Hello, this course is about organometallic
chemistry and my name is A.G. Samuelson and
I work in the inorganic and physical chemistry
department. And I have been there since 1983,
I teach mostly organometallic chemistry and
inorganic chemistry. This course is a result
of my experience with the students in introducing
them to organometallic chemistry.
So, organometallic chemistry is in fact a
very complex field and it is a fascinating
field. What this course does is to have a
structured introduction to it, so that even
if a person has not studied any organometallic
chemistry. In the earlier days, he can boldly
start reading the literature in organometallic
chemistry after going through these few lectures.
So, let us take a look at the slides one by
one.
We have structured introduction as I said,
but it is definitely a very fascinating field.
It was a late comer into the subject field
of chemistry. It was only in 1956 that people
realised that organometallic chemistry is
a discipline. Sub-discipline of its own and
it has a significant amount of material that
would be of great interest, not just in the
laboratories. But also in the industry the
problem with organometallic chemistry is a
the fact that suddenly the student is presented
with a large number of new concepts, which
were not available or necessary to understand
organic chemistry or for that matter even
inorganic coordination chemistry, classical
coordination chemistry. Now, we first answer
the question, why study this particular field,
if it is so complex. If it is interesting
then it must be worth it. So, let us take
a look at some reasons why we should study
it.
I have developed here a few questions that
one should ask when you look at any new discipline
and the first question that I ask is that,
are there any significant discoveries in this
field that has merited? For example, the award
of a noble prize, now at least in the sciences
it is well known that the noble prize is considered
as one of the best prizes, or one of the top
prizes that are given to a scientist, who
has made a very important discovery, which
would benefit mankind and, so we can ask this
question.
Has organometallic chemistry really achieved
that status, so that some people working in
this field would receive the Nobel prize?
Let us see what happens to that question.
Then we ask the question, we look at some
of the journals, which are of importance to
all scientist working in the area of chemistry.
Do we see any organometallic chemistry papers
and what kinds of papers are they. Let us
see what kind of research is funded and then
finally, how much of the economy is really
driven by this topic? So, each one of these
questions will be briefly answered before
we proceed to the subject itself.
So, let us take a look at the Nobel prizes.
Here I have listed for you some of the Nobel
prizes that have been awarded in the general
area of organometallic chemistry. As I said,
the sub-discipline was recognised only in
the late 1950’s and surprisingly by 1963,
Zeigler and Natta were awarded the Nobel prize
for working in the area of titanium. And zirconium
catylyst, catalyst which were capable of producing
high molecular weight polymers.
Especially polyethylene and polypropylene,
and polypropylene was made in a specific fashion,
which resulted in very interesting and very
valuable properties for polypropylene. This
area continues in spite of the fact that the
major discovery was made in 1963. Even today
it continues to be an active area of research
and through the series of lectures. We will
see this, how this is quite important and
what a fundamental reaction an organometallic
chemistry is responsible for this particular
discovery.
Next, let us take the Nobel prize that was
awarded to Wilkinson and Fischer in 1973,
that came just ten years after the first Nobel
prize to Zeigler and Natta and Wilkinson.
And Fischer were actually awarded the Nobel
prize for their discovery of metallocenes.
This discovery was in fact the key discovery
that spurred people onto studying organometallic
chemistry as a sub-discipline. By then it
was very clear that it was this seminal discovery.
The subsequent research work that was done
in this area that was going to contribute
significantly in chemistry.
I next talk about the discoveries made by
Herbert Brown and Wittig and Lipscomb take
these two Nobel prizes together, because it
turns out that all of them contributed to
the chemistry of elements on either side of
carbon. Brown contributed to the chemistry
of boron, Witting to the chemistry of phosphorous,
which is on the next side and the right side
of the periodic table, where carbon is there
in the centre. So, Lipscomb and Brown contributed
significantly to the understanding of the
chemistry of boron and the structures. These
are key discoveries in organometallic chemistry,
because they helped one to understand some
of the structures, which were not available
in the literature in earlier days.
So, next comes the discovery or the Nobel
prize to Fukui and Hoffman. Once again, Fukui
and Hoffman were given the Nobel prize for
their contribution to the structures of compounds
and they used frontier molecular orbital theory,
but Hoffman applied it completely to organometallic
compounds. He was able to explain the structures
of all the new compounds that were discovered
in the 60’s and 70’s by researchers working
in organometallic chemistry and also suggest
a framework for understanding few structures
that would be discovered in the future. So,
that was a seminal contribution to the field
of organometallic chemistry.
