Drawing Good Lewis Structures Molecular Shapechemistry.bd.psu.edu/jircitano/413ch5.pdf ·...
Transcript of Drawing Good Lewis Structures Molecular Shapechemistry.bd.psu.edu/jircitano/413ch5.pdf ·...
1
Always.
Drawing Good Lewis Structures
1. # valence e– in atoms (± charge) must = # e– in structure.
2. Determine connectivity: least EN usually central, avoid small
rings, H always terminal (1 e–).
6. Minimize formal charge (# and distribution of + and –).
4. Remove required e– in pairs from central atom.
5. Move e– pairs from outside atoms to bond with central atom
to complete octet again.
3. Complete octet for each atom (except H); check against #1.
Valence-Shell Electron-Pair Repulsion
VSEPR
regions of e– ρ around central atom repelled as far as possible
Molecular Shape
1957
Ronald J. Gillespie
English
Ronald S. Nyholm
Australian
regions of e ρ around central atom repelled as far as possible
•• = = =
linear trigonal planar tetrahedral trigonalbipyramidal
octahedral
x x x xx
5 6180o 90o 90o 90o 90o
2 3 4
linear
180o
bent120o
trigonal planar
120o
bent109.5o
trigonal pyramidal tetrahedral
109.5o
VSEPR and Deviations
linear T-Shaped see-saw trigonal bipyramidal
120o
square planar square pyamidal octahedral
90o
deviate from ideal when lone-pair involved
bonding-bonding < Lp-bonding < Lp-Lp
0
–2
H2 Molecule
ergy
(eV
)
i t
1
23
0.5
–4
r (Å)1.0
En experiment
Overlap Symmetry
crosssection
crosssection
s p
dSσ > Sπ > Sδ
Valence Bond Treatment: CH4
½(s + px + py + pz)
½(s + px – py – pz)
½(s – px + py – pz)
½(s – px – py + pz)H
4 sp3 orbitals
sp3
2s
2p
weighted average E
E
C
HH
H
2
sp2 Hybrid Orbitals
sp2
trigonal planar
√3
1s +
√3
√2px
√3
1s –
√6
1px +
√2
1py
√3
1s –
√6
1px –
√2
1py
sp2
2s
2pE
2p
sp Hybrid Orbitals
sp
linear
√2
1(s + pz)
√2
1(s – pz)
sp2s
2pE
2p
Other Hybrid Orbitals
s – px + py
s + px
s – px – py
pz + dz2
pz – dz2
s – px – dz2
s + px + dz2
s + pz – dz2 – dx
2– y
2
s + py – dz2 – dx
2– y
2
s – px – dz2 + dx
2– y
2
s – p – d 2 – d 2 2
d2sp3dsp3
axial
equatorial
s – py – dz2 – dx
2– y
2
trigonalbipyramidal
octahedral
Hybrid Orbitals and Bond Strength
bond strength S
sp > sp2 > sp3
SC-C
SC-H
3 2
0.4
0.5
0.6
0.7
0.8
0.9
S
s character S
25% s33% s50% s
sp3 sp2 sp
0.30 20 40 60 80 100
% s character
Multiple Bonds
Multiple bonds from π (and δ) overlap.
ClO3–
O Cl O
O
•••• ••
•• ••
•••• – Cl has low E d orbitals
ClO
O
Osp3
sp2
sp2
sp2
Non-VSEPR Molecule
N(SiH3)3
D3h not C3v
Si
SiSi
SiN
more bonds, lower E
sp2 N
low E d on SiSi Si
N
3
MO Treatment H2
E
gy
no e– density between nuclei – antibonding (u)
1s
E = E
1s 1s
E
En
erg
H2 lower energy
than 2 H by 2 x E.
e– density between nuclei – bonding (g)
1s
MO Treatment H2
1s
Bond Order = (# of bonding e– – # of antibonding e–)/2
gy
1s 1s
1s
En
erg
Bond Order = (2 – 0)/2 = 1
MO Treatment He2
1s
Bond Order = (# of bonding e– – # of antibonding e–)/2
gy
1s 1s
1s
En
erg
Bond Order = (2 – 2)/2 = 0
No energy advantage: He2 does not exist.
–
Molecular Orbitals
π*2p g–
S depends on E and symmetry: SAB > 0, bonding: E stabilizedSAB < 0, antibonding: E destabilizedSAB = 0, nonbonding: no stabilization
Sσ > Sπ > Sδ
+
–
+
+
–
π2p u
*2p u
2p g
*1s u
1s g
+
s-p Energy Separation in First Row Elements
B C N O F
En
ergy 2s
2s
2p2p
2p2p
2p
Complicated by s-p mixing
when s and p close in E.
