Covalent Bonding: Orbitals. Chapter 9

Transcription

1 Covalent Bonding: Orbitals Chapter 9

2 Atomic Orbital (Valence Bond) Approach Advantages of Lewis Dot structures 1. Predict geometries 2. Predict polarities of molecules Disadvantages 1. No information about energies of electrons 2. No information about orbitals used in bonding. Valence bond approach is helpful for these. (Basic idea in valence bond)

3 Atomic Orbital (Valence Bond) Approach A covalent bond is formed from a pair of electrons with opposite spins in overlapped atomic orbitals. Bond formed from half-filled valence orbitals. Atom 1H 1s Atom 9 F 1s 2s 2p Molecule H 2 H H 1s Molecule HF 9 F H 1s 2s 2p

4 Atomic Orbital (Valence Bond) Approach He, Ne Filled orbitals do not form bonds He 1s Ne 1s 2s 2p

5 Atomic Orbital (Valence Bond) Approach Be, B, C Problems with Valence Bond Approach: Be (no bonds?) 1s 2s 2p B (1 bond?) 1s 2s 2p C ( 2 bonds?) 1s 2s 2p

6 Atomic Orbital (Valence Bond) Approach C sp 3 To explain bonding for atoms like these it is assumed electrons are promoted and hybrid orbitals are formed. C 1s 2s 2p bonds form hybrid

7 Atomic Orbital (Valence Bond) Approach CH 4

8 Atomic Orbital (Valence Bond) Approach B sp 2 To explain bonding for atoms like these it is assumed electrons are promoted and hybrid orbitals are formed. B bonds form hybrid 1s 2s 2p

9 Atomic Orbital (Valence Bond) Approach sp 2, p

10 Atomic Orbital (Valence Bond) Approach Be - sp To explain bonding for atoms like these it is assumed electrons are promoted and hybrid orbitals are formed. Be bonds form hybrid 1s 2s 2p

11 Atomic Orbital (Valence Bond) Approach Be, B, C Summary SUMMARY Be 1s 2s 2p hybridized un-hybridized orbitals orbitals? B 1s 2s C 1s 2s 2p 2p

12 Atomic Orbital (Valence Bond) Approach Hybrid orbitals have new properties that are different from the orbitals used to form them. Shape energy

13 Atomic Orbital (Valence Bond) Approach table 1 The geometries are the same as predicted from electron repulsion theory. # electron groups Hybridization Geometry angle

14 Atomic Orbital (Valence Bond) Approach - table 2a # electron groups # non-bonded pairs Hybridization Geometry angle 3 0 sp 2 Planar triangular sp 3 Tetrahedral 1 2

15 Atomic Orbital (Valence Bond) Approach table 2b # electron groups # non-bonded pairs Hybridization Geometry angle 5 0 dsp 3 Trigonal bipyramidal d 2 sp 3 octahedron 1 2

16 Atomic Orbital (Valence Bond) Approach XeF 4

17 Atomic Orbital (Valence Bond) Approach sigma & pi Two types of Covalent Bonds: 1. Sigma bonds - form from hybrid orbitals. They have the e- density symmetrical with bond axis. 2. Pi bonds - form from unhybridized p-orbitals They have the e- density parallel but outside bond axis.

18 Atomic Orbital (Valence Bond) Approach sigma & pi Single bonds have sigma bond(s) pi bond(s) Double bonds have sigma bond(s) pi bond(s) Triple bonds have sigma bond(s) pi bond(s)

19 Atomic Orbital (Valence Bond) Approach CO 2 Double bonds in CO 2

20 Atomic Orbital (Valence Bond) Approach - ethene

21 Valence Bond Approach N2

22 Atomic Orbital (Valence Bond) Approach summary hydbridization HYBRIDIZATION IN MULTIPLE BONDS: The extra electron pairs in multiple bonds (1 extra pair in double, 2 extra pairs in triple) are NOT hybridized. TO DETERMINE AMOUNT OF HYBRIDIZATION OF ATOM: Hybridize enough orbitals to contain: all unshared electron pairs electron pairs to form single bonds one and only one pair in multiple bonds

23 Molecular Orbitals Results of Valence Bond approach to bonding & Lewis Dot Structures Weakness: Inability to predict the correct magnetic properties, O 2 & B 2 Need for resonance to handle special problems Gives no direct information on bond energies Reason - Assumed electrons stayed in atomic orbitals of the individual atoms.

24 Molecular Orbitals Another approach - Linear combination of atomic orbitals to give molecular orbital L.C.A.O. = M.O. BASIC ASSUMPTION OF MOLECULAR ORBITAL APPROACH Orbitals are properties of the molecule not the atoms.

