Chem 104A, UC, Berkeley
Main Group Chemistry
MT Ch. 8
Ref:
Huheey, Keiter & Keiter: Ch 16-18
Chem 104A, UC, Berkeley
Periodic Trends
Generally, atoms with same outer-orbital structure appear
in the same Column.
1
Chem 104A, UC, Berkeley
Group 1: Alkali Metal
Li, Na, K, Rb, Cs, Fr
symbol
Li
Na
K
Rb
Cs
Fr
lithium
sodium
potassium
rubidium
cesium
francium
electron configuration
[He]2s1
[Ne]3s1
[Ar]4s1
[Kr]5s1
[Xe]6s1
[Rn]7s1
Chem 104A, UC, Berkeley
Atomic
Number
Relative Atomic
Melting Point/K Density/kg m-3
Mass
Li
3
6.94
453.7
534
Na
11
22.99
371.0
971
K
19
39.10
336.8
862
Rb
37
85.47
312.2
1532
Cs
55
132.91
301.6
1873
2
Chem 104A, UC, Berkeley
Atomic Radius/nm
Ionic Radius/nm
Li
0.152
0.068
Na
0.185
0.098
K
0.227
0.133
Rb
0.247
0.148
Cs
0.265
0.167
Chem 104A, UC, Berkeley
Ionization Energies/kJ mol-1
1st
2nd
3rd
Li
513.3
7298.0
11814.8
Na
495.8
4562.4
6912.0
K
418.8
3051.4
4411.0
Rb
403.0
2632.0
3900.0
Cs
375.7
2420.0
3400.0
3
Chem 104A, UC, Berkeley
The Solvated Electron
A( NH 3 )  A ( NH 3 )  e  ( NH 3 )
Solvated electron in cavity of 3-3.4 Ǻ diameter
Density of Liquid decreases.
Chem 104A, UC, Berkeley
Charles Pederson
Dupont, 1960s
1987, Nobel Prize
New field: Host-guest chemistry
4
Chem 104A, UC, Berkeley
0.31 nm
Chem 104A, UC, Berkeley
Cation
Ionic diameter
Crown Ether
Hole size
Lithium
1.46
12-crown-4
1.5
Sodium
2.28
15-crown-5
2.3
Potassium
3.04
18-crown-6
3.1
Rubidium
3.4
21-crown-7
3.4
Cesium
3.9
24-crown-8
4.0
5
Chem 104A, UC, Berkeley
Cryptand
2,2,2-crypt = c222
Chem 104A, UC, Berkeley
Electron-pair trapping centers
and channels in K+(cryptand[2.2.2])e-
6
Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
MeLi is better called (CH3Li)4, as it is tetrameric.
methyl
Li
7
Chem 104A, UC, Berkeley
Group 2 Alkaline Earth Metal
Be, Mg, Ca, Sr, Ba, Ra
Chem 104A, UC, Berkeley
8
Chem 104A, UC, Berkeley
Completed d and f shells intervene
Less Effective shielding
Stronger Attraction
Chem 104A, UC, Berkeley
9
Chem 104A, UC, Berkeley
Ih
Total: 50 e
12 B-H bond, 24 e
26 e for skeleton B-B bonds
Projection Operator Method: MO
Exact 13 B-B bonding MOs
Chem 104A, UC, Berkeley
36 e per B12:
26 for skeleton B-B
10 e for linking B12 units
6 2c-2e bonds
6 3c-2e bonds
10
Chem 104A, UC, Berkeley
MO picture for 3c-2e bond
Chem 104A, UC, Berkeley
Borane
Alfred Stock
B2H6
11
Chem 104A, UC, Berkeley
Boron Hydrides




