Welcome to MSB lecture series on main group
elements.
In my previous lecture I was discussing about
group 15 elements.
While discussing I did mention that white
phosphorus having 4 phosphorus atoms with
each one having a pair of electrons can perform
as a ligand.
And of course, many metal complexes of white
phosphorus as such are known.
A couple of examples I displayed in my previous
lecture.
And of course, I would be giving you more
information about the utility of phosphines
or phosphorus compounds as ligands, before
I conclude the chemistry of group 15 elements.
So, let me continue on phosphorus compounds.
And phosphorus can also have ring chain or
cage structure, anionic phosphorus rings,
chains or clusters are also known.
For example, P73-, P82- and P113- are some
of the examples of anionic phosphorus cages
rings or chains.
I will show you the structures of some of
these in my next slide.
You can see here P82- has this structure here.
And also, you can see where the negative charge
is residing.
This is P73- of course, basically what happens,
wherever the charge is ther,e essentially
these phosphorus atoms are two coordinated.
And P113- has a cage structure like this.
And arsenic, antimony and bismuth occur predominantly
as sulphides, these 3 elements essentially
form layer structures very similar to that
of black phosphorus, and arsenic vapour contains
As4 molecules very similar to P4.
And the unstable yellow form of solid arsenic
probably also contains As4 units.
At very low temperatures, antimony vapours
contain molecular SbF4 units.
At room temperature and pressure, arsenic
antimony and bismuth are essentially grey
solids with lattice structures resembling
that of black phosphorus.
And arsenic, antimony and bismuth burn in
air and combine with halogen.
For example, and here M = As, Sb or Bi.
None of these elements reacts with aqueous
alkali.
But arsenic is attacked by fused sodium hydroxide
to form sodium arsenite.
So, this is essentially called sodium arsenite.
Let us look into the compounds of group 15
elements.
The group 15 elements form binary compounds
on direct interaction with many elements.
And the wide variety of possible oxidation
states of group 15 elements can be understood
simply by looking into their electronic configuration,
which is essentially ns2np3. one can anticipate
besides having +3 and +5 states, other oxide
states also.
However, compounds having other than +2 and
+3 are very scarce.
Nitrogen achieves oxide state +5 only with
more electronegative elements such as oxygen
and fluorine.
Of course, when we look into the electronegativity
of all elements in the periodic table, nitrogen
is the 4th most electronegative element.
So, in order to make nitrogen compound exist
in higher positive oxidation state.
Only they (nitrogen) have to be combined with
either oxygen, fluorine or to an extent by
chlorine.
Oxide state +5 is common for phosphorus, arsenic
and antimony.
But rare for bismuth, and most of the base
compounds have +3 as the most stable oxide
state.
And of course, here inert pair effects dominates
the chemistry of bismuth.
As a result, +3 state is more stable for bismuth
compounds.
Since nitrogen is the 4th most electronegative
element, it can also form compounds with negative
oxidation state.
For example N3- ion , nitrides and ammonia
in which ‘N’ is also in -3 state, are
known.
And nitrogen achieves positive oxidation states
only in compounds with more electronegative
elements such as oxygen and fluorine.
As I mentioned, nitrogen does achieve the
group oxidation state +5.
Nitrogen forms simple binary compounds with
other elements.
They are classified as nitrides or azides.
Nitrides are classified as: saline, covalent
or interstitial, depending upon the other
combining elements.
The nitrides of metal can be prepared by direct
interaction of the element with nitrogen or
ammonia, or by thermal decomposition of an
amide.
I will give you some methods of preparation
of various nitrides.
So, this are the few methods of preparation
of nitrides.
So, essentially saline nitrides contain nitride
ion.
This is nitride ion having three negative
charges.
And saline nitrides occur for lithium among
the group 1 elements and also for group 2
elements.
It has composition M3N2.
Whereas, in case of group one essentially
only lithium forms.
Others cannot form stable azides because of
the larger size.
You can see Li3N for group one.
Whereas, in case of group 2 elements.
It is the general formula can be written (M3N2)
because they are divalent in nature.
So, in covalent nitrides due to the presence
of a covalent bond possess a wide range of
properties depending on the element to which
nitrogen is bonded.