There was a long gap since, from 1981 to 2001,
and in 2001 Sharp less Knowles and Noyori
were awarded the Nobel prize for their contributions
to asymmetric catalysis. And this was completely
in the field of organometallic chemistry,
where chiral ligands were used by these workers
to generate chiral compounds, which were very
important in the field of drug discovery.
So, Noyori, Knowles and Sharpless were recognised
for this key contribution in 2001. This was
rapidly followed by another two Nobel prizes
in 2005 and 2010 and in 2005, Schrock, Grubbs
and Chauvin were awarded the Nobel prize for
their work on metathesis chemistry. Interestingly,
this deals completely with carbon-carbon double
bonds and double bond containing compounds
and how they can be transformed from one to
the other. It turns out that the new bonds
that are formed were also double bonds and,
so this turns out to be contribution that
is not parallel in classical organic chemistry.
This was followed by Heck, Negishi and Suzuki
who contributed significantly to the formation
of a carbon-carbon single bond, these two
discoveries in 2005 and 2010. These two Nobel
prizes recognised the contribution of organometallic
chemistry to the formation of carbon-carbon
single bonds and carbon-carbon double bonds.
If you look at the span of say fifty odd years
from or sixty years from 1955 to the 2010.
You see that there are about eight Nobel prizes,
that have been awarded to the field of organometallic
chemistry. And even earlier the discoverer
of Salvarsan, which is inorganic or organometallic
drug that was given to treat syphilis was
also Meone who received the Nobel prize for
in the in the field of drug discovery, but
then it was an organometallic compound.
So, you can see that several compound Nobel
prizes have been given in this field. Now,
we let us take a look at briefly at the journals
and how they give importance to the subject
of organometallic chemistry.
Now, if you look at any one of these common
journals, which are on the top of the list
of any researcher like journal of American
Chemical Society or Chemical Communications
or the Ango Antichemy. These journals are
journals which are publishing in the general
area of chemistry and if you open any issue,
you will find that there are at least two
or three articles on organometallic chemistry.
So, I have put for you to in this, in this
projection here. Just one instance of article,
written by Shilov and Shulpin about c h activation,
this turns out to be a very important reaction,
which has been made possible, because of organometallic
compounds and organometallic chemistry.
So, you can see that once again, inorganic
organometallic chemistry is extremely important
in inorganic chemistry and chemistry in general
once again early transition metals were shown
to be good metals for transition metal c h
activation. But recently it has been shown
that even lead transition metals with appropriate
set of ligands can carry out ligand directed
c h activation reactions, and this work has
been carried out by Melanie, Sanford and her
co workers.
If you look at the field of organic chemistry,
it is not as if everything that needs to be
discovered has been discovered. So, here I
have a article which is appeared on the journal
nature. You find that they have discovered
something extraordinarily new, even in this
last sixty years, what was not known has now
been newly discovered. Here is an example
of a reaction between a molybdenum complex,
where molybdenum is in the 0 oxidation state
and it has got 6 phosphorus ligands around
it.
So, here is a ligand that we are here is a
compound that we are talking about, when its
reacted with this organic compound it forms
a very interesting and novel complex, where
the metal initially coordinates to the ring
where there are 2 nitrogens. So, it is a phenazine
ring that is having the nitrogen on the aromatic
part of the ring, and then after some time
if you heat it further. Then the molybdenum
moves over to the ring which has got only
the carbons.
So, here are some challenges about why this
happens and what are the reasons which initially
stabilise the nitrogen containing ring coordinated
to the molybdenum. And later, what is the
thermodynamic preference for the carbon containing
ring. Interestingly, if you just move from
molybdenum to tungsten, one would expect very
similar reactivity, but surprisingly the same
reaction which was carried out with tungsten,
now suggests that the reactivity can be extremely
different.
And you have extraordinarily novel compounds,
where an aromatic ring system has been broken
and the carbon-carbon ring has been broken,
carbon-carbon bond has been broken and that
to part of an aromatic ring and that forms
a metallacycle. So, here is the set of atoms
that we are talking about and that forms a
ring and that ring turns out that ring is
an aromatic ring. To start with, here is a
ring and that has been broken by the tungsten.
You can see that some extraordinarily novel
complexes can be generated some things that
challenge our understanding of chemistry in
general and help us to think about what could
be coming in the future.
