boranesandcarboranesarun-181103153903.pptx

BORANES AND CARBORANES
&
Polymers containing Phosphorous

CONTENTS Introduction
Boranes
Diborane
Types of boranes
Bonding and structure in boranes
Carboranes
Types of carboranes
Wade’s rule
Applications
Borazines
Boron Nitrides
Polymers containing Phosphorous

INTRODUCTION
The binary compounds of boron and
hydrogen are called boranes. Boranes were first
prepared between 1912 and 1936, by Alfredstock,
who developed vacuum line techniques to handle
these relative materials. Boranes are the name to
class of synthetic hydrates of boron. The molecules of
these compounds are electron deficient and so
reactive with respect to electron donors. Many
boranes synthesised from thermolysis of B2H6 .All
boranes are diamagnetic and colourless.

BORANES
 The parent member BH3 is called borane, is found only in the gaseous state,
and dimerizes to form diborane, B2H6.
 General formula of boranes is BxHy
 The most important boranes are diborane B2H6, pentaborane B5H9, and
decaborane B10H14.
 Boranes are all colourless and diamagnetic, And some are pyrophoric.
 Boranes are highly reactive.
 Common reactions with boranes are: electrophilic substitution, nucleophilic
substitution by Lewis bases, deprotonation by strong bases, cluster building
reactions with borohydrides, and reaction of a nido-borane with an alkyne to

DIBORANE
• Diborane is simplest and most studied of boranes. It is
an electron deficient molecule .
• It has less number of valence electrons than the
number required to form all the electron pair bond in its
structure.2 hydrogen bridges are present.
• The model determined by MOT indicates that the bond
between boron & terminal hydrogen atoms are
conventional 2c-2e electron bonds.
• Having used 2 electrons in bonding to terminal
hydrogen atoms. Each boron has one valence e-
remain for additional bonding. The bridging hydrogen
provides an electron each. Thus B2H2 ring is held
together by 4 electrons.
B
H
B
H
H
H
H
H

 BONDING AND STRUCTURE IN DIBORANE
 The diborane molecule has 2 type of bonds.
• 1. four terminal (2c-2e) B-H bonds.
• 2. two bridged (3c-2e) B-H-B bonds.
• 1. four terminal (2c-2e) B-H bonds.
Each of these bonds are formed by sharing of 2
electrons between boron and terminal hydrogen atoms.
It’s a normal covalent bond.
• 2. two bridged (3c-2e) B-H-B bonds.
Each of these bond is formed by sharing of 2
electrons between 2 B and 1 H atoms .
• It is also called as banana bond

 TYPES OF BORANES
CLOSO BORANES
Closed, triangulated polyhedra structure.
All corners of triangulated polyhedra are
occupied.
General formula: BnHn+2,
Example: [B6H6]2-

 NIDO BORANES
nido boranes are formed when one
corners of the polyhedra is removed.
 nest like, non-closed structure
Example: B5H9, B2H6

 ARACHNO BORANES
 These boranes are formed when two
corners of the polyhedra is removed .
 Web like ,non-closed polyhedra
structure
Example: [B4H10]

 HYPO BORANES
 Hypo- net like
 These have most open clusters in which Boron atoms occupy n corners of(n+3) polyhedron
 General formula: BnHn+8
 these boranes are having complex structures.
 Example : B8H16 and B10H18
 These boranes formed by linking of 2 or more of any other type of boranes.
 These structures are very complex
 General formula : BnHm
 Example: (B5H9)2 , (B4H10)2
CONJUCTO BORANES

They can also be written as having the general formula BnHn2‒, BnHn4‾, BnHn6‾, BnHn8‾, BnHn10‾ for closo,
nido, arachno, hypho and conjuncto respectively.
That is formally subtracting the number of H+ ions from the formula to make the number of boron and
hydrogen atoms equal.

