed. The type of hybrid orbitals obtained varies with the type of atomic orbitals mixed. Types of Hybrid Orbitals sp sp2 sp3 sp3d sp3d2 sp Hybrids Consider BeH2 Central Be needs two hybrid orbitals to accommodate the two bonded H atoms. To aid the visualization of sp hybrid formation use an orbital diagram, shapes of hybrids, and bond angles. Bond angles for sp hybrids = 180o Note: The bonding between the sp hybrid of beryllium and the “s” orbital of the hydrogen atom is considered a sigma bond overlap. sp2 Hybrids Consider BF3 Central boron atom needs three hybrid orbitals to accommodate three bonded fluorine atoms. Visual of hybrid orbital formation theory. Bond angles for sp2 hybrids = 120o Note: The bond involving the sp2 hybrid of boron end-to-end of the “p” orbital of the fluorine atom. sp3 Hybrids Back to CH4 The central carbon atom needs four hybrid orbitals. Other atoms using sp3 Hybrids If the central atom has only two or three (rather than four) bonded atoms, hybrid orbitals may contain lone pairs of electrons. Examples: NH3 , H2O Bond angles between sp3 hybrids = 109.5o sp3d Hybrids Consider PCl5 The central phosphorus atom needs five hybrid orbitals to accommodate the five bonded chlorine atoms. Note that since there is only one “s” orbital and only three “p” orbitals available per energy level, a “d” orbital must be used in the hybrid. Bond angles between sp3d hybrids = 90o (axial) and 120o (equatorial) sp3d2 Hybrids Consider SF6 The six bonded atoms require six hybrids. Bond angles between d2sp3 hybrids = 90o Summary of Hybrid Theory Hybrid Quiz Predict the type of hybrid orbital you would expect in the central atom of the following molecules: SiH4 BH3 AsF5 AlCl3 SF4 Multiple Bonds Double Bond: A=B One bond is a sigma bond The other is a pi bond Sigma bonds are formed by hybrid orbitals overlapping. Pi bonds are formed by unhybridized “p” orbitals overlapping. Double Bond Triple Bond: A≡B One bond is a sigma bond formed from overlapping hybrid orbitals. Two bonds are pi bonds formed from overlapping “p” orbitals. Triple Bond Orbital Overlap and Molecular Rotation Sigma bonds allow free rotation of bonded atoms. Pi bond overlap restricts rotation of bonded atoms. Double bonds lead to cis- and transisomers. Example: CH2Cl2 Quiz Describe the type of hybrid orbitals used by each carbon and oxygen atom in the following molecule: O H C C H H O H Molecular Orbital Theory Basic ideas of MO Theory: 1. Bonded atoms possess molecular orbitals formed from their atomic orbitals. 2. Two types of MOs are possible: Bonding orbitals (lower energy) Antibonding orbitals (higher energy) 3. The total number of MOs equals the number of atomic orbitals combined. Types of MOs “s” sigma molecular orbitals If a “1s” atomic orbital from one atom is combined with a “1s” orbital from another (bonded atom) the result is two molecular orbitals. One MO is called sigma 1s, the other is sigma*1s. Combination of atomic orbitals Hund’s Rule in MO Theory 1. 2. 3. 4. MOs fill low energy first. MOs have a maximum of 2 electrons with opposite spins. Orbitals of equal energy are half-filled before pairing. Bond orbitals must be completely filled before adding electrons to antibonding orbitals. Formation of bonding and antibonding orbitals for H2 Bond Orders The bond order of any molecule (using MO theory) is determined from the number of bonding electrons minus the number of antibonding electrons divided by 2. B.O. = BE – ABE 2 “p” sigma MOs If a “2p” orbital is combined end-to-end with another “2p” orbital from another atom, the result is two molecular orbitals. One is called a sigma 2p MO, the other a sigma*2p. “p” pi MOs There are two possible side-by-side overlaps of “2p” orbitals (recall the x,y,z axes of the three “p” orbitals). A side-by-side overlap results in two pi 2p bonding orbital and two pi*2p antibonding orbital. Formation of sigma and pi MOs from “p” atomic orbitals Put in our own MO diagram so as to not complicate matters Homonuclear Diatomic Molecule MO diagrams for 2nd Period Using MO Theory to Explain Bond Properties Bond orders and bond strengths Paramagnetism and diamagnetism Noble gases are monatomic MO Descriptions of ions Fill in MO diagrams and describe properties of O2 , O2-, and O2+ MO diagrams of heteronuclear diatomic molecules Fill in MO diagrams and describe properties of NO, OF, and BF-.