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Valence Bond Theory

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Valence Bond Theory Valence Bond Theory • A bond is a result of overlapping atomic orbitals from two atoms. The overlap holds a pair of electrons. • Normally each atomic orbital is bringing one electron to this bond. But in a “coordinate covalent bond”, both electrons come from the same atom • In this model, we are not creating a new orbital by the overlap. We are simply referring to the overlap between atomic orbitals (which may or may not be hybrid) from two atoms as a “bond”. Valence Bond Theory Sigma (σ) Bond • Skeletal bonds are called “sigma” bonds. • Sigma bonds are formed by orbitals approaching and overlapping each other head-on . Two hybrid orbitals, or a hybrid orbital and an s-orbital, or two s- orbitals • The resulting bond is like an elongated egg, and has cylindrical symmetry. Acts like an axle • That means the bond shows no resistance to rotation around a line that lies along its length. Pi (π) Bond Valence Bond Theory • The “leftover” p-orbitals that are not used in forming hybrid orbitals are used in making the “extra” bonds we saw in Lewis structures. The 2 nd bond in a double bond nd rd The 2 and 3 bonds in a triple bond /~harding/IGOC/P/pi_bond.html • Those extra bonds form only after the atoms are brought together by the formation of the skeletal bonds made by www.chem.ucla.edu hybrid orbitals. • The “extra” π bonds are always associated with a skeletal bond around which they form. • They don’t form without a skeletal bond to bring the p- orbitals together and “support” them. Valence Bond Theory -- Pi (π) Bond • The pure p-orbitals are perpendicular to the hybrid orbitals. • This causes the two lobes of the p-orbitals on the two atoms to stick out perpendicular to the skeletal sigma bond between them. • Those p-orbitals on the two atoms align and form a “sideways” bond where the lobes of the p-orbitals merge to form a π (pi) bond. www.chem.ucla.edu/~harding/IGOC/P/pi_bond.html Valence Bond Theory -- Pi (π) Bond Pi bonds don’t allow rotation • Since a π bond relies on the alignment of the p-orbitals on two atoms, rotation around the skeletal bond would mean breaking the π bond. • That means π bonds prevent rotation around the skeletal bond over which they formed. No π bond • Breaking a pi bond would require too much energy out of alignment π bond Valence Bond Theory -- Pi (π) Bond The Orbitals for CO 2 sp 2 ππππππ sp 2 OOC σσσ σσσ sp 2 (from hybrid orbitals) sp 2 (from pure p-orbitals) p 2 z sp pz p 2 y py sp sp sp 2 σσσ sp σσσ sp 2 p 2 y py sp 2 sp pz pz (from pure p-orbitals) • The two pi bonds are on perpendicular planes. • If one is on the plane of this page, the “lobes” of the other are sticking into and out of the page. Molecular Orbital Theory • Describes bonding in a different way than the Valence Bond Theory Different from hybrid orbitals • Atomic orbitals disappear, forming molecular orbitals (MO) An MO doesn’t belong to a particular atom It belongs to the entire molecule Molecular Orbital Theory • Molecular electron configurations can be written in much the same way as atomic electron configurations. • Each molecular orbital can hold 2 electrons with opposite spins. • The number of orbitals is conserved. The total number of atomic orbitals coming from different atoms is equal to the number of molecular orbitals created Molecular Orbital Theory Sigma (σ) molecular orbitals (MOs) . The electron probability of both molecular orbitals is centered along the line passing through the two nuclei. It has the same “symmetry” as a stick (or a cigar, or an egg) . If you spin it along its length, you won’t see a change. Formed by “head-on” approach of atomic orbitals . Spherical s atomic orbitals have no choice but approach each other head-on and form sigma molecular orbitals . We will first use the case of sigma molecular orbitals formed by s atomic orbitals to talk about “bonding” and “anti-bonding” molecular orbitals Molecular Orbital Theory Example: Combination of Hydrogen 1 s Atomic Orbitals to form MOs https://cnx.org/resources/92f991702d0b48e34361c055884b247df2391425/CNX_Chem_08_04_ssigma.jpg Two atomic orbitals create two molecular orbitals Molecular Orbital Theory Again, the number of orbitals are conserved Remember? When we considered the hybrid atomic orbitals, the number of pure atomic orbitals going into the mix was equal to the number of hybrid atomic orbitals created. Same here: Two atomic orbitals create two molecular orbitals. # of atomic orbitals used = # of MOs created Molecular Orbital Theory We will consider only the MOs formed by valence shell atomic orbitals . MOs made by core orbitals don’t contribute significantly to bonding and