Here is another challenge that has come up
very recently in 2005, Graham ball and co
workers have shown that even xenon, which
is 0 group element or an inert gas. What was
originally considered as something that would
not react with any other element, what was
called inert gases or the 0 group that turns
out to be a element, which can form an organometallic
compound. Let us see what they have actually
done.
Here is the reaction that they have carried
out the rhenium complex, which was excited
with a xenon flash lamp absorbs light and
in the excited state loses one of the ligands
if it loses the xenon. If it loses the carbon
monoxide then this xenon appears to coordinate
to the rhenium. In this particular state,
it is possible to see the xenon phosphorus
and xenon fluorine-coupling constant.
So, this was carried out by labelling the
xenon, so they carried out a reaction in the
presence of 128 xenon, which is NMR active
and they were able to show that. In fact there
is a covalent interaction between the xenon
and the rhenium that is the only way in which,
you can have this type of a weak coupling
constant between the xenon and the fluorine.
So, you can see that there are surprises galore
and there are challenges that have to be addressed
in the field of organometallic chemistry.
If you ask these two last questions, to see
what research is funded, how much of it is
driving the industry?
Once again organometallic chemistry comes
out on the top, you can take all the journals
that publish organometallics. And you can
see that the number of papers published are
significantly large close to 2,400 papers
have this concept of organometallic chemistry
mentioned in their papers, or if you look
at the key papers if you look at the topics
that are mentioned. You see that organometallics
are there in at least 1400 papers every year.
So, it turns out that this is a very active
field of research.
And even if you look at the industry it looks
as.
If you have a large number of industries which
employ organometallic compounds either as
catalysts, or as additives and as part of
the process which is there in the industry.
So, here I have listed some of the silicones
and organolithium compounds. Organolithium
compounds are used significantly in fine chemical
synthesis, silicones are used in various industrial
applications and there elastomers and so on.
Metal complexes are very commonly used for
making polymers. Specially, poly propylene
and poly ethylene which are made in tonnage
quantities.
These molecules are made using organometallic
catalysts which are basically the result of
Zeigler, Natta’s work and then there are
other small molecules like acetic acid acetaldehyde.
Several other small molecules which are generated
from carbon monoxide, hydrogen and readily
available starting materials and this is again
made possible by organometallic compounds.
We will some of these reactions and how these
are how these unique reactions are made possible,
because of organometallic chemistry.
So, we have looked at a variety of questions,
are there Nobel prizes, are there important
papers published in journals, is this research
well funded and what is practised in the industry?
In all these cases we have come up with positive
answers for the field of organometallic chemistry.
Now, we ask the question why is this subject
so important? Are we dealing with a special
element? Organic chemistry, for example deals
with this very special element of carbon.
What about organometallic chemistry, is it
the same fact is it because of carbon?
So, if I ask the question, what is special
about carbon you would probably come up with
these answers, which I have listed here carbon
is unique because it forms bonds to other
carbon. It forms strong multiple bonds and
it forms strong multiple bonds to in a cyclic
fashion, so that the molecules are called
aromatic. So, if I list all these things I
would still say this is not the reason why
organometallic chemistry is so special?
Carbon has in fact, an electronic configuration
of 1 s 2 2 s 2 and 2 p 2. This is in fact
special, why the answer is that when it forms
4 covalent bonds, in order to generate the
nobel gas configuration the molecule can do
so by hybridising 2 s and 2 p. The gap between
2 s and 2 p orbitals are just right. So, the
promotion of the electrons from the 2 s and
2 p orbitals are easy and 4 equivalent covalent
bonds are formed. That leaves carbon with
no extra electrons and no vacant orbitals,
this is extremely important and this probably
a fundamental fact which makes carbon a unique
element.
Now, when it comes to transition metals, you
will notice that transition metals are exactly
the opposite. They always have vacant orbitals
almost always or they have extra electrons,
rarely do they keep a Nobel gas configuration.
In the case of transition metals the Nobel
configuration would usually be an 18 electron
configuration. Here I have shown for you some
of the stable coordination compounds that
are available in the literature most of them
are octahedral complexes formed by the 3 d
transition elements 3 d 2 plus ions or 4 d
2 plus ions or 3 plus ions. You will notice,
that many of them or most of them are not
18 electron species to look at the valence
electron count, you just count the number
of valence electrons on the cobalt.