Changes relative MO E’s.
B C N O FB C N O F
2s
2s
2s
E (eV) 5.7 8.8 12.4 16.5 21.6
En
ergy
*2p2p
*2s2s
Homonuclear Diatomic MO Diagram
2px 2py 2pz 2pz 2py 2px
*2p
*2p
2p
2s 2s
2p
2p
*2s
2s
Complicated by s-p mixing
when s and p are closer in
E (early elements).
4
Homonuclear Diatomic MO Diagram
6u
2g
5g2px 2py 2pz 2pz 2py 2px
No longer named after
AO. Numbered and
symmetry (u or g) given.
1u
2s 2s
g
4u
3g
MO Diagram: Li2, Be2, B2, C2, N2
6u
2g
5gLi
2px 2py 2pz 2pz 2py 2px
1u
2s 2s
g
4u
3g
Li2
bond order
Li Li
MO Diagram: Li2, Be2, B2, C2, N2
6u
2g
5gB
2px 2py 2pz 2pz 2py 2px
1u
2s 2s
4u
3g
Be2
bond order
Be Be
MO Diagram: Li2, Be2, B2, C2, N2
6u
2g
5gB
2px 2py 2pz 2pz 2py 2px
1u
2s 2s
g
4u
3g
B2
bond order
B B
MO Diagram: Li2, Be2, B2, C2, N2
6u
2g
5gC
2px 2py 2pz 2pz 2py 2px
1u
2s 2s
g
4u
3g
C2
bond order
C C
MO Diagram: Li2, Be2, B2, C2, N2
6u
2g
5gN
2px 2py 2pz 2pz 2py 2px
1u
2s 2s
g
4u
3g
N2
bond order
N N
5
MO Diagram: O2, F2, Ne2
*2p
*2p
2pO
2px 2py 2pz 2pz 2py 2px
2s 2s
2p
2p
*2s
2s
O2
bond order
O O
MO Diagram: O2, F2, Ne2
*2p
*2p
2pF
2px 2py 2pz 2pz 2py 2px
2s 2s
2p
2p
*2s
2s
F2
bond order
F F
MO Diagram: O2, F2, Ne2
*2p
*2p
2pN
2px 2py 2pz 2pz 2py 2px
2s 2s
2p
2p
*2s
2s
Ne2
bond order
Ne Ne
Bond Length 1/Bond Order
*2p
*2p
2p
Superoxide dismutase (SOD)
O +
order length, pm
2p
2p
*2s
2s
O O
O2+
O2
O2–
O22–
x2–y2 z2xzyzxy z2 xz yz x2–y2xy
Cr2: Bonds
bond order*z2
*xz, yz
*s
*x2–y2, xy
Cr2
y yy y yy
4s3d
4s3d
z2
xz, yz
s
x2–y2, xy
z2
xz, yz
x2–y2, xy
Cr Cr
2s
nb
2p
2p
Carbon Monoxide MO Diagram
2s
2s
nbC O
6
b1*
a1*
b22p
A1
B1
H2O
O
HH
O
HH
O
2a1
2s
b2
b1
1a1
p
O 2 H
A1
A1 B1 B2
O
HH
O
HH
HH
O
HH(~98% O)
CH4 SALC
A1= H1 1s + H2 1s + H3 1s + H4 1s
T = H1 1s – H2 1s + H3 1s – H4 1s
T = H1 1s – H2 1s – H3 1s + H4 1s
T = H1 1s + H2 1s – H3 1s – H4 1s
bonding orbitals
A1 T
BH
H
H
BH
H
H H: A1g= 1s
B SALC: A1g= sp3 + sp3
B2u= sp3 – sp3
D2h
Diborane, B2H6: 3-Center, 2 e– Bond
H
B, B
A1g
B2unb
A1g
3 0
3.5
4.0
4.5
y BrCl
F
Electronegativity: Periodic Property
Fr
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20 40 60 80 100Atomic Number
Ele
ctro
neg
ativ
ity
At
Cs
I
Rb
Br
KNaLi
70
95
120
y (M
J/m
ole)
NeFO
Cl
Total Energy
–5
20
45
–1 0 1 2 3
Oxidation State
Tot
al E
ner
gy
E = q + q2
E = IE or EA
q = ionic charge
where
6
8
10
12
14
tal E
ner
gy
Cl
N
Mulliken-Jaffe Electronegativity
Cl larger than Na
–4
–2
2
4
0 1–1
Charge
Tot Na