Greenwood & Earnshaw
2 nd Edition
Chapter 6
Boron
Elemental Boron
Many complex structures, little tendency to form
B=B or B=C, strong affinity for oxygen, fluorine.
Strong resemblance to silicon. Unreactive to air in bulk, but unoxidized powders pyrophoric.
Four valence orbitals but only three valence electrons. Adept at multi-center bonding, coordination numbers of 1-8 known. Catenates extensively by two- and three-center bonding and multi-center bonding.
Reacts with oxygen, nitrogen, carbon at high temperatures producing intractable mixtures.
Boron Rich Borides - Cubic MB
6
Group 1 metals – Borides are insulators, the metal electrons are localized in the B
6 octahedra.
Group 2 metals – Borides are semi-conductors,
B
6 octahedra are just satisfied.
Group 3, lanthanide & early transition metals are metallic conductors, additional metal electrons go into delocalized metallic bonding.
The later, smaller lanthanide metals (Ho, Er, Tm,
Lu) are too small for the 24-coordinate metal site and form the tetragonal MB
4
, chains of B
6
– B
2
.
1

The Boron Hydrides
closo – Closed B n
“deltahedra” – B n
H n
2-
nido – “nest” , closo minus one apical boron,
B n
H n+4
, B n
H n+3
, B n
H n+2
2-
arachno – “spider web”, closo minus a B-B,
B n
H n+6
, B n
H n+5
, B n
H n+4
2-
hypho - closo minus a B
3 deltahedra, rare.
B n
H n+8
, examples B
8
H
16
, B
10
H
18
conjuncto – B-B bonded combinations of the above structures, same or mixed.
The Boron Hydrides – Bonding Elements
Ψ
3
B B B B
Ψ
3
Ψ
2
B
H
B
Ψ
1
2B
B B
B
H
B
H
Ψ
2
Ψ
1
B
B
B
2B
B B
B
B
B
B
Also: B
H
H
Bonding Topology – The STYX Numbers
S – The number of B-H-B 3c-2e bonds.
T – The number of closed B
3
3c-2e bonds.
Y – The number of B-B 2c-2e bonds.
X – The number of BH
2 groups; 2 2c-2e bonds.
B
2
H
6
(2002)
H
H
B
H
H
B
H
H
B
5
H
9
(4120)
H
B
H
H
B
H
B
H
H
B
H
H
B
H
2

Bonding in the closo-Hydridoboranes, B
6
H
6
2-
H sigma bonding
(2-)
B
H
H
B B strongly bonding a
1g lowest energy MO
H
B
B
H
B
H exocyclic B-H bonding weakly antibonding t
1u a'
1g + t'
1u + e' g e g strongly antibonding
Bonding in the closo-Hydridoboranes, B
6
H
6
2-
H
(2-)
Molecular Orbital Diagram
*t
1g B
H
H
*t
2u
B B
*e g exocyclic set of 6 MO's
12
ππππ
B B
H
B
H
H the two t1u MO's can mix since they have the same symmetry
*t
1u t
1u
LUMO
HOMO
6
σσσσ t
2g
Pi Bonding a
1g t
2g bonding t
1u t
2u antibonding t
1g
Relationship of closo- to nido-
Hydridoboranes
H
H
B
B
H
B
B
H
B
H closo
-B
6
H
6
2-
(2-)
B
H
-BH 2+
+4 H+
H
B
H
H
B
B
H
H
B
H H
B
H
H
nido-
B
5
H
9
3

Relationship of nido- to arachno-
Hydridoboranes
H
B
H
H
B
B
H
H
B
H
H
B
H
H
-BH
2+
+2 H+
H
H
B
H
H
B
H
H
B
B
H
H
H
H
nido-
B
5
H
9
arachno-
B
4
H
10
Chemistry of Pentaborane-9; nido-B
5
H
9
H I
H
B
2-I-
H
B
B
H
H
B
H H
nido-
nido-
B
5
B
H
H
5
8
H
9
B
B
H
H
I
2
/ AlCl
3
N
N
N
H
H
B
B
H
H
B
H H
B
H
I
N
H
H
B
H
B B
H
H H
B B
H H
1-I-
nido-
B
5
H
8
Electrophilic methods cause apical substitution.
Nucleophilic methods and rearrangement using bases cause equatorial substitution.
Chemistry of Pentaborane-9; nido-B
5
H
9
H
B
H
H
H
B
H
B
H
nidoB
5
H
9
B
H
H
B
H
H
H
H
3
C
N
3
C
CH
2
CH
2
N
CH
3
CH
3
"hard" base attack
B
H
H
H
H
B
H
B
P(CH
3
)
3
2 P(CH
3
)
3
"Soft" base attack, not a strong nucleophile
H
H
B
B
B
A hypho-borane adduct results.
H
H
H
B
B
B
H
H
H
3
C CH
3
N
CH
2
B
H
H
H
3
C
CH
2
N
CH
3
P(CH
3
)
3
H
H H
4