If it is cobalt 2 plus then you would have
seven electrons and with 6 water molecules
it each one of them would donate 2 electrons
each. So, if you add up you would end up with
a valence electron count of 19, so that is
1 more than 18. Similarly, for nickel it would
be 20 for the vanadium aquo complex it would
be 15, so very few complexes would really
have 18 electrons. So, it neither has a full
shell and it always many times it has less
than 18 or more than 18.
So, what happens in transition metals, to
the transition metal, in transition metal
organometallics? If you look at most of the
compounds, they follow the 18 electron rule
so much, so that it is important for us to
look at this rule briefly, to see how this
rule can be verified, when we write a structure
to see if it would be a stable compound. Most
complexes with transition metal carbon bonds
force this preference, so they always have
an 18 electron count. When carbon combines
with the transition metal, both the metal
and the carbon lose their identity. In what
way has carbon lost its identity?
So, here is the an organometallic compound
that I want to show you for just illustration
of the fact that carbon, in this molecule,
is in a very strange environment very rarely
would carbon or rather almost always, it would
have a 4 coordinate geometry and that also
it would be a tetrahedral geometry. If it
has only single bonds, but here is a molecule
where carbon is having 5 atoms around itself.
Now, look at the coordination geometry around
the iron that is no longer octahedral either,
so that has got geometry, which is non octahedral
and the carbon has got a geometry which is
non tetrahedral.
So, both carbon and the transition metal lose
their identity. So, here is another example,
we all know that cyclobutadiene is a rectangular
molecule, because if it is square it would
be anti aromatic. So, it turns out to be equilibrating
between 2 rectangles structures and when it
forms a iron carbonyl complex lone be hold
it changes its character, it keeps a strictly
square structure of 4 carbon atoms. So, you
can see that now carbon has lost its character
of being anti aromatic when it has got 5 n
pi electrons.
So, here is another example which is from
an aromatic molecule like chlorobenzene, if
you treat it with ammonia and heat it any
amount, it will not carry out such a nucleophilic
substitution. Nucleophilic substitution for
an aromatic compound is not possible in classical
organic chemistry, but if you do this with
a transition metal attached to the aromatic
ring, it is now possible to carry out this
simple nucleophilic substitution. So, nucleophilic
substitutions which were not possible in classical
organic chemistry now becomes easy when you
have a transition metal coordinated to the
ring system.
So, are we dealing with a special element?
The answer is probably no, we are dealing
with a special combination of elements.
The combination of transition metals and carbon.
So, there are very interesting challenges
which this combination throws at us. First
of all, we can synthesise some new and interesting
molecules, which have extraordinarily new
interesting reactivity patterns, what was
not possible with classical organic chemistry.
Activation of inert molecules for example,
methane or even carbon dioxide or the carbon
fluorine bond, turn out to be challenges which
are still being handled by organometallic
chemists. There are other things also like
catalysis.
Catalysis by organometallic chemistry is quite
popular it is indeed feasible, but can we
approach the efficiencies of enzymes? Can
we use organometallic compounds to catalyse
reactions at extremely fast rates? Finally,
asymmetric induction and catalysis is also
a major challenge and as I had mentioned just
briefly, it was for this contribution that
three people received the Nobel prize in 2005.
I want introduce to you now to the organisation
of this course as much as possible we have
kept it modular based on the ligand systems.
Now, so one would ask the question how are
the ligands classified in organometallic chemistry?
The ligands are classified according to the
hapticity. Hapticity is indicated by the symbol
eta and eta indicates the number of carbon
bonds, number of carbon metal bonds that can
be seen when you have a transition metal organometallic
compound.
If I have a 3 carbon framework and if all
3 carbons are within bonding distance if all
3 carbons are within bonding distance to the
metal. Then I would call this an eta 3 compound
whereas, if the same compound is present in
such a way that the metal is interacting with
only 1 carbon. If it is interacting with only
1 carbon then I would call it a eta 1 compound.
So, the hapticity of the ligand is really
a key to understanding and unravelling organometallic
compounds and many times the hapticity can
change. This is called fluxional behaviour
and this is something, which we deal with
during the course of this particular set of
lectures.
Let me just now show you how we have organised
the lectures. In many text books synthesis
structure and bonding of organometallic compounds
with common ligands are shown in initially
and then we have reactions and we have properties.
Now, in this course during the course of these
lectures, we will not strictly follow this
order, but let me introduce to you the way
in which the ligands with metal carbon bonds
are classified. Let us take a look at that
first and then we will see how we have presented
the topics in this particular course.