Some examples of covalent nitrides are essentially
boron nitride.
So, boron nitride has composition 1:1 and
cyanogen, phosphorus nitride, tetrasulphur
nitride.
Now so, disulphur dinitride, so the largest
category of nitrides consists of interstetial
nitrides of the ‘d’ block elements with
formula MN or M2N or M4N that depends on the
oxidation state of the metal.
Essentially d block element where the ‘N’
occupies some or all the octahedral sites
with in the cubic or hexagonal close packed
lattice of metal atoms.
And transition metal nitrides are hard and
inert in nature.
So, they are essentially used as refractory
materials, and find applications as crucibles
high temperature reaction vessels and thermocouple
sheets.
Nitride ion is often found as a ligand in
transition metal complexes as well.
I am going to show one or two examples of
metal complexes having nitride as a ligand.
As one of the ligands, it has high negative
charge, small size and ability to serve as
a good π donor as well as a σ donor.
Means that it can stabilize metals in higher
oxidation state.
Very similar to oxygen, the short coordinate
bond between the ion and the metal atom is
often represented as M-N.
An example is the complex of this type.
One can write like this or one can also write
something like this.
So, one example is; I will show you the structure
of this compound here.
You can see the structure of pentamine osmium
nitride complex.
It has an octahedral geometry where one of
the actual positions is occupied by ‘N’.
And essentially, we have Os-N triple bond
here.
Another important class of nitrogen compounds
are essentially azides.
Azides are toxic and unstable.
And they are used as detonators in explosives.
So, one should not confuse between azide and
nitride; so the N3- (single negative charge)
whereas, in case of nitride we have one nitrogen
with three negative charges (N3-).
So, this is azide and this is nitride.
So, azide in which nitrogen is present as
N3- , may be synthesized by the oxidation
of sodium amide with either NO3- ion or N2O
at elevated temperature.
For example, H2N- reacts with NO3- to give
N3-.
Or one can also start from N2O.
In N3- the average oxidation number of ‘N’
is -1/3.
The ion is N3- ion.
So, this is for, I am telling N3- ion.
So, here each one is three minus only, one
here 3 nitrogen’s are there the oxidation
number can be considered as -1/3 for each
nitrogen.
This azide ion is isoelectronic with N2O as
well as CO2.
And of course, azide is also a very good ligand
towards transition metal ions.
And heavy metal complexes salts such as Pb(N3)2
or Hg(N3)2 are shock sensitive detonators
and decompose to produce the metal and nitrogen.
Ionic azides such as NaN3, sodium azide are
thermodynamically unstable, but kinetically
inert.
They can be handled at room temperature.
They can also be used as source of azide for
various other preparation such as organic
azide and inorganic azides.
Sodium azide is toxic.
And is used as a chemical preservative and
in pest control.
And when alkali metal azides are heated or
detonated by impact, they explode liberating,
nitrogen depending upon the quantity of metal
azide involved in it.
This reaction is used in the inflation of
airbags in cars in which the heating of the
azide is by electrical means.
Of course, I elaborated more about airbag,
and the utility of sodium azide in it for
inflation.
Besides azide, nitrogen compounds containing
polynitrogen cations are also known for example,
N5+ is synthesized from species containing
N3- and N2F+ ions.
For example, if you consider [Na2F][AsF6]
in solution, when it is treated with HN3,
it forms N5+and H-F formation will be there.
You may be surprised to know the structure
of N5+.
This how it looks like.
So, overall charge will be plus.
So, overall charge will be plus here and here
this angle is 111° and this distance is 129
pm.
And this is 110 pm.
And whereas, this angle is 166°.
And similar to nitrogen, phosphorus forms
compounds with almost all elements in the
periodic table.
Phosphides maybe metal rich or phosphorous
rich depending upon the composition.
And that is based on the oxidation state of
the metal or the other element with which
phosphorus is combined.
The phosphides of other elements can be prepared
by heating the appropriate element with red
phosphorus in inert atmosphere.
For example, here phosphorus we are considering
is red phosphorus.
So, there are many varieties of phosphides
with formulas ranging from M4P to MP15.
They include a metal rich phosphides in which
metal to phosphorus ratio is greater than
1.