These form one of the most
C has 4 valence e,
structurally diverse series of
H H
compounds.
H has 1, so C2H6
H
C
C
H
has enough electrons
Simplest is diborane, B2H6.
(8+6) for 7 2c2e
Similar formula to ethane,
H H
bonds.
but structurally very
different because it is
electron deficient.
B2H6 only has
H 6+6=12 electrons.
Gets around the problem by H
H
forming delocalized bonds.
B H B
This makes an
H ethane-like
structure impossible
H
Chem 104A, UC, Berkeley
BH3



B2H6 is a dimer of
boron trihydride.
This is a fugitive
species, present in low
concentration in
diborane at high T.
Important in
mechanisms of
reactions of B2H6 at
high T.
H
H
H
H
B
H
H
B H B
H
H
12
Chem 104A, UC, Berkeley
Bonding in Diborane




The B-H-B unit is held
together by 2e.
This is called a 3 centre - 2
electron bond (3c2e).
The orbital basis can be
made up of two sp3 hybrids
of the B atoms and two
H(1s) orbitals.
The remaining boron
orbitals form normal 2c2e
bonds to the terminal H’s.
H
H
H
B H B
H
H
H
B
B
H
Chem 104A, UC, Berkeley
3c2e Bonds in Diborane

The two electrons
occupy the fully
bonding combination,
so that the overall bond
order between the B
and the bridging H is
1/2.
13
Chem 104A, UC, Berkeley
3c2e Bonds


3c2e bonds are
occasionally shown in
structural diagrams like
this:
Bond Energies:
BH 381 kJ/mol
BHB 441 kJ/mol
H
H
H
B
B
H
H
H
1.19 Å
H
H
H
H
B H B
1.32 Å H
Chem 104A, UC, Berkeley
Electron Deficiency




All boranes are electron
deficient.
The need to form 3c2e
bonds (BHB and BBB)
causes the molecules to
‘curl-in’ on themselves.
The more electron
deficient the more
‘spherical’ a molecule
becomes.
For example [B6H6]2- is
more electron deficient
than B4H10
14
Chem 104A, UC, Berkeley
Electron Counting




Just how electron deficient
a borane is can be derived
by counting the number of
skeletal pairs of electrons.
Each HB has 4 valence
electrons. One pairs used
for a 2c2e bond (e.g a
terminal BH).
The remaining 2e are used
for delocalized cluster
bonding.
Any remaining H contribute
1e to the cluster

[B6H6]2



write as (BH)62Each BH unit
contributes 2e
Plus the 2- charge
gives 14 electrons
6 boron atoms in the
cluster bonded with 7
pairs (6+1).
Chem 104A, UC, Berkeley
Electron Counting




Just how electron deficient
a borane is can be derived
by counting the number of
skeletal pairs of electrons.
Each HB has 4 valence
electrons. One pairs used
for a 2c2e bond (e.g a
terminal BH).
The remaining 2e are used
for delocalized cluster
bonding.
Any remaining H contribute
1e to the cluster

B4H10:





Write as (BH)4H6
Each BH => 2e (8e in
all)
Each additional H gives
1e (6e in all)
Total number of
electrons = 14
4 Borons in cluster
bonded by 7 pairs of
electrons (4+3).
15
Chem 104A, UC, Berkeley
Electron Counting




Just how electron deficient
a borane is can be derived
by counting the number of
skeletal pairs of electrons.
Each HB has 4 valence
electrons. One pairs used
for a 2c2e bond (e.g a
terminal BH).
The remaining 2e are used
for delocalized cluster
bonding.
Any remaining H contribute
1e to the cluster

B5H9:
(BH)5H4
 10 + 4 = 14 electrons
 5 Boron atoms
bonded by 7 electron
pairs (5+2).
 In terms of electron
deficiency
B6H62- > B5H9 > B4H10
 All have 7 e pairs for
skeletal bonding (ie cluster
bonding).