Ligands with metal carbon bonds, if we only
look at the metal carbon bonds, you have a
very large number, but based on hapticity
you can see that many of them or a large significant
number of them have got a single carbon attached
to the metal. These are all eta 1, eta 1 carbon
ligands. So, they can be alkyl aryl or for
that matter vinyl and you can also have small
ligands like carbon monoxide and carbenes,
carbon monoxide and carbenes are all single
carbon bonded systems.
Then you have the odd number of carbons attached
to the metal on the top, these are odd and
then I have listed for you even number of
carbons attached to the metal on the lower
half. So, you will notice that the odd carbon
attached metal complexes are usually indicated
as anionic ligands and we will talk about
the nature of this interaction in a few minutes.
On, the other hand even carbon atoms that
are linked to the metal are usually neutral
ligands and many of them are known with transition
metal, are interacting with them.
Many of them are known where transition metals
are interacting with even number of carbons
and then there are non carbon ligands which
support organometallic chemistry. This is
the situation where there is 1 metal carbon
bond, but the rest of the metal complex has
got either a ligand with a phosphorous or
a hydride or a nitrosyl. These do not have
a metal carbon bond by themselves, but nevertheless
they support organometallic chemistry or modify
the properties of the organometallic chemistry,
so much, so that we need to know a little
bit about the chemistry of these molecules.
If we have to understand organometallic chemistry.
So, we are going to deal with these ligands
as well.
Now, if you look at the reactions once again
we can go from the simple ligands. The reactions
of the simple ligands to the more complex,
but it is more convenient to classify them
by the type of reactions that they undergo.
The simplest of course is, substitution and
isomerisation. These are listed for you on
the top followed by what is unique to transition
metal organometallic chemistry which is called
insertion reaction. So, normally insertion
of an anionic ligand occurs when the anionic
ligand migrates to the neutral ligand.
We will look at these reactions and we will
do that right after we talk about metal carbon
containing compounds, where there is only
a single carbon attached to the metal. Subsequently,
we can also see that there can be an oxidation
state change in the transition metal, when
you have an organometallic compound. So, you
have oxidative coupling of neutral ligands
like a carbene and an olefin that is the basis
for the famous metathesis reaction. Then you
can also have oxidative additions of molecules
like methyl iodide and elimination of such
molecules including the elimination of two
atoms of hydrogen. For example, on a metal
that would result in reductive elimination.
Here there is a oxidation state change on
the transition metal, so initially we will
consider those reactions where there is no
change in oxidation change, and then we will
look at those where there is a change. If
you look at the properties there are several
properties which are very interesting for
organometallic compounds, we will not be always
dealing with these with with these examples.
But towards end of the course we will deal
with some cases, where organometallic chemistry
has turned out to be an extremely important
and valuable addition to the arsenal of the
chemist.
So, in the current approach we will avoid,
we will try to deal with these ligands in
a systematic fashion going from the simple
to the complex ligands until, we discuss the
complex ligands, we will not introduce them
as much as possible in the early part of the
course. We will try to integrate the discussion
on reactions with the new structure types.
I found that this keeps the interest of the
person studying this subject, so that first
we do not deal with completely structures
and then move completely over to reactions.
So, there has been an integration done of
the reactions and the structures, and you
will see the value of doing that in a structured
fashion.
So, when it comes to text books that are available
in the literature, there are several new text
books that have appeared, but I would strongly
recommend that if you are a serious if you
are a serious organometallic chemistry student.
Serious chemistry student that you would have
at least one book on organometallic chemistry
and in recent times there is a book on organometallic
chemistry that has been written by B D Gupta
and Anil Elias, which has been published by
Universities Press, which is quite affordable
and available ready available for the chemist.
So, let us take a look at electron counting,
the organometallic chemist way and unfortunately,
as I had hinted earlier there are two ways
in which the organometallic chemist or some
organometallic chemist count the number of
d electrons, which are accessible to the metal.
So, let us take a look at some of these methods.
First, we should mention that these counting
rules are only to keep track of the number
of electrons, they are not to be confused
with the charge that is present on the metal
or for example, on the ligand.
Nevertheless, we will talk about ligands as
if they are ionic or as if they are neutral
and this book keeping is important only for
us to find out, if the molecule will be stable
or will it undergo some reactions or will
it undergo some redox reactions with other
oxidants or reductants. When you want to look
at a metal complex which has got a net charge,
these electrons are usually, if you have excess
electrons then they are added to the metal
atom, so that you have a the total net charge
of the species, which is inside the square
bracket matches, the charge which is present
outside the square bracket.
