Chapter 9 Molecular Geometry & Bonding Theories I .

Chapter 9Molecular Geometry & Bonding TheoriesI) Molecular Geometry (Shapes)Chemical reactivity of moleculesdepends on the nature of thebonds between the atoms as wellon its 3D structureMolecular GeometryArrangement or positions ofatoms relative to each otherBond AnglesAngles made by lines joiningthe nuclei of atoms bonded1

A) Basic ABn Arrangements2

Various molecular shapes can arisefrom the 5 basic ABn shapes.3

II) VSEPR TheoryValence-Shell Elecron-Pair Repulsione! pair: lone pair e! or bonding e!(single, double & triplebonds treated same)- really consideringregions of e! density (domains)VSEPR:e! pairs arrangethemselves as far apartas possible to minimizerepulsions between them- controls geometryaround central atom4

A) Types of Geometry1) Electron-Domain Geom.arrangement of bonding and!nonbonding e pairs (domains)about the central atom2) Molecular Geom. (Shapes)arrangement of bonded atomsabout the central atomdescribed using ONLY the ATOMSDistinction is very important!5

Electron-Domain Geom6

ED and MG for AB2, AB3 & AB4 EDs7

!B) 2 e Pairs8

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Why is the bond angle not exactly 120 ?Lone-pair e! (nbe) not trapped betweentwo atoms and thus spread out and takeup more space. Repulses bonding pairsand reduces the bond angles.11

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ED and MG for AB5 & AB6 EDs15

!E) 5 e Pairs DomainsTwo “different” bonds.3 equatorial bonds forminga trigonal planar arrangementw. 120 angles2 axial bonds which are perpendicularto the trigonal planar equatorialbonds (90 angles)16

4 Molecular Geometries1) trigonal bipyramidalAngles:120 & 90 2) seesawAngles:- 120 & - 90 3) T-shapedAngles:- 90 4) linearAngle:180 17

!a) Lone-pair e & Bonding PairsIn 2, 3 and 4:lpe! wind up in the equatorialpositions to maximize separationand reduce repulsions.In 2 & 3 lpe! pushes bonding pairscloser together and reduces angles18

!F) 6 e Pair DomainsOctahedral structure19

3 Molecular Geometries1) octahedralAngles:90 2) square pyramidalAngles:- 90 3) square planarAngles:90 20

G) Shapes of Larger MoleculesSame rules apply to individual atomsin larger molecules.21

III) Molecular Shape and PolarityMUST have polar bondsMUST consider shapeIf the centers of and – charges donot coincide, the molecule is polar.A) Diatomic MoleculesA diatomic molecule w. apolar bond is polar* HCl*!22

B) Polyatomic MoleculesFor polyatomic moleculesgeometry is very important inpredicting if the centers of and – charges coincide.The dipole moment isfor the entire moleculevector sum of ALL of theindividual bond dipole moments.23

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5) PCl526

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IV) Covalent Bonding and Orbital OverlapWave Interference:e! behave like any other wave &when 2 waves meet they can interactconstructively or destructively.Constructive interference:waves add together andget a bonding orbitalDestructive interference:waves subtract from each otherand get an antibonding orbital28

A) Sigma (F) Bondse! density concentrated betweennuclei along the internuclear axisResults from overlap of 2 “s” orb.,“s” & “p” orb., 2 “p” orb. end-to-end,“s” & hybrid orb., 2 hybrid orb (end on)s s Fs p Fp p F29

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A) Pi (B) Bondse! density above and belowinternuclear axisResults from sideways overlapof parallel p orbitals31

V) Hybrid Orbitals - Valence Bond TheoryBonds are created by orbital overlapto produce F or B bondsTo explain many observed moleculargeometries, pure “s” and “p” atomicorbitals are combined to produce a setof “hybrid” orbitals on atoms.These hybrid orbitals then form bondsbetween atoms producing the correctgeometry.32

A) sp Hybrid OrbitalsBeF2 linear with 2 single bondsBe atom:[He] ¼¿2s2pShould not form bonds- no singly occupied orbitalsAs it forms bonds it can absorbenough energy to “promote” one2s e! to a 2p orbital.[He] ¼2s¼2p33

The s and p orbitals then mix or“hybridize” to form two degeneratesp hybrid orbitals.These sp hybrid orbitals have twolobes like a p orbital.One of the lobes is larger and morerounded as is the s orbital.34

These two degenerate orbitals alignthemselves 180 from each other:linearConsistent with the observedgeometry of Be compounds.35

2B) sp Hybrid OrbitalsBF3:trigonal planar, 120 36

3C) sp Hybrid OrbitalsCH4:tetrahedral, 109.5 37

D) Hybrid Orbitals - Summary38

VI) Multiple BondsOverlap of hybrid orbitals with s or por other hybrid orbitals (end-to-end):F bonds.e! density is symmetric about theinternuclear axis of F bond, groups canrotate about the bond without breaking it.- free rotation about F bondsSingle bonds are F bondsHHHHHC CHHC CHHH HH39

Multiple bonding requires B bondsA) Double BondsLook at ethylene: C2H4F bonds between C and H and bothC atoms using sp2 hybrid orbitalsleaves ”p” orbitals on each C whichcan overlap sideways to form B bonds40

Trigonal planar around each C atom- whole molecule is planarB bond is perpendicular to planeNo free rotation between C atomsDouble bond / 1 F 1 B41

B) Triple BondsLook at acetylene: C2H2F bonds between C and H and bothC atoms using sp hybrid orbitalsleaves 2 sets of ”p” orbitals on eachC which can overlap sideways toform 2 sets of B bonds42

Linear around each C atomTriple bond / 1 F 2 B43

C) Resonance & Delocalized BondingLocalized F and B bondscan’t explain resonance.Instead can think of atomsforming delocalized B bonding.Benzene:Each C atom is sp2 hybridized andhas 1 atomic p orbital left over- form a delocalized B bond44

VII) Molecular OrbitalsSome things not explained by VB theoryIn MO theory orbitals are constructed ascombination of AOs from ALL atomsinthe molecule.The MO can span more than 2 atoms.Each MO can still only contain 2 e!In VB theory orbitals are mixedon individual atoms 1st thenbonded together as neededIn MO theory the orbitals of all atomsmix and are then used to form the lowestenergy molecular orbitals.45

Chapter 9 Molecular Geometry & Bonding Theories I) Molecular Geometry (Shapes) Chemical reactivity of molecules depends on the nature of the bonds between the atoms as well on its 3D structure Molecular Geometry Arrangement or positions of atoms relative to each other Bond Angles Angles made by lines joining the nuclei of atoms bonded