Part 1. Reading: Gray: (4-1), (4-2), and (4-4) OGN: (16.2)

Transcription

1 Part 1 Reading: Gray: (4-1), (4-2), and (4-4) OGN: (16.2)

2 The story so far: MO-LCAO works great for diatomic molecules! But... What about other numbers of atoms? Will MO-LCAO work for polyatomic molecules?

3 Let s try 2 We know: 2 is a linear molecule, by VSEPR. Bond angle: 180 The electron configuration of 2 is : 2 2 : 1 2p

4 Make MO s: First bond and to form σ* π n σ b σ b 2p Next bond and to form 2 σ* σ* σ* π n π n σ b σ b 2 σ b The MO s for for 2 are complicated and hard to to work with.

5 Attempt to make a graphical model of 2 : : 2 2 : 1 : 1? Are any of these models reasonable?

6 Attempt to make a graphical model of 2 : : 2 2 : 1 : 1? These two are no good: One orbital can t hold four electrons

7 Attempt to make a graphical model of 2 : : 2 2 : 1 : 1 90? This one is no good: According to VSEPR, the bond angle should be 180.

8 Attempt to make a graphical model of 2 : : 2 2 : 1 : 1? Unfortunately, none of these models make sense.

9 hybridized the orbitals... Maybe if we blended the orbitals together... 2 sp hybrids

10 ybridization in terms of electron configuration: Unhybridized 2p 2p y 2p x Decouple electron pair

11 ybridization in terms of electron configuration: 2p p hybrids Create hybrid orbitals ybridized p z p z- 2p y 2p x

12 Bonding with a ybrid Orbital: The sp hybrids act individually to form MO s with each hydrogen atom: σ b Ψ E Ψ E Ψ E

13 Bonding with a ybrid Orbital: The sp hybrids act individually to form MO s with each hydrogen atom: σ * Ψ E Ψ E Node Ψ E

14 Let s try 2 : : 2 2 : 1 : 1 sp ybridization σ* σ b p p σ* σ b p p Success!

15 Rules for ybridization Example: 2 1) Assign Geometry Using VSEPR Theory 2 SN = 2 Geometry: Linear 2) Write electronic configuration of the atom to be hybridized : () 2 () 2 (2p) 0 3) Draw energy diagram for said atom and decouple paired electrons 2p 2p Sp ybrids

16 Rules for ybridization Example: 2 4) Take linear combinations of the atomic orbitals participating in the bond to make hybrid orbitals 5) Combine hybrid orbitals with other atom s orbitals using diatomic MO theory

17 Make B 3 : We know: The geometry, from VSEPR: B B Trigonal Planar Bond angle: 120 The electron configurations: B: 2 2 2p 1 : 1 2p

18 Make B 3 : What does this look like in terms of orbitals? Possibilities: 2p x p x 90 2p y 2p y WRONG: These shapes don t match those predicted by VSEPR.

19 Use what we learned about hybrids: B 3 1) Assign Geometry Using VSEPR: B 3 SN = 3 Geometry: Trigonal planar 2) Write electron configuration of atom to be hybridized: B: 2 2 2p 1 3) Draw energy diagram for the atom and decouple paired electrons: 2p 2p sp 2 ybrids

20 4) Take linear combinations of the atomic orbitals participating in the bond to make hybrid orbitals: 2p y B We know that if we start with three orbitals,we must finish with three orbitals. Thus, they will be 120 apart: 120 The orbitals are centered at the node between the green and red lobes, so the orbitals overlap.

21 4) Take linear combinations of the atomic orbitals participating in the bond to make hybrid orbitals: 2p y B 1/ 3 2/ 6 1/ 3-1/ 6 1/ 2 1/ 3-1/ 6-1/ 2

22 4 ) Take linear combinations of the atomic orbitals participating in the bond to make hybrid orbitals: sp 2 sp 2 B 2p y 5) Combine hybrid orbitals with other atoms orbitals using diatomic MO theory: σ* σ b B p 2 B sp 2 sp 2 sp 2 sp 2 B p 2 σ* σ b B 3 B p 2 σ* σ b

23 Make C 4 : We know: The geometry, from VSEPR: C C Bond angle: Tetrahedral The electron configurations: C: 2 2 2p 2 : 1 2p

24 Make C 4 : What does this look like in terms of orbitals? Possibilities: 90 2p x p y WRONG: This shape doesn t match the one predicted by VSEPR.

25 Use what we learned about hybrids: C 4 1) Assign Geometry Using VSEPR: C 4 SN = 4 Geometry: Tetraheadral 2) Write electron configuration of atom to be hybridized: C: 2 2 2p 2 3) Draw energy diagram for atom and decouple paired electrons: 2p 2p sp 3 ybrids

26 4) Take linear combinations of participating atomic orbitals to make hybrid orbitals: We know that if we start with four orbitals, we must finish with four orbitals. Thus, they will be apart: The orbitals are centered at the node between the green and red lobes, so the orbitals overlap.

27 4) Take linear combinations of participating atomic orbitals to make hybrid orbitals: 1/2 1/2 1/2 1/2 1/2-1/2-1/2 1/2 1/2-1/2 1/2-1/ /2 1/2-1/2-1/2

28 4) Take linear combinations of participating atomic orbitals to make hybrid orbitals: Notice the tetrahedron formed by the sp 3 hybrid orbitals. p This is the shape predicted for C 4 by VSEPR. p 3 p 3 p Exploded view

29 5) Combine hybrid orbitals with other atoms orbitals using diatomic MO theory: σ* C p 3 σ b sp 3 C p 3 σ* σ b σ* sp 3 sp 3 C C p 3 p 3 σ b C 4 All the angles are correct. All the orbitals have the same energy. σ* σ b

30 Make N 3 : We know: It has one lone pair. The geometry, from VSEPR:.. N Lone pair Bond angle: <109.5 The electron configurations: N: 2 2 2p 3 : 1 2p

31 Make N 3 : What does this look like in terms of orbitals? Possibilities: Lone pair in orbital 2p x Lone pair in 2p orbital < p x 2p y 2p y WRONG: The shapes don t match those predicted by VSEPR.

32 Make N 3 : N 3 has a steric number of 4. This suggests we should use sp 3 hybridization 2p sp 3 hybridize p 3 Result: sp 3.. Notice one of the sp 3 orbitals has the lone pair Lone pair sp 3 sp 3 Slightly less than because lone pairs are fat!

33 Make 2 O: We know: The molecule has two lone pairs. The geometry, from VSEPR:.... Lone pairs The electron configurations: O: 2 2 2p 4 : 1 O Bond angle: <109.5 (104.5 ) 2p

34 Make 2 O: What does this look like in terms of orbitals? Possibilities: Lone pair in 2p orbital Lone pair in orbital.... 2p x < 90 > 90.. Lone pair in 2p orbital.. 2p x <135 Lone pair in 2p orbital 2p y 2p y WRONG: These shapes don t match those predicted by VSEPR.

35 Make 2 O: 2 O has a steric number of 4. This suggests we should use sp 3 hybridization 2p sp 3 hybridize p 3 Result: sp 3.. Notice two of the sp 3 orbitals have lone pairs Lone pairs sp 3.. sp 3 Even smaller than because two lone pairs are fatter!

36 End of Part 1 Reading: Gray: (4-1), (4-2), and (4-4) OGN: (16.2)