BONDING IN BORANES
 2c-2e bond B. .
H
 3c-2e bond B------H.------.B
B.------.H------B
 In boranes 4 types of bonds are present
H
1. 3c-2e B-H-B B B Bent
2. 3c-2e B-B-B B B B Open
B
B B Closed
3. 2c-2e B-B Normal
4. 2c-2e B-H Normal

 STYX CODES
The over all boranes bonding can be represented by 4 digital numbers
S = number of B-H-B bonds (1st
digit )
T = number of B-B-B bonds (2nd
digit)
Y = number of B-B bond (3rd
digit)
X = number of B-H2 groups (4th
digit)
Ex: B2H6 (Diborane)
S = number of B-H-B bonds = 2
T = number of B-B-B bonds = 0
Y = number of B-B bond = 0
X = number of B-H2 groups = 2
Hence for diborane styx code is 2002.
Example – B4H10 styx code is 4012
B
H
B
H
H
H
H
H

CARBORANES
• Carboranes are mixed hydrides of boron and carbon in which carbon and
boron atoms occupy the vertices of triangulated polyhedron.
• Carboranes are most important heteroboranes.
• Carboranes are member of a class of organometallic compounds containing
carbon (C), boron (B), and hydrogen (H).
• General formula of carboranes is C2BnHn+m, where n is an integer
• carboranes with n ranging from 3 to 10 have been characterized.
• Boranes and carboranes have same number of electrons in their bonding
framework , will have similar structure.

 TYPES OF CARBORANES
CLOSO CARBORANES
 Closed, triangulated polyhedra structure.
 General formula: C2BnHn+2,
 Total number of electron in bonding framework
is (2n+2)e-
. i.e. (n+1) e pairs.
 Example-1,6-C2B8H10

 One corner of triangulated polyhedra is
removed
 General formula: C2BnHn+4,
 Total number of electron in bonding framework
is (2n+4)e-
. i.e. (n+2) e-
pairs.
 Example- C2B4H8, [1,7-C2B9H12]-
 NIDO CARBORANES

 ARACHNO CARBORANES
• Two corners of triangulated polyhedra is removed.
• General formula: C2BnHn+6,
• Total number of electron in bonding framework is
(2n+6)e-
. i.e. (n+4) e-
pairs.
• Example- 1,3-C2B7H13

 WADE’S RULE
In chemistry polyhedral skeletal electron pair theory provides electron
containing rules useful for predicting structures of boranes and carboranes. It is
formulated by Wade and hence it is called as Wade’s rule.
In Wade’s rule
i. Each BH unit donates 2e-
to skeletal structures.
ii. Each CH unit donates 3e-
.
iii. -ve charge on borane gives an electron.
iv. Additional hydrogen atom gives one electrons each.
Types Formula Skeletal Electron
Pairs
Example
Closo BnHn n + 1 [B5H5]2-
Nido BnHn+4 n + 2 B2H6
Arachno BnHn+6 n + 3 B4H10
Hypho BnHn+8 n + 4 None

boranesandcarboranesarun-181103153903.pptx

Important features of Bonding
1. The triangular-faced polyhedra shown in Fig.1 form the basis for the structures of boranes and
carboranes.
2. The skeletal boron or carbon atoms occupy all, all but one, or all but two of the vertices of the
appropriate polyhedron in closo, nido or arachno compounds, respectively.
3. Each skeletal boron or carbon atom has a hydrogen atom (or some other singly bonded ligand)
terminally attached to it by a bond radiating outward (exo) away from the center of the polyhedron to
which a pair of electrons is allocated.
4. The remaining valence shell electrons (b pairs) are regarded as skeletal bonding electrons.
5. Each skeletal boron or carbon is considered to provide three atomic orbitals for skeletal bonding.
6. The symmetries of the polyhedra are such as to generate (n+1) skeletal bonding molecular orbitals
from these atomic orbitals, (where n= number of polyhedron vertices [n=(b-1)].
7. Compounds with a skeletal atoms and b skeletal bond pairs adopt closo structures if b=(a+1), nido
structures if b=(a+2) and arachno structures if b=(a+3).
O Carboranes are compounds with the basic structural unit containing a number of C and B
atoms arranged on the vertices of a triangular polyhedron.
O The structures of carboranes are closely related to those of the isoelectronic boranes.
BH=B- =C; BH2 =BH- = BL =CH
O Structure of carboranes can be predicted with the help of Wade-Mingo rules.
O The C and B atoms in closo-structures are numbered in such a manner that the top vertex
is given 1 and the remaining vertices are numbered in clockwise direction.