25 Molecular Orbital Model Main Ideas MAIN IDEAS OF MOLECULAR ORBITAL MODEL: 1. Same number of molecular orbitals as the # atomic orbitals that were combined 2. Molecular orbitals can hold two e- with opposite spins 3. square of molecular orbital function indicates e- probability 4. Important properties of orbitals: size, shape, and energy (See fig for shape and size) 5. Molecular orbital configurations can be written much like e- configurations for atoms

26 Molecular Orbital Model H 2 MAIN IDEAS OF MOLECULAR ORBITAL MODEL: 1. Same number of molecular orbitals as the # atomic orbitals that were combined 2. Molecular orbitals can hold two e- with opposite spins Ex. Hydrogen 1s A σ1s* 1s B σ1s

27 Molecular Orbital Model H 2 Ex. Hydrogen σ1s* 1s A 1s B σ1s The orbitals described above are both sigma (σ) molecular orbitals bonding molecular orbital -(σ1s) antibonding molecular orbital -(σ1s*)

28 Molecular Orbital Model H 2 MAIN IDEAS OF MOLECULAR ORBITAL MODEL: 5. Molecular orbital configurations can be written much like e- configurations for atoms Ex. Hydrogen ex. H 2 : σ1s* 1s A 1s B σ1s

29 Molecular Orbital Model H 2 - Example: Predict the molecular orbital configuration in H 2- using the diagram below σ1s* 1s A 1s B ex. H 2- : a) Is this ion stable (does it have lower energy that its separated parts)? b) How do you expect this bond strength to compare to H 2? σ1s

30 Molecular Orbital Model Bond Order Bond Order - the difference between the number of bonding e- and the number of antibonding e- divided by two Bond Order = (# bonding e- ) - (# antibonding e-) 2 Calculate the Bond order for H 2 and H 2 - Bond order is an indication of The larger the bond order the the bond

31 Molecular Orbital Model Bond Order He 2 Ex. Predict the bond order and stability of He 2

32 Molecular Orbital Model Bond Order Li 2 and Be 2 Bonding in Homonuclear Diatomic Molecules: Homonuclear diatomic molecule Ex. Predict the Bond Order and stability of Li 2 and Be 2 σ2s* 2s A 2s B σ2s* 2s A 2s B σ2s σ2s

33 Molecular Orbital Model Li 2 NOTE: In order to participate in molecular orbitals, atomic orbitals must overlap.

34 Molecular Orbital Model π vs. σ Pi (π) molecular orbitals - (see Fig for shapes of p and s bonding p-orbitals) How would you expect the p orbitals to compare in energy to the s orbitals? π σ

35 Molecular Orbital Model expected MO E Expected MO E diagram:

36 Molecular Orbital Model B2

37 Molecular Orbital Model B2

38 Molecular Orbital Model Expected MO E diagram: What is the molecular orbital configuration for σ2p* σ2p* π2p* π2p* π2p* π2p* π2p π2p π2p π2p σ2p σ2p σ2s* σ2s* σ2s a) F 2? b) B 2? σ2s

39 Molecular Orbital Model - Paramagnetism Paramagnetic - Diamagnetic

40 Molecular Orbital Model - Paramagnetism The expected E level diagram needs to be modified slightly to account for the magnetic properties of B 2. This change results from p-s mixing, thus the π2p and σ2p orbitals are reversed Because the importance of p-s mixing becomes less important across the period, the π2p and σ2p orbitals revert to the order expected in absence of p-s mixing for O 2 and F 2.

41 Molecular Orbital Model - Paramagnetism This change results from p-s mixing, thus the π2p and σ2p orbitals are reversed Because the importance of p-s mixing becomes less important across the period, the π2p and σ2p orbitals revert to the order expected in absence of p-s mixing for O 2 and F 2.

42 Bonding in Heteronuclear Diatomic Molecules - heteronuclear diatomic molecule - When two atoms are near each other in the periodic table, we can use the MO diagram for homonuclear molecules. Ex. NO (like N 2 ) CN -

43 Bonding in Heteronuclear Diatomic Molecules - When the two atoms are different, a new diagram must be used. Ex. HF

44 Bonding in Heteronuclear Diatomic Molecules - Because 2p is lower in energy than the hydrogen 1s orbital, the electrons prefer to be closer to the fluorine atom.

45 Combining Localized Electron & Molecular Orbital Models - In molecules that require resonance, the s bond is localized while the p bonding is delocalized. Ex. NO 3- and C 6 H 6

46 Combining Localized Electron & Molecular Orbital Models -

47 Combining Localized Electron & Molecular Orbital Models -