Theories of Bonding and Structure CHAPTER 10
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Theories of Bonding and Structure CHAPTER 10 Chemistry: The Molecular Nature of Matter, 6th edition By Jesperson, Brady, & Hyslop CHAPTER 10: Bonding & Structure Learning Objectives VESPR theory: Determine molecular geometry based on molecular formula and/or lewis dot structures. Effect of bonded atoms & non-bonded electrons on geometry Molecular polarity & overall dipole moment Assess overall dipole moment of a molecule Identify polar and non-polar molecules Valence Bond Theory Hybridized orbitals Multiple bonds Sigma vs pi orbitals Molecular Orbital Theory Draw & label molecular orbital energy diagrams Bonding & antibonding orbitals Predict relative stability of molecules based on MO diagrams 2 Molecular Geometry Basic Molecular Geometries Linear 3 atoms Trigonal Planar or Planar Triangular Trigonal Bipyramidal 6 atoms 4 atoms Tetrahedral: 5 atoms Octahedral: 7 atoms Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 3 VESPR Definition Valence Shell Electron Pair Repulsion Model Electron pairs (or groups of electron pairs) in the valence shell of an atom repel each other and will position themselves so that they are far apart as possible, thereby minimizing the repulsions. Electron pairs can either be lone pairs or bonding pairs. Tetrahedral arrangement of electron pairs Bent geometry Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E http://chemistry-desk.blogspot.com/2011/05/prediction-of-shape-of-molecules-by.html 4 VESPR Definition Valence Shell Electron Pair Repulsion Model Electron pairs (or groups of electron pairs) in the valence shell of an atom repel each other and will position themselves so that they are far apart as possible, thereby minimizing the repulsions. Text uses “electron domain” to describe electron pairs: Bonding domain: contains electrons that are shared between two atoms. So electrons involved in single, double, or triple are part of the same bonding domain. Nonbonding Domain: Valence electrons associated with one atom, such as a lone pair, or a unpaired electron. Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 5 VESPR Basic Examples 2 bonding domains 3 bonding domains 5 bonding domains 4 bonding domains 6 bonding bonding domains Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 6 VESPR When Lone Pairs or Multiple Bonds Present Including lone pairs: • Take up more space around central atom • Effect overall geometry • Counted as nonbonded electron domains Including multiple bonds (double and triple) • For purposes of determining geometry focus on the number of atoms bonded together rather then the number of bonds in between them: ie, treat like a single bond. • Treat as single electron bonding domain Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 7 VESPR Electrons that are Bonding & Not Bonding Bonding Electrons – More oval in shape – Electron density focused between two positive nuclei. Nonbonding Electrons – More bell or balloon shaped – Take up more space – Electron density only has positive nuclei at one end Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 8 VESPR 3 atoms or lone pairs Number of Bonding Domains Number of Nonbonding Domains 3 0 Planar Triangular (e.g. BCl3) All bond angles 120 1 Nonlinear Bent or V-shaped (e.g. SnCl2) Bond <120 2 Structure Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E Molecular Shape 9 VESPR Number of Bonding Domains 4 4 atoms or lone pairs Number of Nonbonding Domains Structure Molecular Shape Tetrahedron (e.g. CH4) All bond angles 109.5 0 Trigonal pyramidal 3 (e.g. NH3) Bond angle less than 109.5 1 Nonlinear, bent 2 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E (e.g. H2O) Bond angle less than109.5 10 VESPR 5 atoms or lone pairs Trigonal Bipyramidal • Two atoms in axial position – 90 to atoms in equatorial plane • Three atoms in equatorial position – 120 bond angle to atoms in axial position – More room here – Substitute here first 90 120 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 11 VESPR 5 atoms or lone pairs Number of Bonding Domains Number of Nonbonding Domains 5 0 4 Structure Molecular Shape Trigonal bipyramid (e.g. PF5) Ax-eq bond angles 90 Eq-eq 120 Distorted Tetrahedron, or Seesaw (e.g. SF4) 1 Ax-eq bond angles < 90 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 12 VESPR • • • • 5 atoms or lone pairs Lone pair takes up more space Goes in equatorial plane Pushes bonding pairs out