Let us take a few examples and then this would
become fairly clear. If you take a simple
complex like Cp 2 cI Cl 2 Cp stands for the
cyclopentadienyl ligand. So, cyclopentadienyl
ligand is a system which has got 5 carbons
and because it is c 5 h 5 minus, it is obvious
see that this has got 6 pi electrons and if
it has 6 pi electrons it is an aromatic ligand
system. Now, it turns out that this aromatic
ligand system will readily coordinate to a
variety of metals and if it coordinate 2 of
them coordinate to titanium you can have this
complex Cp 2 Ti Cl 2.
Now, let us take look at the set of ligands
where we have for us the two methods listed
out. So, let us go from the simple ligands.
If you have a ligand like hydrogen then in
the ionic method you would assign it a charge
of minus 1, if it has a charge of minus 1
hydrogen already has 1 electron around it
and, so a minus 1 charge will necessarily
mean that it has got 2 electrons, so this
turns out to be a 2 electron donor.
Whereas, in the neutral method, this is the
neutral method, you would only count the number
of electrons which are there on the on the
ligand to start with, you would not assume,you
would not attribute a charge to the ligand
if when it is not there. Here, for example,
for hydrogen we would indicate that it is
a 1 electron ligand. Now, you might wonder
why do we confuse this issue by having h minus.
Very often hydrogen is more electronegative
than the metal, so when it is coordinated
to the metal the assumption is that hydrogen
acquires 1 electron from the metal. So, the
metal becomes plus 1 and hydrogen becomes
minus 1, so 1 electron is transferred from
the metal to the hydrogen, so this turns out
to be correct.
For example, in the next case when you have
a halogen, it is very obvious that halogen
is extremely electronegative and if so, when
it coordinates to an electropositive element
like a metal, then it will acquire a negative
charge and it will become x minus. So, all
halogens are indicated as 2 electron donors,
but in the neutral method you still count
it as a 1 electron ligand.
You can see that none of these methods either
the ionic method or the neutral method is
a perfect method both methods have their advantages
and disadvantages, but what is interesting
and what is important is that you use the
methods consistently. You could not use the
ionic method for some ligands in a metal complex
and the neutral method, for some other ligands
in the same complex, that would lead to a
wrong result.
So, let us take a look at some of the other
ligands which are listed here. Cyanide for
example, is a electronegative group and it
is it is indicated as CN minus and it is usually
bonded through the carbon it is in the ion
neutral method it is considered still as 1
electron ligand. Each of the metal radicals
or any alkyl radicals it is indicted as an
anionic species, this again turns out to be
confusing at times, because carbon can be
just as electronegative as a metal atom or
sometimes it can even be less electronegative.
But still we consider by convention carbon
to be more electronegative and indicated as
an anionic ligand.
All these species turn out to be simple species
where you have a 1 electron donor and the
neutral method and 2 electrons in the ionic
method. Next comes carbon monoxide. Let us
leave nitric oxide for the moment and deal
with carbon monoxide. Carbon monoxide has
got a lone pair on the carbon and of carbon
monoxide and that means it donates 2 electrons.
Here we have no confusion between the ionic
method and the neutral method both of them
suggest that 2 electrons are donated from
the ligand to the metal.
Similar is the case for ammonia or water they
are 2 electron donors, whether there are neutral
or ionic it does not matter which method you
use a carbene which is, what is indicated
here carbene is also a 2 electron donor. So,
you can imagine a carbene as a carbon containing
a lone pair of electrons, that can donate
2 electrons in either method. Subsequently,
we should look at isocyanides.
Here are isocyanides and isocyanides again
have got a lone pair of electrons situated
on the carbon. So, they are they are not confusing
there will be 2 electron donors whether you
consider them in method a which is ionic method
or the method b which is the neutral method.
So, ethylene is a molecule which is an unsaturated
organic molecule, which has got a pair of
electrons in the pi orbital and this can donate
a pair of electrons. So, we have a neutral
species which is giving 2 electrons. Now,
let us come back to nitric oxide.
Nitric oxide, because nitrogen is more electronegative,
it can transfer an electron from the metal
to the NO and then it will become NO minus.
And this 1 electron that was sitting in the
anti bonding orbital will now have another
electron in the same orbital and these 2 electrons
can be donated to the metal. So, NO, if it
is specially if it is in a bent form. So,
if the metal is having a bent NO then we end
up calling it as a nitrosyl, bent nitrosyl
which can donate 2 electrons and it is counted
as a negative ligand.
