chem101/3, wi2010 po 13‐1 chem101/3, wi2010 po 13‐2 General
CB VI
VB Theory = combination of ideas from
AO, Lewis & VSEPR theories
Valence Bond (VB)
Theory
• identifies AO’s ( & associated e–’s)
involved in bonding
• overlapping orbitals, occupied by e–’s
responsible for bonding
(incl. AO Hybridization)
• e–’s are “localized” in these bonding orbitals
(rather then spread over the molecule as whole)
Ref
11: 1 - 2
Prob
FUP: 11: 1, 2
• in CHEM101/3 use qual. approach
• VB concepts/language widely used;
esp. idea of AO hybridization
E of C: 11: 5, 6, 11 - 18
Adv Rdg 11: 3
chem101/3, wi2010 po 13‐3 chem101/3, wi2010 po 13‐4 chem101/3, wi2010 po 13‐5 chem101/3, wi2010 po 13‐6 chem101/3, wi2010 po 13‐7 chem101/3, wi2010 po 13‐8 Hybridization of AO’s
Hybridization Process
• concept needed to explain
pure AO’s
bonding geometry of most molecules
various combinations
• hybrid AO’s formed
hybrid AO’s
by various combination of pure AO’s
(after all they are waves, can interact)
• produces orbitals that
overlap more
increasing bond strength, stability
optimizing occupation of 3D space
rule:
# of hybrid orbitals =
# of originating, “pure” AO’s
chem101/3, wi2010 po 13‐9 po 13‐10 Pet.Fig.11.9 sp hybridization
Common Hybridization Cases
original AO’s
chem101/3, wi2010 hybrid AO’s
1.) s, p
sp, sp
(each is ½ s & ½ p)
linear geometry
e.g., BeCl2, see Pet.Fig. 11.9
has (sp,p)σ bond
sp2, sp2, sp2
2.) s, p, p
(each is
1
3
s &
2
3 p)
trigonal geometry
e.g., BH3, see Pet.Fig. 11.8
has (sp2,s) σ bond
chem101/3, wi2010 po 13‐11 2
Pet.Fig. 11.8 sp hybridization
chem101/3, wi2010 (blank by design)
po 13‐12 chem101/3, wi2010 po 13‐13 chem101/3, wi2010 po 13‐14 chem101/3, wi2010 po 13‐16 common …
3.) s, p, p, p
sp3, sp3, sp3, sp3
(each is ¼ s & ¾ p)
tetrahedral geometry
e.g., CH4, see Pet.Fig. 11.5, 11.6
has (sp3,s) σ bond
4.) s, p, p, p, d
sp3d, sp3d, sp3d, sp3d, sp3d
trigonal bipyramidal geometry
e.g., PCl5, see Pet.Fig. 11.11
has (sp3d,p) σ bond
5.) s, p, p, p, d, d
(sp3d2) x 6
octahedral geometry
e.g., SF6, see Pet.Fig. 11.11
has (sp3d2,p) σ bond
chem101/3, wi2010 po 13‐15 chem101/3, wi2010 po 13‐17 Formation of π bonds
chem101/3, wi2010 po 13‐18 Pet.Fig. 11.13 Double bond formation
(involving sp2 and sp hybridized atoms)
• sp2 and sp hybridized atoms still have 1 or 2 “unused’’ p orbitals
• these can get involved in π bond formation
if the neighbor atom also has a p orbital.
Ex.1
ethylene, C2H4 has Lewis structure:
H
H
C
H
C
H
each C has three sp2 orbitals and one p orbital
sp2 orbitals form σ bonds w/ s orbitals of H
and sp2 orbital of other C;
p orbitals form π bond
resulting in the following bonds:
C–H
(sp2,s)σ
(sp2,sp2)σ
C–C
(p,p)π
Note: two non-identical bonds in C,C double bond
chem101/3, wi2010 po 13‐19 Pet.Fig. 11.15 Triple bond formation
Ex.2
acetylene, C2H2, Lewis structure
H C C H
each C has two sp orbitals and two p orbitals,
sp orbitals form σ bonds w/ s orbitals of H
and w/ sp orbital of other C,
p orbitals form π bonds
resulting in the following bonds:
C–H
(sp,s)σ
(sp,sp)σ
C–C
chem101/3, wi2010 (p,p)π
(p,p)π
Note: one σ and two π bonds in C,C triple bond
po 13‐20 chem101/3, wi2010 po 13‐21 chem101/3, wi2010 Correlation between VSEPR & VB
po 13‐22 Practice Examples
A.) Bonding in NH3
1. Lewis
e– arrangement
acc. to VSEPR
H
hybridization
N
H
H
octahedral
3 2
sp d
2. VSEPR: 4 e– groups
3
trigonal pyramidal
sp d
tetrahedral
sp3
tetrahedral e– arrangement
3. VB: sp3 hybridization
2
trigonal planar
sp
linear
sp
lone pair
bonded pair
see also Pet. Fig. 11.7
chem101/3, wi2010 Pet. Fig. 11.7 Bonding in NH3
po 13‐23 chem101/3, wi2010 po 13‐24 Practice …
B. Bonding in XeF4
F
1. Lewis: Nt = 36, No = 32
4 extra e–’s go on Xe
2. VSEPR predicts geometry
F
Xe
F
octahedral e– arrangement
square planar atom arrangement
F
F
Xe
F
F
3. VB: sp3d2 hybridization on Xe,
four sp3d2 orbitals involved in σ bonding
w/ p orbitals of F atoms,
two sp3d2 orbitals occupied by lone pairs
F
chem101/3, wi2010 po 13‐25 Hybridization in Terminal Atoms
chem101/3, wi2010 po 13‐26 Review of Bond Terminology
Order, Length, Strength
somewhat controversial
• order (B.O.)= # of bonds
previously:
1 = single
VSEPR concepts were applied to terminal atoms
2
e.g., O is sp hybridized in CO2
2 = double
3 = triple
(note: can have fractional numbers, like 1.5 )
now:
• length = distance between nuclei of bonded atoms
no hybridization on terminal atoms;
length↓ as B.O.↑
C,O bond consists of (sp,p)σ and
(p,p)π bond
• strength ≈ “bond energy” (BE)
(p,p)π
O
C
strength↑ as B.O.↑
O
(sp,p)σ
chem101/3, wi2010 Pet. Table 10.2 Bond Lengths
see Pet.Tables 10.2 and 10.3 for detailed values
po 13‐27 chem101/3, wi2010 Pet. Table 10.3 Bond Energies
po 13‐28 chem101/3, wi2010 po 13‐29 chem101/3, wi2010 po 13‐30 Some Extras/Repeats
Bonds in Resonance Hybrids
Condensed Structures
property (such as strength, length, order, ... )
H
- CH3
H
C
is approx. weighted avg. of contributing structures.
H
means
- CH2CH3
–
Ex. nitrate, NO3 , exists as resonance hybrid
H
H
C
C
H
H
H
Resonance Structures & Hybrids
O
O
O
O
O
O
N
N
N
O
O
O
H
O
C
H
C
H
O
C
H
H
C
H
resonance hybrid
all 3 structures contribute equally
therefore, B.O. between N and any one O:
2 +1 +1
B.O. for any N,O bond =
= 1.33
3
Bond length & strength could be estimated from
this by consulting tables using interpolation.
(bond length closer to N–N than N=N, ~130 pm)
resonance structure
major
Constitutional Isomers
H
O
C
H
Once again:
all N,O bonds are identical,
they are the same type
minor
C
H
H
H
H
,
have different connectivities
C
H
O
C
H