Chemistry of Pentaborane-9; nido-B
5
H
9
H
H
B
H
B
B B
H
H
H
H NaH B B
H H
H
H
B B
H B
B
H H H
H H
nidoB
5
H
9
LiH
H
A fluxional anion, equatorial hydrogens & bridge-hydrogens are equivalent on nmr timescale.
Anion behaves as a formal
2e donor ligand & synthon.
The
µµµµ
-H's are more acidic than terminal
H's; the apical H's are the most hydridic.
H
B
B
B
H
H
H Li
B
B
H
H H
Cluster Expansion Reactions
H H
H
B
H
B
H
H
B
H
1/2 B
2
H
6
Et
2
O -78 C
HCl
H
H
H
B
B
H
H
B
H
B
H
B
H
H
B
B
H
B
H
ClP(CH
3
)
2
Li H P(CH
3
)
2
H
B B
H H H
Me
2
O
-78 C
B
2
H
6
H
LiBH
4
B
H
H
B
H
H
B
H
B
H
H arachno-
B
6
H
12
H
B
B
B
H
H
H
B nido-
H B
6
H
10
B
H
H
Boron Halides
BF
3
, BCl
3
, BBr
3
, BI
3 are all monomeric. Mixtures will scramble halogens by bridging intermediates.
BX
4
will not scramble unless BX
3 is present.
B-F bond is much shorter than the single bond covalent radii predict. Dative
ππππ delocalization of the fluorine lone pairs to the vacant p orbital of boron. Effect is much reduced for Cl, Br, and I.
Dative
ππππ bonds occur for oxygen and nitrogen.
BX
3
, X = halogen are “hard” acids. BH
3 is a
“soft”acid forming stronger adducts with P, As, S...
5

Boron Subhalides
B
2
X
4 are planar in the solid state, only B
2
F remains planar in the gas phase. The formation of an “extended
ππππ system” is more
4 important than steric effects for the small F.
120 o
F
132 pm
F
B B
F
172 pm
F
120 o
Cl
173 pm
Cl
B B
Cl
175 pm
Cl
170 pm
120 o
Cl
B B
Cl
Cl
Cl
175 pm
Boron Subhalides – Clusters
B
4
Cl
4
– Regular tetrahedron;
B n
X n
; n = 8-12 X = Cl; n = 7-10 X = Br. All are
closo- and hyper electron deficient.
Cl
Cl
B
B
Cl
B
170 pm
171 pm
B Cl
Boron
Chlorine
B
9
Cl
9 a tricapped trigonal prism
Boron - Oxygen – Boric Acid – A Lewis Acid
HO B
OH
+ 2 H
2
O
OH
H
3
O
+
+
HO
HO
B
OH
(-) pK a
= 9
OH
HO B
OH
OH
+ 2 CH
2
CH
2
OH OH
H
2
O
H
3
O
+
+
CH
2
O
CH
2
O
(-)
B
OCH
2 pK a
= 5
OCH
2
A chelating alcohol, glycol, or sugar can increase the acidity by 10 4 .
6

Boron – Oxygen : Borates & Silicates
B(OH)
3
Heat
[B(O)OH] x
H
2
O
+ network polymers
Compare with:
Si(OH)
4
Heat
[SiO n
(OH)
4-2n
] x
H
2
O
+ network polymers
Bonding Elements:
O 3-
B O d = 120 pm
O
B
O d = 128 to 143 pm may be planar or 3D
O
O
B
O
O d = 143 to 155 pm tetrahedral, T d
5-
H
H
Aminoboranes – Alkene Mimics
H
B N
H
H
H
C C
H
H
B
N
N
B
B
N C
C
C
C
C
C
Borazine Organometallics
B
N
N
B
B
N C
C
C
C
C
C
H
3
C
H
H
3
C
N
B
3
C
O
N
C
Cr
C
B
CH
3
N CH
3
B
CH
3
C
O
O
H
3
C
H
H
3
3
C
C
O
C
Cr
C
O
C
CH
3
CH
O
3
CH
Crystal structures are quite similar, but the borazine complex is slightly puckered in solution. The hexaethylborazine shows puckering in the crystal.
3
7