of way Result: distorted tetrahedron Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 13 VESPR 5 atoms or lone pairs Number of Number of Bonding Nonbonding Domains Domains 3 2 2 3 Structure Molecular Shape T-shape (e.g. ClF3) Bond angles 90 Linear (e.g. I3–) Bond angles 180 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 14 VESPR 6 atoms or lone pairs Number of Number of Bonding Nonbonding Domains Domains 6 0 5 1 Structure Molecular Shape Octahedron (e.g. SF6) Square Pyramid (e.g. BrF5) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 15 VESPR 6 atoms or lone pairs Number of Number of Bonding Nonbonding Domains Domains 4 Structure Molecular Shape Square planar (e.g. XeF4) 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 16 VESPR Determining 3-D Structures 1. Draw Lewis Structure of Molecule – Don't need to compute formal charge – If several resonance structures exist, pick only one 2. Count electron pair domains – Lone pairs and bond pairs around central atom – Multiple bonds count as one set (or one effective pair) 3. Arrange electron pair domains to minimize repulsions • Lone pairs – Require more space than bonding pairs – May slightly distort bond angles from those predicted. – In trigonal bipyramid lone pairs are equatorial – In octahedron lone pairs are axial 4. Name molecular structure by position of atoms—only bonding electrons Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 17 Molecular Polarity Polar Molecules • Have net dipole moment – Negative end – Positive end • Polar molecules attract each other. – Positive end of polar molecule attracted to negative end of next molecule. – Strength of this attraction depends on molecule's dipole moment – Dipole moment can be determined experimentally • Polarity of molecule can be predicted by taking vector sum of bond dipoles • Bond dipoles are usually shown as crossed arrows, where arrowhead indicates negative end Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 18 Molecular Polarity Molecular Shape & Polarity • Many physical properties (melting and boiling points) affected by molecular polarity • For molecule to be polar: – Must have polar bonds • Many molecules with polar bonds are nonpolar - Possible because certain arrangements of bond dipoles cancel - For molecules with more than two atoms, must consider the combined effects of all polar bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E http://wps.prenhall.com/wps/media/objects/3081/3155729/blb0903.html 19 Molecular Polarity Symmetrical Nonpolar Molecules • Symmetrical molecules – Nonpolar because bond dipoles cancel • All five shapes are symmetrical when all domains attached to them are composed of identical atoms Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 20 Molecular Polarity Symmetrical Nonpolar Molecules Cancellation of Bond Dipoles In Symmetrical Trigonal Bipyramidal and Octahedral Molecules • • • All electron pairs around central atom are bonding pairs and All terminal groups (atoms) are same The individual bond dipoles cancel Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 21 Molecular Polarity Polar Molecules Molecule is usually polar if – All atoms attached to central atom are NOT same Or, – There are one or more lone pairs on central atom Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 22 Molecular Polarity Polar Molecules Water and ammonia both have non-bonding domains Bond dipoles do not cancel Molecules are polar Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 23 Molecular Polarity Polar Molecules: Exception Exception to these general rules for identifying polar molecules: Nonbonding domains (lone pairs) are symmetrically placed around central atom Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 24 Problem Set A 1. For the following molecules: a. b. c. d. e. Draw a lewis dot structure. Determine the molecular geometry at each central atom. Identify the bond angles. Identify all polar bonds: δ+ / δAssess the polarity of the molecule & indicate the overall dipole moment if one exists AsF5 AsF3 SeO2 GaH3 ICl2- SiO4-4 TeF6 25 VB Theory Review: Modern Atomic Theory of Bonding Modern Atomic Theory of Bonding is based on wave mechanics and gave us: – Electrons and shapes of orbitals – Four quantum numbers – Heisenberg uncertainty principle • Electron probabilities – Pauli Exclusion Principle Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 26 VB Theory Valence Bond Theory & Molecular Orbital Theory Valence Bond Theory • Individual atoms, each have their own orbitals and orbitals overlap to form bonds • Extent of overlap of atomic orbitals is related to bond strength Molecular Orbital Theory • Views molecule as collection of positively charged nuclei having a set of molecular orbitals that are filled with electrons (similar to filling atomic orbitals with electrons) • Doesn't worry about how atoms come together to form molecule Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 27 VB Theory Valence Bond Theory & Molecular Orbital Theory Both Theories: • Try to explain structure of molecules, strengths of chemical bonds, bond orders, etc. • Can be extended and refined and often give same results Valence Bond Theory Bond between two atoms formed when pair of electrons with paired (opposite) spins is shared by two overlapping atomic orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 28 VB Theory H2 H2 bonds form because 1s atomic valence orbital from each H atom overlaps Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 29 VB Theory F2 • F2 bonds form because atomic valence orbitals overlap • Here 2p overlaps with 2p • Same for all halogens, but different np orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 30 VB Theory HF HF involves overlaps between 1s orbital on H and 2p orbital of F 1s 2p Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 31 VB Theory H2S • Assume that unpaired electrons in S and H are free to form paired bond • We may assume that H—S bond forms between s and p orbital Predicted 90˚ bond angle is very close to experimental value of 92˚. Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 32 VB Theory Need to Change Approach to Explain Bonding in CH4 Example: CH4 C 1s 22s 22p 2 and H 1s 1 • In methane, CH4 – All four bonds are the same – Bond angles are all 109.5° • Carbon atoms have – All paired electrons except two unpaired 2p – p orbitals are 90° apart – Atomic orbitals predict CH2 with 90° angles Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 33 VB Theory Hybridization • Mixing of atomic orbitals to allow formation of bonds that have realistic bond angles. – Realistic description of bonds often requires combining or blending two or more atomic orbitals • Hybridization just rearranging of electron probabilities Why do it? • To get maximum possible overlap • Best (strongest) bond formed Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 34 VB Theory Hybrid Orbitals • Blended orbitals result from hybridization process • Hybrid orbitals have – New shapes – New directional properties – Each hybrid orbital combines properties of parent atomic orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 35 VB Theory Hybrid Orbitals • Symbols for hybrid orbitals combine the symbols of the orbitals used to form them – Use s + p form two sp hybrid orbitals – Use s + p + p form three sp 2 hybrid orbitals • One atomic orbital is used for each hybrid orbital formed • Sum of exponents in hybrid orbital notation must add up to number of atomic orbitals used Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 36 VB Theory Hybrid Orbitals Mixing or hybridizing s and p orbital of same atom results in two sp hybrid orbitals Two sp hybrid orbitals point in opposite directions Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 37 VB Theory Ex: sp Hybridized Orbitals: BeH2 • Now have two sp hybrid orbitals • Oriented in correct direction for bonding • 180 bond angles – As VSEPR predicts and – Experiment verifies • Bonding = – Overlap of H 1s atomic orbitals with sp hybrid orbitals on Be Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 38 VB Theory Hybrid Orbitals Hybrid Atomic Orbitals Used sp sp2 sp3 sp3d sp3d2 Electron Geometry s+p Linear Bond angles 180° s+p+p Trigonal planar Bond angles 120° s + p+ p + p Tetrahedral Bond angles 109.5° s + p+ p + p + d Trigonal Bipyramidal Bond angles 90° and 120° s + p+ p + p + d + d Octahedral Bond angles 90° Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 39 VB Theory Bonding in BCl3 • Overlap of each halffilled 3p orbital on Cl with each half-filled sp2 hybrid on B Forms three equivalent bonds Trigonal planar shape 120 bond angle Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 2p sp2 40 VB Theory Bonding in CH4 Overlap of each half- filled 1s orbital on H with each halffilled sp3 hybrid on carbon Forms four equivalent bonds Tetrahedral geometry 109.5 bond angle Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E sp3 41 VB Theory Hybrid sp