Molecular Orbitals Chapter 9 Orbitals and Covalent Bond The overlap of atomic orbitals from separate atoms makes molecular orbitals Each molecular orbital has room for two electrons Two types of MO Sigma ( σ ) between atoms Pi ( π ) above and below atoms 1 2 Sigma bonding orbitals Sigma bonding orbitals From s orbitals on separate atoms From p orbitals on separate atoms + + s orbital s orbital + + + + Sigma bonding molecular orbital p orbital p orbital 3 4 Sigma bonding molecular orbital Pi bonding orbitals p orbitals on separate atoms Sigma and pi bonds All single bonds are sigma bonds A double bond is one sigma and one pi bond A triple bond is one sigma and two pi bonds. Pi bonding molecular orbital 5 6 1
7 Atomic Orbitals Don t Work to explain molecular geometry. In methane, C 4, the shape is tetrahedral. The valence electrons of carbon should be two in s, and two in p. the p orbitals would have to be at right angles. The atomic orbitals change when making a molecule 8 ybridization We blend the s and p orbitals of the valence electrons and end up with the tetrahedral geometry. We combine one s orbital and 3 p orbitals. sp 3 hybridization has tetrahedral geometry. 9 10 11 In terms of energy ybridization sp 3 12 ow we get to hybridization We know the geometry from experiment. We know the orbitals of the atom hybridizing atomic orbitals can explain the geometry. So if the geometry requires a tetrahedral shape, it is sp 3 hybridized This includes bent and trigonal pyramidal molecules because one of the sp 3 lobes holds the lone pair. 2
sp 2 hybridization C 2 4 Double bond acts as one pair. trigonal planar ave to end up with three blended orbitals. Use one s and two p orbitals to make sp 2 orbitals. Leaves one p orbital perpendicular. 13 14 In terms of energy ybridization sp 2 15 16 Where is the P orbital? Perpendicular The overlap of orbitals makes a sigma bond (σ bond) Two types of Bonds Sigma bonds from overlap of orbitals. Between the atoms. Pi bond (π bond) above and below atoms Between adjacent p orbitals. The two bonds of a double bond. 17 18 3
C C sp 2 hybridization When three things come off atom. trigonal planar 120º One π bond, σ + lp =3 19 20 21 What about two When two things come off. One s and one p hybridize. linear 22 sp hybridization End up with two lobes 180º apart. p orbitals are at right angles Makes room for two π bonds and two sigma bonds. A triple bond or two double bonds. In terms of energy CO 2 ybridization sp C can make two σ and two π O can make one σ and one π O C O 23 24 4
N 2 N 2 25 26 Breaking the octet PCl 5 The model predicts that we must use the d orbitals. dsp 3 hybridization There is some controversy about how involved the d orbitals are. Trigonal bipyrimidal can only σ bond. can t π bond. basic shape for five things. dsp 3 27 28 Can t tell the hybridization of Cl PCl 5 Assume sp 3 to minimize repulsion of electron pairs. d 2 sp 3 gets us to six things around Octahedral Only σ bond 29 30 5
Molecular Orbital Model Localized Model we have learned explains much about bonding. It doesn t deal well with the ideal of resonance, unpaired electrons, and bond energy. The MO model is a parallel of the atomic orbital, using quantum mechanics. Each MO can hold two electrons with opposite spins 31 Square of wave function tells probability get two orbitals MO 2 = 1s A - 1s B MO 1 = 1s A + 1s B 32 What do you get? Solve the equations for 2 A B 33 The Molecular Orbital Model The molecular orbitals are centered on a line through the nuclei MO 1 the greatest probability is between the nuclei MO 2 it is on either side of the nuclei this shape is called a sigma molecular orbital 34 The Molecular Orbital Model In the molecule only the molecular orbitals exist, the atomic orbitals are gone MO 1 is lower in energy than the 1s orbitals they came from. This favors molecule formation Called an bonding orbital MO 2 is higher in energy This goes against bonding antibonding orbital The Molecular Orbital Model MO 2 2 1s 1s MO 1 35 36 The Molecular Orbital Model We use labels to indicate shapes, and whether the MO s are bonding or antibonding. MO 1 = σ 1s MO 2 = σ 1s * (* indicates antibonding) Can write them the same way as atomic orbitals 2 = σ 1s 2 6
The Molecular Orbital Model Each MO can hold two electrons, but they must have opposite spins Orbitals are conserved. The number of molecular orbitals must equal the number atomic orbitals that are used to make them. 1s 2 - σ 1s * 1s σ 1s 37 38 39 Bond Order The difference between the number of bonding electrons and the number of antibonding electrons divided by two Bond Order = # bonding-#antibonding 2 40 Only outer orbitals bond The 1s orbital is much smaller than the orbital When only the orbitals are involved in bonding Don t use the σ 1s or σ 1s * for Li 2 Li 2 = (σ ) 2 In order to participate in bonds the orbitals must overlap in space. Bonding in omonuclear Diatomic Molecules Need to use omonuclear so that we know the relative energies. Li 2 - (σ ) 2 (σ *) 1 B 2 Be 2 (σ ) 2 (σ *) 2 What about the p orbitals? ow do they form orbitals? 41 Remember that orbitals must be conserved. 42 7
B 2 σ * π * σ Expected Diagram σ * π * π * π π σ σ * 43 π 44 σ B 2 B 2 (σ ) 2 (σ *) 2 (σ ) 2 Bond order = (4-2) / 2 Should be stable. This assumes there is no interaction between the s and p orbitals. ard to believe since they overlap proof comes from magnetism. 45 46 Magnetism Magnetism has to do with electrons. Remember that spin is how an electron reacts to a magnetic field Paramagnetism attracted by a magnet. associated with unpaired electrons. Diamagnetism repelled by a magnet. associated with paired electrons. B 2 is paramagnetic. 47 48 Magnetism The energies of of the π and the σ are reversed by p and s interacting The σ and the σ * are no longer equally spaced. ere s what it looks like. 8
Correct energy diagram σ * π * π * B 2 σ * π * σ σ π π π σ * σ * 49 σ 50 σ Patterns As bond order increases, bond energy increases. As bond order increases, bond length decreases. Supports basis of MO model. There is not a direct correlation of bond order to bond energy. O 2 is known to be paramagnetic. Movie. 51 52 Magnetism Ferromagnetic strongly attracted Paramagnetic weakly attracted Liquid Oxygen Diamagnetic weakly repelled Graphite Water Frog C 2 N 2 O 2 F 2 P 2 Examples eteronuclear Diatomic Species Simple type has them in the same energy level, so can use the orbitals we already know. Slight energy differences. NO 53 54 9
55 NO NO + CN - You try What if they come from completely different orbitals and energy? F Simplify first by assuming that F only uses one if its orbitals. F holds onto its electrons, so they have low energy 56 1s σ Paramagnetic Consequences Since is lower in energy, favored by electrons. Electrons spend time closer to fluorine. Compatible with polarity and electronegativity. 57 σ 58 59 Names sp orbitals are called the Localized electron model σ and π Μolecular orbital model Localized is good for geometry, doesn t deal well with resonance. seeing σ bonds as localized works well It is the π bonds in the resonance structures that can move. π delocalized bonding C 6 6 60 10
C 2 6 NO 3-61 62 11