Chem 104A, UC, Berkeley
Wade’s Rules
6+1
n+1
Closo
5+2
n+2
Nido
4+3
n+3
Arachno
16
Chem 104A, UC, Berkeley
Closo –[BnHn]2-
Nido –[BnHn]4-
Arachno –[BnHn]6-
n=4-12
Closed n-vertex
Polyhedral
n=4-11
“nest” n+1 vertex
Polyhedral
n=4-10
“web” n+2 vertex
Polyhedral
2n+2 B-B electrons
Missing one vertex
Missing 2 vertices
2n+4 B-B electrons
2n+6 B-B electrons
Chem 104A, UC, Berkeley
17
Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
18
Chem 104A, UC, Berkeley
For a regular polyhedron having n vertices,
there will be
n+1 bonding molecular orbitals.
Chem 104A, UC, Berkeley
7 bonding MOs
19
Molecular Orbitals of closo-B6H62- (OChem
104A, UC, Berkeley
h)
Oh
E
8C3
6C2
6C4
3C2
i
6S4
8S6
3h
6d
r()
6
0
0
2
2
0
0
0
4
2
H
H
B
r() = A1g + Eg + T1u; orbitals of these symmetries suitable for -bonding can be
formed by six s or six pz atomic orbitals (two sets of six “radial” orbitals result)
2-
B
B H
H B
B
H
B
r()
12
0
0
0
-4
0
0
0
0
0
H
r() = T1g + T2g + T1u + T2u ; orbitals of these symmetries suitable for B-B -bonding
can be formed by six px and six py orbitals (twelve “tangential” orbitals)
d
S4, C4, C2
C2
z
x1
h
y1
S 6, C 3
d
x
basis set for -bonding;
vectors x and y are in
h planes
y
basis set for -bonding
Chem 104A, UC, Berkeley
Character table for Oh point group
20
Chem 104A, UC, Berkeley
Molecular Orbitals of closo-B6H62-. “Radial” group orbitals
6B 2pz symmetry adapted atomic orbitals
6H and 6B 2s symmetry adapted atomic orbitals
eg
eg
t1u
3a1g(2s+2pz-1s)
1a1g(2s+2pz+1s)
2a1g(2s-2pz)
a1g(2s+2pz)
t1u
+
a1g
a1g
(2pz)
3a1g
(2s)
2a1g
a1g
(1s)
a1g
1a1g
Note that only one of the six 2pz boron group orbitals, namely a1g, is bonding
Six 2s and six 2pz boron group orbitals will mix to form two sets of radial orbitals.
One of these two six-orbital sets will be used to combine with six 1s hydrogen group orbitals
to form six bonding and 6 antibonding MO’s (B-H bonds)
Chem 104A, UC,
Berkeley
Molecular Orbitals of closo-B6H62-. “Tangential”
group
orbitals
• Remaining twelve 2px and 2py boron orbitals form four sets of triply degenerate
“tangential” group orbitals of t1g, t2g, t1u and t2u symmetry.
• Only two of these sets , t2g and t1u, are suitable for B-B -bonding in closo-B6H62-. They
form six -bonding MO’s (B-B -bonds).
Bonding and antibonding 6B 2py and 2px symmetry adapted group orbitals
...
t1u
t2u
...
t2g
t1g
21
Chem 104A, UC, Berkeley
B-B and B-H bonding MO’s of closo-B6H62