Orbitals Two sp hybrids Linear Three sp2 hybrids All angles 120 Planar Triangular Four sp3 hybrids All angles 109.5 Tetrahedral Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 42 VB Theory Expanded Octet Hybridization Hybridization When Central Atom has More Than Octet • If there are more than four equivalent bonds on central atom, then must add d orbitals to make hybrid orbitals Why? • One s and three p orbitals means that four equivalent orbitals is the most you can get using s and p orbitals alone So, only atoms in third row of the periodic table and below can exceed their octet • These are the only atoms that have empty d orbitals of same n level as s and p that can be used to form hybrid orbitals • One d orbital is added for each pair of electrons in excess of standard octet Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 43 VB Theory Expanded Octet Hybridization Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 44 VB Theory Hybridization in Molecules with Lone Pairs CH4sp3 tetrahedral geometry 109.5° bond angle NH3 107° bond angle H2O 104.5° bond angle • Angles suggest that NH3 and H2O both use sp3 hybrid orbitals in bonding • Not all hybrid orbitals used for bonding e– – Lone pairs can occupy hybrid orbitals • Lone pairs must always be counted to determine geometry 2p hybridize form bonds 2s Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E sp3 lone pair bonding electrons 45 VB Theory 2p Ex: H2O Hybridization hybridize form bonds 2s lone sp3 bonding pairs electrons Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 46 VB Theory Multiple Bonds • So where do extra electron pairs in multiple bonds go? – Not in hybrid orbitals – Remember VSEPR, multiple bonds have no effect on geometry • Why don’t they effect geometry? Two types of bond result from orbital overlap • Sigma () bond – Accounts for first bond • Pi () bond – Accounts for second and third bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 47 VB Theory Sigma () Bonds • Head on overlap of orbitals • Concentrate electron density concentrated most heavily between nuclei of two atoms • Lie along imaginary line joining their nuclei s+s p+p sp + sp Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 48 VB Theory Pi () Bonds • Sideways overlap of unhybridized p orbitals • Electron density divided into two regions – Lie on opposite sides of imaginary line connecting two atoms • Electron density above and below bond. No electron density along bond axis bond consists of both regions Both regions = one bond Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 49 VB Theory Pi () Bonds • Can never occur alone – Must have bond • Can form from unhybridized p orbitals on adjacent atoms after forming bonds • bonds allow atoms to form double and triple bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 50 VB Theory Multiple Bonds Ex: Ethene (C2H4) • Each carbon is – sp 2 hybridized (violet) – has one unhybridized p orbital (red) • C=C double bond is – one bond (sp 2 – sp 2 ) – one bond (p – p) p—p overlap forms a C— C bond Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 51 VB Theory Conformations • C—C single bond has free rotation around the C—C bond • Conformations – Different relative orientations on molecule upon rotation Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 52 VB Theory Conformations Ex: Pentane, C5H12 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 53 VB Theory Properties of -Bonds • Can’t rotate about double bond • bond must first be broken before rotation can occur Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 54 VB Theory Ex: Bonding in Formaldehyde H • C and O each – sp 2 hybridized (violet) – Has one unhybridized p orbital (red) C O H Unshared pairs of electrons on oxygen in sp2 orbitals C=O double bond is one bond (sp2 – sp2) one bond (p – p) sp2—sp2 overlap to form C—O bond Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 55 VB Theory Bonding in Ethyne, C2H2 H C C H Each carbon is sp hybridized (violet) Has two unhybridized p orbitals, px and py (red) CC triple bond one bond sp – sp two bonds px – px py – py Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 56 VB Theory Ex: Bonding in N2 Each nitrogen NN triple bond one bond sp hybridized (violet) Has two unhybridized p orbitals, px and py (red) sp – sp two bonds px – px py – py Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 57 Problem Set B 2. 3. 4. 