And monophosphides.
So, one is with metal to phosphorus ratio
greater than 1 and another class is metal
to phosphorus ratio equal to 1.
And another set of compounds; metal to phosphorus
ratio greater than 1.
They are essentially phosphorus rich horse
phosphides.
And here they are metal rich phosphides, the
structures have a trigonal prismatic arrangement
of 6, 7, 8, 9 metal ions around a phosphorus
atom.
Monophosphides essentially adopt a variety
of structures depending on the relative size
of other atoms.
For example, aluminium phosphide adopts, the
zinc blend structure, tin phosphide adopts
rock salts structure.
And vanadium phosphide adopts the nickel arsenide
structure.
And phosphorus rich phosphides have lower
melting points.
And are less stable than metal rich phosphides
and also mono phosphides.
I will show you some of these phosphides in
this slide.
This is a tin phosphide.
You can see here, the ratio is 1:1.
And here we have gallium phosphide, and you
can see the red ones are gallium and grey
ones are phosphorus.
And this is indium phosphide.
So, other hydrides of group 15 are essentially
phosphines, formally phosphanes.
Essentially the hydrides of group 15, we have
ammonia and of course, with phosphorus they
are called phosphines or phosphanes having
the formula PH3.
And similarly, we have arsine AsH3 and stibine
SbH3.
All are poisonous gases.
The difficulty of oxidising Bi3 to Bi5 by
chlorine or bromine is an example of the inert
pair effect.
Essentially if you take any of the trivalent
bismuth compound, if we try to make an attempt
to oxidize using chlorine or bromine, it is
not easy because of the inert pair effect.
The complete combustion of phosphorus yields
phosphorus pentoxide; that is, P4O10.
Combustion in a limited supply of oxygen results
in the formation of P(III) oxide or P4O6;
here phosphorous is in +3 state and here phosphorous
is in +5 state.
And essentially; that means, combustion of
white phosphorus in a limited supply of oxygen
gives P4O6 phosphorous trioxide.
Whereas the complete combustion gives phosphorus
pentoxide.
So, this how P4O6 looks here.
And it is very easy to write the structure
of P4O6 or P4O10 starting from P4.
For example, we all know that 
this how the structure of white phosphorus.
So, one can simply like this here.
And then connect them with ‘O’.
So, this is P4O6, partially oxidized one,
you can see all phosphorus atoms are in trivalent
state.
And if excess of oxygen is there, this essentially
forms P4O10.
So, all phosphorus are going to +5 oxidation
state; this is P4O10.
So, indium and gallium arsenides and antimonides
are essentially semiconductors.
The compounds formed between metals and arsenic
antimony and bismuth can be prepared by direct
reaction of the elements.
For example, nickel when it is treated with
arsenic it gives nickel arsenide.
Gallium arsenide is the most important and
is used to make devices such as integrated
circuits, light emitting diodes and laser
diodes.
Gallium arsenide: its band gap is similar
to that of silicon and larger than those of
other group 13 and 15 semiconductors.
As a result, gallium arsenide finds application
and it is most sought after compared to others.
Gallium arsenide integrated circuits are commonly
used in mobile phones; satellite communications
and some radar systems.
Let me begin the discussion on hydrides of
group 15 elements.
All the group 15 elements form binary compounds
with hydrogen and are toxic, having the composition
like EH3.
Nitrogen also forms a catenated hydride besides
NH3 (Ammonia), N2H4 that is called hydrazine.
You can see there, in addition to ammonia,
nitrogen forms the hydrides, N2H4 or diazene
N2H2.
And of course, HN3, it is called hydrazoic
acid.
There should not be any confusion between
ammonia NH3 and hydrazoic acid HN3.
Ammonia is produced by a Habers process.
It is used to manufacture fertilizers.
And many other useful nitrogen containing
chemicals.
So, in this process nitrogen and hydrogen
combined directly at high temperature of 450
°C and high pressure of (100 atm) over a
promoted iron catalyst to form ammonia.
So, let me stop here, and continue the discussion
on hydrides of group 15 elements in my next
lecture.
Until then have a pleasant reading of inorganic
chemistry.
Thank you.