In the ionic method in the neutral method
of course, we will consider that only as a
one electron donor oxo and sulphide oxido
and sulphido ligands are rarely encountered
in organometallic chemistry, but they are
all 2 electron donors, when it comes to the
neutral method and when it comes to the ionic
method. Now, it will have to be considered
as 4 electron donors. So, I guess this set
of this classification helps you to follow
the general trend, which is there in the literature.
They might either use the ionic method or
the neutral method in order to assign the
electron count. Now, in this next panel I
have now for you a linear nitric oxide. A
linear nitric oxide is a system where 1 electron
from the nitric oxide anti bonding orbital
is transferred from the ligand completely
to the metal. Sometimes in the ionic, not
sometimes, but always in the ionic method
you will consider that as NO plus. So, this
is a first example that we are considering
where the ligand is positively charge.
Although, NO is more electronegative as a
group than a metal atom. If it is a linear
nitric oxide then it turns out that you can
transfer the electron completely to the metal
and you consider that as a 2 electron donor
which is positively charged. Similarly, an
allyl group can be considered as a 2 electron
donor if it is positively charged and that
is quite obvious, because you would have a
double bond and then a positive charge at
the allylic position, so it would have only
2 electrons which it can donate to the metal.
Carbines are unusual we will leave them for
the moment and look at other examples.
These are classical ligands ethylenediamine,
bipyridine, butadiene is just an extension
of ethylene. So, all of them will give 4 electrons
each, whether they are in the ionic method
or the neutral method. Similarly, benzene
will give 6 pi electrons and that is again
for the sake of electron count, it is not
as if the electron is completely transferred
to the metal. Where as in the case of C 5
H 5 minus, it is obvious that the metal will
be positively charged and the ring system
will be negatively charged. So, you have a
6 electron donor in the case of the cyclopentadienyl
anion. In the neutral method they do a bit
of violence to this concept and we say that
it gives only 5 electrons.
Now, there are some advantages in using the
neutral method, because it is easy to forget
the fact that the metal is changing its role
or the ligand is changing its role, we just
count the number of atoms which are coordinated
to the metal and keep that as a number of
electrons that are donated to the metal. So,
C 7 H 7 for example, we would not have to
consider whether it is aromatic or not.
If all 7 carbon atoms are bonded to the metal
then we consider that as a 7 electron donor.
Now, let us come back to the nitride case.
This is a example where in a simple minded
fashion if you use a neutral method you would
think that there are 3 electrons, which are
donated to the metal. So, in other words there
are totally 5 electrons which are there for
the nitrogen.
This is the valence shell, but if you consider
this as a nitrido ligand, then you would write
it with a triple bond to the metal. So, here
is a metal and it is forming a triple bond
with these 3 electrons and you have only a
lone pair on the nitrogen, which is present
on the nitrogen side and so, you have a metal
which is coordinated with a triple bond to
the nitrogen, then in the neutral method,
you take it as a 3 electron donor in the ionic
method. You look at as if it is a N 3 3 minus.
So, you would end up with a total of 6 electrons
on the nitrogen. This is an unusual bonding
situation, but that is the way we count it.
Surprisingly, for a carbine a similar situation
should occur, but in terms of electron count
people have always used a 3 electron count
for the ionic and the neutral methods, so
that is why I have marked it with a red colour.
Since, that is an exception which is not following
the convention that we have followed right
through in this whole in this list of ligands.
So, when you count electrons you either should
follow the method a, which would have charges
on the ligands. When it is appropriate or
you should stick to method b, where you would
ignore the oxidation state of the metal you
count the total number of d electrons for
the metal as if it is in the 0 oxidation state.
So, you just assume that it is in the 0 oxidation
state, and you add the number of electrons
that are coming from the ligand irrespective
of what you might think is the charge on the
ligand.
Let us take a few examples and then this will
become easy to follow. And what is interesting
or what is important is that with either method,
we should end up with the same result. That
is the most important fact and it is important
that you practice with some of the compounds,
so that you will understand what is going
on.
So, let us take a look at the first compound
H MnCO5 that we are going to talk about HNnCO5
hydrogen, because as I mentioned to you in
many cases it should be considered as a more
electronegative element than the metal. And
because we cannot figure it out the ionic
method always takes it as H minus, and if
it is H minus it is a 2 electron ligand. So,
it is a 2 electron donor then if this is H
minus and if the complex is having no charge
that means something else in the metal complex
should have a charge. So, if H is H minus
then we have to put a plus charge on the manganese
because we know that carbon monoxide is a
neutral ligand.