5. What is the hybridization of oxygen in OCl2? For the species and XeF4O, determine the following: a. electron domain geometry (geometry including non-bonding pairs) b. molecular geometry c. Hybridization around central atom d. Polarity How many and bonds are there in CH2CHCHCH2, and what is the hybridization around the carbon atoms? Draw & list the bonding orbitals for HCN. 58 Problem Set B 2. sp3 3. XeF4O: octahedral, square pyramid, sp3d2, polar 3. 9, 2, sp2 3. HCN: C will be create a σ bond to H and N with sp2 hybridized orbitals and use 2 p orbitals to participate in 2 π bonds with N. N will participate in the σ bond with C with an sp2 hybridized orbital, the other will hold the N lone pair, and then N will use 2 p orbitals to π bond with C. 59 MO Theory Molecular Orbital Theory Molecular Orbital Theory Views molecule as collection of positively charged nuclei having a set of molecular orbitals that are filled with electrons (similar to filling atomic orbitals with electrons) Doesn't worry about how atoms come together to form molecule 1. 2. 3. Molecular orbitals are associated with entire molecule as opposed to one atom Allows us to accurately predict magnetic properties of molecules Energies of molecular orbitals determined by combining electron waves of atomic orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 60 MO Theory Bonding Molecular Orbitals • Come from various combinations of atomic orbital wave functions • For H2, two 1s wave functions, one from each atom, combine to make two molecular orbital wave functions 1sA + 1sB Combined Bonding MO + Constructive interference of waves Energy of bonding MO lower than atomic orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 61 MO Theory * Antibonding Molecular Orbitals • Number of atomic orbitals used must equal number of molecular orbitals • Other possible combination of two 1s orbitals: 1sA – 1sB + • • Destructive interference of the 1s waves Energy of the bonding molecular orbital is higher than energy of parent atomic orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 62 MO Theory Summary of MO from 1s Atomic Orbital • Bonding molecular orbital – Electron density builds up between nuclei – Electrons in bonding MOs tend to stabilize molecule • Antibonding molecular orbital – Cancellation of electron waves reduces electron density between nuclei – Electrons in antibonding MOs tend to destabilize molecule Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 63 MO Theory MO Diagram for H2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 64 MO Theory Rules for Filling in MO Energy Diagrams 1. Electrons fill lowest-energy orbitals that are available – Aufbau principle applies 2. No more than two electrons, with spin paired, can occupy any orbital – Pauli exclusion principle applies 3. Electrons spread out as much as possible, with spins unpaired, over orbitals of same energy – Hund’s rules apply Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 65 MO Theory Bond Order • Measure of number of electron pairs shared between two atoms (number of bonding e – ) - (number of antibonding e – ) Bond order = 2 electrons/bond • H2 bond order = 1 • A bond order of 1 corresponds to a single bond Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 66 MO Theory MO Diagram for He2 • Four electrons, so both and * molecular orbitals are filled • Bond order 2-2 Bond order = =0 2 • There is no net bonding • He2 does not form Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 67 MO Theory Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 2p Molecular Orbitals 68 MO Theory 2nd Row Periodic Table MO Diagrams Li2 N2 2p Lower in energy than 2p O2, F2 and Higher 2p Lower in energy than 2p Can ignore filled 1s bonding & antibonding and focus on valence electrons Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 69 MO Theory MO Diagram for Li2 2p Lower in Energy than 2p Diamagnetic as no unpaired spins * s2pz * , p*2py p2px Bond order = (2 – 0)/2 =1 2p s2pz p2px, p 2py 2p * s2s Li 2s Li2 s2s Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 2sLi 70 MO Theory MO Energy Diagram for F2 2p Lower in Energy than 2p F electron configuration = [He]2s22p5 Diamagnetic as no Bond order = (8 – 6)/2 =1 2p F – F single bond stable molecule * s2pz unpaired spins * , p*2py p2px p2px, p 2py 2p s2pz * s2s 2s F s2s Jesperson, Brady, Hyslop. Chemistry: The F2 of Matter, 6E Molecular Nature 2s F 71 MO Theory Heteronuclear Diatomic Molecules • If Li through N 2p below 2p • If O, F and higher atomic number, then 2p below 2p Example – BC both are to left of N • so 2p below 2p – OF both are to right of N • so 2p below 2p – What about NF? • Each one away from O so average is O and 2p below 2p Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 72 MO Theory B-C and N-F * s2pz * , p*2py p2px * s2pz * , p*2py p2px s2pz p2px, p 2py 2p lower p2px, p 2py s2pz * s2s * s2s s2s s2s 2p lower BC NF Number of valence e = 3 + 4 = 7 Bond Order = (5 – 2)/2 = 1.5 Number of valence e = 5 + 7 = 12 Jesperson, Brady, Hyslop. Chemistry: Bond The Molecular Nature of Matter, 6E Order = (8 – 4)/2 = 2 73 MO Theory N-O * s2pz * , p*2py p2px • • • • • Bond Order for NO tricky N predicts 2p lower O predicts 2p lower Have to look at experiment Shows that 2p is lower p2px, p 2py s2pz * s2s 2p lower s2s Number of valence e = 5 + 6 = 11 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E Bond Order = (8 – 3)/2 = 2.5 74 MO Theory • Same diagram • Different number of e– • NO+ has 11 – 1 = 10 valence e Bond order = (8 – 2)/2 = 3 • NO has 11 + 1 = 12 valence e Bond order = (8 – 4)/2 = 2 N-O+ and N-O– NO+ * s2pz * , p*2py p2px NO * s2pz * , p*2py p2px p2px, p 2py s2pz p2px, p 2py s2pz * s2s * s2s s2s s2s Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 75 Relative Stability of N-O, N-O+ and N-O– MO Theory • Recall that as bond order increases, bond length decreases, and bond energy increases Molecule or ion NO+ Bond Order 3 Bond Length (pm) 106 Bond Energy (kJ/mol) 1025 NO NO 2.5 2 115 130 630 400 So NO+ is most stable form Highest bond order, shortest and strongest bond Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 76 Problem Set C 6. What is the MO Energy Diagram for B2? How many unpaired electrons does B2 have? 7. What is the bond order & number of unpaired electrons in O , O+ , and O- ? 2 2 2 8. Draw the MO Energy Diagram for BN. 77 Bonding VB vs MO Theory • Neither VB or MO theory is entirely correct – Neither explains all aspects of bonding – Each has its strengths and weaknesses • MO theory correctly predicts unpaired electrons in O2 while Lewis structures do not • MO theory is a difficult because even simple molecules have complex energy level diagrams • MO theory is a difficult because molecules with three or more atoms require extensive calculations Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 78 Bonding VB vs MO Theory Successes of MO Theory • MO theory is particularly successful in explaining paramagnetism of B2 and O2 – One electron each in 2px and 2py (for B2) – One electron each in *2px and *2py (for O2) • • • • Successes of VB Theory Based on simple Lewis structures and related geometric figures Three dimensional structures based on electron domains without massive calculations Simple hybrid orbitals invoked where experimental evidence shows the need Integer bond orders are often correct Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 79 Resonance VB Theory Treatment of Resonance • Formate anion, HCOO– • C has three electron domains (all bonding pairs) so – sp2 hybridized; trigonal planar • Each O has three electron domains (one bonding pair and two lone pairs) – so sp2 hybridized; trigonal planar Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 80 Resonance VB Theory Treatment of Resonance • Have two resonance structures • Have lone pair on each O atom in unhybridized p orbitals as well as empty p orbital on C • Lewis theory says – Lone pair on one O – Use lone pair of other O to form (pi) bond – Must have two Lewis structures Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 81 Resonance MO Theory Treatment of Resonance Bonding MO delocalized over all three atoms This is also our resonance hybrid picture This is the best view of what actually occurs and can be obtained from both VB and MO theory Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 82 Resonance • • • • MO / VB Theory Treatment of Resonance: Benzene Six C atoms, each sp2 hybridized (3 bonds) Each C also have one unhybridized p orbital (6 total) So six MOs, 3 bonding and three antibonding So three bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 83 Resonance • • • • • • MO / VB Theory Treatment of Resonance: Benzene Can write benzene as two resonance structures But actual structure is composite of these two Electrons are delocalized Have three pairs of electrons delocalized over six C atoms Extra stability is resonance energy Functionally, resonance and delocalization energy are the same thing Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 84