 APPLICATIONS OF BORANES AND CARBORANES
• Boranes are used as rocket fuels in supersonic aeroplanes.
• Used as catalyst in polymerisation reaction
• Used as reducing agent in organic reactions
• Boranes can be used as vulcanising agent for natural and synthetic rubber.
• Used in borane coating

 CONCLUSION
• Boranes are colourless and gaseous at room temperatures.
• Boranes are highly reactive and reacts violently with water.
• In boranes 3c-2e bonds are present; It is unique type of bond present in
boranes only.
• Carboranes are mixed hydrides of boron and carbon. And have same
structure like boranes.
• Carboranes derived from pyrolysis of boranes

Structure borazine molecule.
molecule is isoelectronic with benzene and hence its structureure is the same as that of benzene.
Thus, like benzene, borazine has a planar hexagonal structure containing six-membered ring, in which B and N
atoms are arranged alternately.
It is because of the similarity between the structures of borazine and benzene that borazine is called Inorganic
benzene.
 In the structure of borazine, both B and N atoms are sp2 hybridised.
 Each N-atom has one lone of electrons, while each B-atom has an empty p-orbital.
 (B-N) π-bond in borazine is a dative bond, which arises from the sidewise overlap, between the filled p-
orbitals of N-atom and empty p-orbitals of B-atom.
 Since borazine is isoelectronic with benzene, both the compounds have aromatic π-electron cloud (electrons
in π-orbitals).
 Due to greater difference in the electronegativity values of B and N-atoms, the π-electron cloud in B3N3 ring
of borazine molecule is partially delocalised, while in case of benzene ring, the π-electron cloud is
completely delocalised.
 In fact, complete delocalisation of π-electron cloud in B3N3 ring in borazine molecule, cannot be expected,
since N - π orbitals are of lower energy than the B - π orbitals. Molecular orbital calculations have indicated
that π-electron drift from N to B is less than the 𝜎-electron drift from B to N, due to greater electronegativity
of N-atom.
 In C6H6 molecule, C = C bonds are nonpolar, while in case of B3N3H6 molecule, due to the difference in
electronegativities between B and N atoms, B-N bond is polar.

The ring structure of borazine molecule is the same as the layer lattice structure of boron nitride, (BN)n.
 Due to the partial delocalization of the π-electron cloud that π-bonding in B3N3 ring is weakened.
 In addition, N-atom retains some of its basicity and boron atom retains some of its acidity. Polar species
like HCl, therefore, attack the double bond between N and B. Thus, borazine, in contrast to C6H6, readily
undergoes addition reactions.
 In these reactions, more electronegative atom (e.g., Cl in HCl molecule) is generally attached with B-
atom, which is less electronegative than N in B-N bond.
 In borazine, B-N bond length is equal to 1.44 Å, which is between the calculated single B-N bond (= 1 54
Å) and double bond, B = N (= 1.36 Å) distances. The angles are equal to 120°. In benzene C-C bond
length is equal to 1.42 Å.

Structure:
 The lattice of boron nitride consists of different layers.
 Each layer has a hexagonal arrangement of B and N atoms.
 Different layers are arranged in such a manner that B-atoms in one layer are immediately above the N-atoms in
the adjascent layer.
 The (B-N) distances are equal to 1.45 Å and the distance between the two layers is 8.33 Å.
 The (B-N) bonds in boron nitride are formed by the overlapping of sp2 hybrid orbitals of boron and nitrogen
atoms. The remaining electrons form π-bonds.
 BN molecule is iso-electronic with two adjacent C-atoms in graphite (BN=3+5=8, C2 =4+4=8) and hence the
structure of (BN)n is similar to that of graphite, Cn.
 In graphite π-electrons are completely delocalized over all the C-atoms of the lattice while in boron nitride the π-
electrons are only partially delocalized over B-atoms, leading to unequal (B-N) bond lengths.
 Boron nitride, like graphite, is able to act as host, i.e., it is able to occlude oxides, sulphides, oxy-halides and
chlorides in the same way as graphite does.
 Boron nitride, when heated to 3000 °C at 70,000 atm pressure gives a cubic form of boron nitride which has
diamond like structure in which all the B and N-atoms attain tetrahedral (four) coordination.
 This cubic form is superior to diamond, in its mechanical strength and hardness, and suffers only superficial
oxidation in air at high temperatures.