So, 5 carbon monoxides give you 5 into 2,
10 electrons total and H minus is given 2
electrons and we have removed 1 electron from
the manganese, we have removed 1 electron
from the manganese and manganese had 7 electrons
to begin with, so we end up adding 6 electrons
as a contribution of manganese and the total
turns out to be 18 electrons. We achieved
this 18 electron rule by counting it in this
particular fashion.
Now, let us take a look at the neutral method,
so this is a typographical error, so this
should be the neutral method. The neutral
method, we have manganese as if it is in the
0 oxidation state, it has a electronic configuration
of 3 d 5 4 s 2 which adds up to a total of
7 electrons. So, then you have hydrogen which
we will now consider artificially as if it
is H dot, so that will contribute only 1 electron
and 5 carbon monoxides will give 10 electrons
and the total will again be 18 electrons.
So, you can see that whether you use the neutral
method or the ionic method, you end up with
a total of 18 valence electrons.
Now, let us take another example where there
is a charge on the metal, because of the net
charge on the complex. So, this is a complex
which has got a net negative charge there
is a charge on the complex, which is indicated
as a charge after the square bracket. This
charge adds to the total number of electrons
that are present in the valence shell. So,
carbon monoxide, 3 carbon monoxides are there,
so we end up adding 6 electrons for the same
PF 3 is like PR 3, it would donate 2 electrons,
2 valence electrons on the phosphorous will
be donated to the metal.
Now, we have a net negative charge on the
complex and neither of these 2 ligands systems
are charged and so what we have to do is to
add this extra electron on to the rhodium
which is a metal. So, this negative charge
is added on to the metal and that is why we
consider rhodium as rhodium minus 1. Rhodium
had 9 electrons to start with valence electrons
and with 1 extra electron it becomes a 10
electron species. So, the total adds out to
be 18 valence electrons.
Now, in a similar fashion in the neutral method,
what we do is to take the rhodium which had
9 electrons in the valence shell, PF 3 2 electrons.
And 3 carbon monoxides 6 electrons and because
we had an extra charge on the whole complex
that extra charge is added separately and
as a result we would again end up with 18
electrons. Now, let us just think about this
for the for a moment, if there was a net charge
plus 1 indicated by a plus charge in a metal
complex m, l, n. Then we would have to subtract
1 electron from the valence shell in the neutral
method, in the neutral method we would have
to subtract 1 electron. If there is a negative
charge m, l, n, and then we have a negative
charge then we end up adding this 1 electron.
So, you will it is important for us to note
that both of them come at the same result
as a result of this addition. So, once again
ferrocene, the analog of ferrocene is ruthenocene
that what we have here we have C 5 H 5, which
would donate 5 electrons in the neutral method.
So, 5 into 2, 10 and we have 18 electrons
from ruthenium and it ends up with 18 valence
electrons for the ruthenocene. Similarly,
C 5 H 5 minus would be an anionic ligand.
So, 6 electrons from each C 5 H 5 minus that
gives us 12 plus 6 and that is 18 valence
electrons. So, either method gives us the
same number of valence electrons, but only
thing that we have to remember is that we
have to be consistent as we go through this
electron counting.
So, during the course of this series of lectures
we will always be consistent and either use
the neutral method or the anionic method.
Nitrosyls are 1 example, where the situation
is very complicated and one needs to know
whether the nitrosyl is in a linear fashion
or whether it is in a bent fashion. When it
is in a linear fashion it is close to 180
degrees. The angle is 180 degrees. Then in
the ionic method it is considered as a positive
ligand NO is considered as NO plus and that
is very important. Then it is considered as
a 2 electron donor.
Whereas, in the case of the neutral method
it is considered as NO and then it is considered
as a 2 electron donor, but the net result
would be the same. So, with this we end today’s
lecture, where we were considered the classification
of various ligands and how we look at them
by their hapticity. The hapticity is a number
of carbon atoms which are attached to the
transition metal.
We have also seen that the number of electrons
that are donated can either be based on the
ionic method, which makes carbon hydrogen
and nitrogen and halogens more electronegative.
So, they are assigned negative charges, artificially
given negative charges. In the case of the
neutral method, we consider them as if they
contribute 1 electron each. So, we will continue
with this series of lectures where we will
talk about one ligand after the other starting
with carbon 1 containing systems.