closo-B6H62- has 7 core bonding orbitals, 6 of them are - (t1u & t2g) and one is -MO (a1g).
In boron cages of the formula closo-(BH)x (x = 5, … 12) the optimum number of the core
electron pairs is x+1 (all bonding orbitals are filled). That explains enhanced stability of
dianionic species closo-(BH)x2-.
1.9 eV
-1.1 eV
t2g
2t1u
eg
B6-core -orbitals
B6-core -orbitals
-4.4 eV
-5.0 eV
-7.3 eV
2a1g
1t1u
BH bond orbitals
BH bond orbital
BH bond orbitals
-15.3 eV
1a1g
B6-core -orbital
Chem 104A, UC, Berkeley
22
Energy not to scale
Chem 104A, UC, Berkeley
Bonding and antibonding 6B 2py and 2px symmetry adapted group orbitals
...
t2u
t1u
...
t1g
t2g
6B 2pz symmetry adapted atomic orbitals
eg
t1u
a1g
6H and 6B 2s symmetry adapted atomic orbitals
6H and 6B 2s symmetry adapted atomic orbitals
eg
eg
t2g
2t1u
B6-core -orbitals
B6-core -orbitals
-4.4 eV
eg
-5.0 eV
-7.3 eV
2a1g
1t1u
BH bond orbitals
BH bond orbital
BH bond orbitals
-15.3 eV
1a1g
B6-core -orbital
1.9 eV
-1.1 eV
t1u
t1u
a1g
a1g
B6
[B6H6]2-
H6
Chem 104A, UC, Berkeley
23
Chem 104A, UC, Berkeley
For a regular polyhedron having n vertices,
there will be
n+1 bonding molecular orbitals.
Chem 104A, UC, Berkeley
Closo –[BnHn]2-
Nido –[BnHn]4-
Arachno –[BnHn]6-
N=4-12
Closed n-vertex
Polyhedral
N=4-11
“nest” n+1 vertex
Polyhedral
N=4-10
“web” n+2 vertex
Polyhedral
2n+2 B-B electrons
Missing one vertex
Missing 2 vertices
2n+4 B-B electrons
2n+6 B-B electrons
24
Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
25
Chem 104A, UC, Berkeley
Wade’s Rules: Example 1

B6H10





(BH)6H4
12 + 4 = 16e = 8 pairs
8 pairs = 6B + 2
Nido cluster
Remove one vertex from
7-vertex polyhedron.
Chem 104A, UC, Berkeley
Wade’s Rules: Example 2

B5H11





(BH)5H6
10 + 6 = 16 = 8 pairs
5 B atoms, 8 pairs
n+3 arachno cluster
based on seven vertex
polyhedraon via removal
of two vertices.
26
Chem 104A, UC, Berkeley
Zintl ions
First in 1891
Na
Pb(s) --------------
4Na+ + [Pb9]4NH3 (l)
Many such ions were made in 1930s
Structures established after cryptand ligands enabled crsytallization (J. Corbett)
[Pb9
]4-
222-Crypt
+Pb --------
2[Pb5]2NH3(l)
in [Na(C222)]2[Pb5]
Chem 104A, UC, Berkeley
Wade’s Rules: Example 3
Isolobal B-H & Sn, Pb


[Sn9]4- Zintl ions
Each Sn has a lone
pair and contributes 2e
to cluster bonding,



18 + 4 = 22 e
9 atoms, 11 pairs
Nido cluster, remove 1
vertex from 10 vertex
polyhedron.
Bi-capped
square antiprism
27
Chem 104A, UC, Berkeley
Wade’s Rules: Example 4

[Pb5]2




Pb has 1 lone pair
2e/Pb for cluster
bonding
10 + 2 = 12e
5 Pb, 6 pairs
Closo structure
Chem 104A, UC, Berkeley
Synthesis of Boranes: Diborane






Hf = +80 kJ/mol, so direct combination of B
and H is not possible.
2NaBH4 + I2  B2H6 + 2NaI + H2
2NaBH4 + 2H3PO4  B2H6 + 2NaH2PO4 + 2H2
4BF3 + 3LiAlH4  2B2H6 + 3LiAlF4
Air and moisture must be rigorously excluded:
diborane is highly pyrophoric!
Boranes burn with a characteristic green flash
(decay of excited state of BO)
28
Chem 104A, UC, Berkeley
Higher Boranes


Made by controlled pyrolysis of B2H6
Highly specific and not at all predictable.
B2H6
160-200°C
slow hot tube
pyrolysis
B10H14
80°C/200 atm/5hr
B4H10
H2/200-240°C/rapid hot tube pyrolysis
B5H9
Chem 104A, UC, Berkeley
29
Chem 104A, UC, Berkeley
Typical Reactions 1: Lewis Base Cleavage