chemistry. Also, we consider only the molecular orbitals formed by atomic orbitals of similar (or same) energy; otherwise the resulting MO would be essentially the same as the atomic orbital with the much lower energy, and not be useful as a “molecular orbital” description. We will mostly deal with homonuclear (same element) diatomic species, where MOs are formed by valence orbitals of equal energy, coming from each atom. Molecular Orbital Theory Energies of MOs: Anti-bonding MO > parent atomic orbitals Bonding MO < parent atomic orbitals Anti bonding molecular orbital: higher in energy bad! Bonding molecular orbital: lower in energy good Copyright © Cengage Learning. All rights reserved 13 Molecular Orbital Theory MO Energy-Level Diagram for the H 2 Molecule 1s orbital of hydrogen A and 1s orbital of hydrogen B disappear, and form two molecular orbitals: One with higher energy (anti-bonding) One with lower energy (bonding) decreased electron density between nuclei! Anti- bonding Atomic orbital Atomic orbital of hydrogen B of hydrogen A Bonding Molecular orbitals of H2 molecule decreased electron density between nuclei! Molecular Orbital Theory The molecular orbital model produces electron distributions and energies that agree with our basic ideas of bonding. The labels on molecular orbitals indicate their symmetry (shape), and whether they are bonding or antibonding. “Sigma” ( σ) refers to the cylindrical/egg-shaped symmetry This corresponds to the sigma bonds we discussed earlier. We can’t see a change if we spin the object along its length Copyright © Cengage Learning. All rights reserved 15 Molecular Orbital Theory Example: H 2 If there is an asterix, it is “antibonding” σ∗ 1s Egg-shaped, formed by “head- on” approach of … Made from 1 s atomic orbitals Molecular Orbital Theory Bond Order • Larger bond order means greater bond strength. • Bonding electrons make a bond stronger • Antibonding electrons make a bond weaker • It’s the excess (net) bonding electrons that gives the bond its strength • Two net bonding electrons correspond to a “bonding pair” in one bond in the Lewis structures • We can have fractional bond order (0.5, or 1.5, or 2.5) in the MO model # of bonding e− − # of antibonding e − Bond order = 2 Copyright © Cengage Learning. All rights reserved 17 Molecular Orbital Theory – Bond order Example: H 2 0 electrons in antibonding orbital(s) 2 – 0 Bond Order = = 1 2 2 electrons in bonding orbital(s) Molecular Orbital Theory – Bond order – Example: H 2 2 – 1 1 Putting an extra electron on Bond Order = = H makes the bond weaker! 2 2 2 1 electron in antibonding orbital(s) 2 electrons in bonding orbital(s) Molecular Orbital Theory – Bond order Populating molecular orbitals There is no good reason to keep writing the atomic orbitals, once we learn the energy order of the molecular orbitals formed by the valence orbitals of the atoms. We then simply populate the MOs with the valence electrons in the order of increasing energy, from lower to higher. The number of electrons in those MOs is the total number of valence electrons brought by the individual atoms, minus the charge on the species. Molecular Orbital Theory – Bond order – Example: H 2 We just populate molecular orbitals with the valence electrons. H charge ‒ H2 has 3 valence electrons: (2)(1) – (-1) = 3 Easy enough to list and σ∗ 1 electron in order the MOs when 1s antibonding orbital(s) we only have s-orbitals making molecular σ 2 electrons in orbitals 1s bonding orbital(s) Molecular Orbital Theory Sigma and Pi orbitals Sigma ( σ) MO (when it is the “bonding” kind) corresponds to the σ bond we created using head-on overlap of hybrid atomic orbitals. But no hybrid orbitals here! Just pure atomic orbitals forming MOs. Pi ( π) MO (when it is the “bonding” kind) corresponds to the π bond we created using the pure atomic p-orbitals. Same here! πππ sideways πππ head-on σσσ πππ sideways πππ Molecular Orbital Theory Expected energy diagram for Molecular Orbitals Pi orbitals from the inefficient sideways-overlap of p-orbitals are expected to have smaller “reward” (lowering of energy) and “penalty” (raising of energy) for being bonding and “anti-bonding” σ∗ 2p π∗ 2p 2p π 2p 2p σ 2p σ∗ 2s 2s 2s σ 2s Molecular Orbital Theory -- Energy Diagrams The expected energy diagram for MOs can predict the bond orders for the diatomic species just fine B2 C2 N2 O2 F2 6 val. e ‒ 8 val. e ‒ 10 val. e ‒ 12 val. e ‒ 14 val. e ‒ 4 – 2 6 – 2 8 – 2 8 – 4 8 – 6 B.O.= = 1 B.O.= = 2 B.O.= = 3 B.O.= = 2 B.O.= = 1 2 2 2 2 2 σ ∗ 2p π∗ 2p π 2p σ 2p σ ∗ 2s σ 2s Molecular Orbital Theory -- Energy Diagrams The expected energy diagram for MOs can predict the bond orders for the diatomic species just fine + ‒ 2‒ 2+ C2 C2 N2 O2 7 val.
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