1. Chain polymers: These polymers include
(i) polyphosphazines, [NPR2]n in which R = CI,OCH3 or OC2H5.
Polyphosphazines may be of three types namely
(a) polyphosphonitrilic chlorides, [NPCI2]n
(b) polydimethoxy-phosphazines, [NP(OCH3)2]n
(c) poly diethoxy-phosphazines, [NP(OC2H3)2]n
(ii) polyphosphoryl chlorides, Pn O2n-1, Cln+2
(iii) vitreous polyphosphates (phosphate glasses)
(iv) crystalline polymetaphosphates. Examples are Maddrell's and Kuroll's salts
(v) platicised and flexible phosphates
(vi) polyphosphoric acids.
2. Network polymers: These polymers include
(i) polymeric phosphorus pentoxide
(ii) polymeric boron orthophosphates, [BPO4)n
(iii) polymeric aluminium orthophosphates [AIPO4]n
(iv) polymeric silver orthophosphate, [Ag3PO4]n,
(v) polymeric ferrous orthophosphate, [Fe3(PO4)2.8H2O)n
(vi) sulphur-phosphorus polymers
(vii)ultraphosphate glasses
(viii)boron phosphate glasses.
Polymers containing phosphorus

Polyphosphonitrilic chlorides, [PNCI2]n (n = 3 to 7) :
These polymers are also called inorganic rubbers. These compounds can be represented by the general
structure,
Thus, these compounds contain as a repeating unit.
triphosphonitrilic chloride, [PNCI2]3; tetraphosphonitrilic chloride, [PNCI2]4 etc.
Properties. (1) (NPCI2)3; melts at 114° C, boils at 256°C at 1 atm pressure and is readily soluble in benzene, ether and
carbon tetrachloride. (NPCI2)4 melts at 123.5°C, boils at 328.5°C and has a lower solubility in benzene, ether and carbon
tetrachloride than (NPCI2)3.
(2) (NPCI2)3 and (NPCI2)4 are irritating and somewhat toxic also.
(3) Cyclic trimer. [NPCI2]3 or tetramer, [NPCI2]4, when heated in vacuum at 250°C gives a chain polymer.

Structure of (NPCI2)3 molecule:
 (NPCI2)3 molecule has a planar six-membered ring structure (Structure I) in which each N-atom is sp2
hybridised and each P-atom is sp3 hybridised.
 The lone pair of electrons on each N-atom resides in one of the three sp2 hybrid orbitals. It is this lone pair
of electrons on which makes (NPCI2)3 molecule to show basic properties. The bond angles are as shown in
the structure. Resonance structures can also be drawn, as in case of C6H6 molecule, indicating aromaticity
in the ring.
 Unlike benzene which involves extensive (pπ - pπ) bonding, (N3P3CI6) molecule involves (dπ - pπ) bonding.
The extent of (dπ - pπ) bonding appears to be quite appreciable as the N-P distance (- 1.6 Å) is
considerably shorter than the N-P single bond distance (1.75 -1.80 Å).
 Whether there is complete delocalisation of π-electron charge cloud on all the ring atoms as in C6H6
molecule or there are intensely-localised islands of the electron cloud within the PNP segments cannot be
answered with certainty.

This molecule has a tube-like puckered structure
(NPCI2)4
Linear phosphonitrilic halides or inorganic rubber: When PCI5 and NH4CI are heated together in a refluxing S-
tetrachloroethane, a product is obtained. Some portion of this product is insoluble in petroleum ether and has the
composition, [NPCI2]n. PCI5, where n =10 to 15. This product is rubber-like and hence is called inorganic rubber.
The structure of this compound can be represented as shown below.

REFERENCES
• Inorganic chemistry, Peter Atkins, Tina Overton, Jonathon Rourke, Mark
Weller, Fraser Armstrong, 5th
Edition, Oxford University Press, UK, 2013.
• Inorganic chemistry, Gary L. Miessler, Donald A Tarr, 3rd
Edition, Pearson
Education, NewDehli ,India, 2004.
• Inorganic chemistry-Principles of Structures and Reactivity, James E.
Huheey, Ellen A. Keiter, Richard L. Keiter, 4th
Edition, Pearson, India Edition,
NewDehli ,India, 2004.
• Advance Inorganic Chemistry, F. Albert Cotton, Geoffrey, Wilkinson, Carlos A.
Murillo, Manfred Bochmann, 6th
Edition, Wiley Student Edition, John Wiley and
Sons, INC, NewYork, 2004.

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