Boranes are electron
deficient.
Lewis bases add
electrons
Small boranes may
cleave:
H B H B H
H
H
H
NMe3
H
H B
H
Me
N Me
Me
Chem 104A, UC, Berkeley
Reactions of B2H6with Bases
[BH2(NH3)2]+[BH4]NH3
B2H6
H-
NMe3
H3BNMe3
CO
H3BCO
BH4-
30
Chem 104A, UC, Berkeley
Wade’s Rules: Example 5




Heteroatoms:
B10C2H12
BH contribute 2e
CH contribute 3e





(BH)10(CH)2
20 + 6 = 26 e
12 atoms in cluster
13 pairs
Closo 12-vertex
polyhedron
Chem 104A, UC, Berkeley
31
Chem 104A, UC, Berkeley
1,2-dicarba-closo-dodecaborane
ortho
Chem 104A, UC, Berkeley
1,7-dicarba-closo-dodecaborane
meta
32
Chem 104A, UC, Berkeley
1,12-dicarba-closo-dodecaborane
para
Chem 104A, UC, Berkeley
33
Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
34
Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
[C2B9H11]2-
Cp 
cyclopentadienide
35
Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
36
Chem 104A, UC, Berkeley
MgB2, superconductor, Tc=39 K
Chem 104A, UC, Berkeley
CaB6
37
Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
Noble Gas Chemistry
He, Ne, Ar, Kr, Xe
Inert, not discovered until 1800’s by Sir William Ramsay.
Prof. Neil Bartlett (Berkeley, Chemistry), 1962, chemistry of PtF6
O2  PtF6  O2 [ PtF6 ]
O2  O2  e 
H  1175kJ / mol
IE Xe  1169kJ / mol
Xe  PtF6  Xe  [ PtF6 ]
38
Chem 104A, UC, Berkeley
History of Noble Gas Compounds
1962, Bartlett and Lohmann:
• demonstrated the great oxidizing strength of PtF6 in
producing O2+PtF6• IP(Xe) ≈ IP(O2)
Xe
+
PtF6
RT
XePtF6 + Xe(PtF 6)2
- dependent on reactant ratio
- red-tinged yellow solid
Graham, L.; Graudejus, O.; Jha, N. K.; Bartlett, N. Concerning the nature of XePtF6. Coord. Chem. Rev. 2000, 197, 321-334.
Chem 104A, UC, Berkeley
Molecular Orbital Theory

MO Theory does not involve outer orbitals


Example: Xe uses 5p (5s less important)


Too much energy is required to excite e- to these
orbitals to fill them so bonding can occur
F uses 2p
So for XeF2 have three three-center MOs
39
Chem 104A, UC, Berkeley
XeF2

Three AOs goes to three MOs.



Xe 5px and 2 F 2px
Best overlap occurs when is centrosymmetric or D∞ h
symmetry (choose them to be on x-axis)
Xe contributes 2e- (1 to each), each F contributes
1e-
Chem 104A, UC, Berkeley
MO Diagram
Anti-Bonding
Non-Bonding
Bonding

Net bond order of 1
40
Chem 104A, UC, Berkeley
VSEPR




This theory implies outer orbital involvement in
the bonding
Each bond between ligand and central atom
involves an electron pair
All non-bonding valence electrons have a steric
effect
MO theory proves to be just as effective as
VSEPR for less than 6 coordinate complexes

VSEPR correctly predicts XeF6 as non-octahedral
Chem 104A, UC, Berkeley
VSEPR cont.
41
Chem 104A, UC, Berkeley
VSEPR oxides
Chem 104A, UC, Berkeley
XeF2
• first prepared 1962
• colorless as solid, liquid, or gas
• homogeneous reaction
Xe
+
F2
cat. HF
F
Xe
F
electric discharge, heat,
UV light, sunlight
• thermal heterogeneous reaction using solid NiF2
• production favored with low F pressures and high temp
42
XeF2
Chem 104A, UC, Berkeley
• large crystals at RT
• body-centered tetragonal
• strong interactions between XeF2
molecules (high ∆Hsub)
• -0.5F-Xe+1-F-0.5
• packing structure distances F
from equatorial nonbonding
electrons on Xe
F
Xe
Unit cell
F
Zemva, B. Noble Gases: Inorganic Chemistry. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; John Wiley & Sons:
New York, 1994; pp 2660-2680.
Chem 104A, UC, Berkeley
XeF4
• first noble gas binary fluoride synthesized
F
Xe
1
+
:
F2
5
673 K
closed nickel can
tot pressure 0.6 MPa
F
Xe
F
F
• colorless as crystals, liquid, or vapor
• strong oxidative fluorinator, but has high kinetic inertness
like XeF2
43
XeF4
Chem 104A, UC, Berkeley
• square planar in gas phase
• nearly square planar as a solid
• strong electrostatic interactions between molecules in
solid
Molecular packing, projection down b axis
Zemva, B. Noble Gases: Inorganic Chemistry. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; John Wiley & Sons:
New York, 1994; pp 2660-2680.
Chem 104A, UC, Berkeley
Xenon Oxides
• XeO3
• colorless, hygroscopic, detonatable solid
XeF6 (g)
+
3 H2O (l)
low temp
6 HF (aq)
+
XeO3 (aq)
• XeO4
• pale yellow solid
• unstable
• tetrahedral in gas phase
• great oxidizing agent
• gas phase XeO
44
Chem 104A, UC, Berkeley
Xenon Oxyfluorides
• all possible Xe(IV), Xe(VI) oxyfluorides are known
• XeOF2 (light-yellow solid)
• XeOF4 (colorless, liquid at RT, most thermally stable
compound with a Xe-O bond)
O
F
F Xe
F
F
C4v
• almost all possible Xe(VIII) oxyfluorides are known
• XeO2F4
Chem 104A, UC, Berkeley
The Amazing [AuXe4]2+






Seidel and Seppelt: 2000, Goal: AuF
AuF3 + HF/SbF5
 dark red solution
-78°C : AuXe42+ (Sb2F11-)2
Bond = 272.8 – 275.1 pm
Stable up to -40°C
Raman: 129 cm-1 Au-Xe
45
Chem 104A, UC, Berkeley
Krypton Compounds

Krypton Difluoride


Krypton Oxide



First synthesized by Turner and Pimentel in 1963.
KrF2 hydrolized by moist air to KrO.
Unstable and decomposes explosively.
Krypton (II) Compounds


Cationic salts, KrF+ / Kr2F3+
Molecular adducts of KrF2
Chem 104A, UC, Berkeley
KrF2

Characteristics




Thermodynamically unstable
Colorless as solid or gas
Decomposes at above 250 K
Methods of synthesis



Electric discharge, near-UV light, frequency
discharge, thermal decomposition, or sunlight
Low temperature synthesis (~77 K)
Most efficient method yields 1 g/h
46
Chem 104A, UC, Berkeley
KrF2



Lowest average bond
energy of any fluoride
compound.
D∞h symmetry
Unit Cell


Molecules aligned perp.
Places negatively charged
F atoms close to positively
charged krypton atoms.
Zemva, B. Noble Gases: Inorganic Chemistry. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; John Wiley & Sons:
New York, 1994; pp 2660-2680.
Chem 104A, UC, Berkeley
HArF





Räsänen and co-workers, 2000.
Neutral covalent molecule (ArH+)(F-)
Stable at low temperatures in a matrix
Elimination of HF calculated to be a 8
kcal/mol barrier.
Possibility of ArF+ salt complexes existing

Anions need to have high ionization potentials
and be poor fluoride donors.
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