Bonding in Metal Carbonyls

What do you know about carbonyl ligands? Or is it the first time you

are actually coming across this term? Well, do not worry! We will

help you in that. In this chapter, we will look at the concept of

bonding in various metal carbonyls. However, before we proceed, let

us first look at what carbonyls are?

Brief about Metal Carbonyls

We are aware of many complex compounds having carbonyl ligands,

We call them homoleptic carbonyls. We can, otherwise, call them

metal carbonyls. So, which elements form these carbonyls? Maximum

of the transition metals specialise in forming homoleptic carbonyls or

metal carbonyls. These have simple and very well-defined structures.

For example, their structures are usually tetrahedral, octahedral etc.

In a metal carbonyl, the metal-carbon bond has the characteristics of

both σ and π bonds. The bond between the carbonyl molecule and the

metal becomes stronger by the synergic effect that the metal-ligand

bond produces. We will explain the two types of bonding in metal

carbonyls in the section below.

Structure of Metal Carbonyls

The formation of a metal-carbon σ bond takes place because of the

donation of electrons by the carbonyl molecules. To what do they

donate these electrons? They donate it to the vacant orbitals of the

metal. This is one way of formation of the metal carbonyls.

The other form is the creation of a metal-carbon π bond because of the

donation of a pair of electrons from a filled d orbital metal into the

vacant antibonding π* orbital of carbonyl ligand.

Stability of Carbonyl Compounds

How stable are these coordination compounds? Well, we have

practical evidence that suggests these compounds to break down or

dissociate in various solutions. The stability of a coordination

compound in a solution mainly depends on upon the degree of

association between the two species involved in the state of

equilibrium.

Quantitatively speaking, the stability of any complex is the magnitude

of the equilibrium constant for the formation of the compound. A

general example is given below:

A + 4B → AB4

In the above example, the amount of AB4 molecule in the solution

depends on upon the value of the equilibrium constant, k. This k is

also what we know as the stability constant. On the other hand, the

instability constant or the dissociation constant of complexes is

nothing but the reciprocal of the equilibrium constant of the formation

reaction.

Solved Example for You

Q: Write a note on the physical characteristics of carbonyl

compounds.

Ans:

● Most of these carbonyl compounds are colourless or pale

yellow in colour and are generally volatile in nature.

● We can find them as liquids or solids that are flammable and

toxic. For example, Vanadium hexacarbonyl, which is a stable

17-electron metal carbonyl, is a blue-black solid. It is a unique

compound. Di- and polymetallic carbonyls have deeper colours

as compared to the others. The crystalline metal carbonyls

usually undergo sublimation in the vacuum.

● Metal carbonyls are soluble in both non-polar and polar organic

solvents. These include benzene, diethyl ether, and carbon

tetrachloride. Some salts of metal carbonyls are soluble in

water or lower alcohols.

Crystal Field Theory

We have heard of the Valence bond theory, VSEPR theory and even

Werner’s theory. But what is the Crystal field theory? Sounds new?

Well, it will become familiar once you read this topic. We will cover

all of it. But first, we will start with what the theory is all about!

What is the Crystal Field Theory?

The valence bond theory could not explain the stability of the

coordination compounds. It also failed to throw a light on the

differences between strong and weak ligands. Therefore, scientists

proposed the crystal field theory.

According to this theory, the metal-ligand bond acts as an ionic bond

arising purely from the electrostatic interactions between the metal

ions and ligands. This theory takes anions as point charges and neutral

molecules as dipoles. When the transition metals do not bond to any

ligand, their d orbitals degenerate. This means that they have the same

amount of energy.

When they start bonding with other ligands, the d orbitals split apart

and become non-degenerate. This bonding occurs mainly due to

different symmetries of the d orbitals and the inductive effect of the

ligands on the electrons. The pattern of the splitting of d orbitals

depends on upon the nature of crystal field.

Splitting of Crystal Fields

In case of an octahedral coordination compound, there are six ligands

that surround the metal atom/ion. In these cases, we observe repulsion

between the electrons in d orbitals and ligand electrons. This repulsion

is more in the case of dx2-y2 and dz2 orbitals. This is because they point

towards the axes along the direction of ligand. Hence, their energy is

higher as compared to the average energy in spherical crystal field.

While, dxy, dyzand dxz orbitals experience lower repulsions as they are

directed between the axes. Hence, these three orbitals possess lower

energy than the average energy in spherical crystal field. Thus, we get

two energy levels:

● t2g– set of three orbitals (dxy, dyz and dxz) with lower energy

● eg – set of two orbitals (dx2-y2 and dz2) with higher energy

Crystal Field Splitting

It is the process of the splitting of degenerate level in the presence of

ligand. The difference between the energy of t2g and eg level is

denoted by “Δo”. There are some ligands producing strong fields and

causing large crystal field splitting. On the other hand, others produce

very weak fields.

Thus, the crystal field splitting depends on upon the field produced by

the ligand and the charge on the metal ion. We can arrange ligands in

order of their field strength as:

I– < Br– < Cl– < SCN– < F– < OH– < C2O42-< H2O <NCS– < EDTA4- <

NH3 < en < CN–< CO

Solved Example for You

Q: How does the filling of d-orbitals take place, according to the

crystal field splitting theory?

Ans: Filling of d-orbitals takes place as follows:

The first three electrons arrange in t2g level as per the Hund’s rule.

The fourth electron can either enter into t2g level to result in a

configuration of t2g4eg0 or can enter the eg orbital to give a

configuration of t2g3eg1.

Definition of Some Important Terms Pertaining to Coordination Compounds

To understand the concept of coordination compounds better, it is

important to know certain terms and their definitions. Let’s take a look

at these terms and what they mean in coordination chemistry.

Coordination Chemistry

An assembly where a central atom or ion is bound to a fixed number

of molecules or ions is a coordination entity. For example, in the

coordination entity [CoCl3(NH3)3], the central cobalt ion is bound to

three ammonia molecules and three chloride ions surrounding it. Other

examples in coordination chemistry are [Ni(CO)4], [PtCl2(NH3)2],

[Fe(CN)6]4–, [Co(NH3)6]3+.

Browse more Topics under Coordination Compounds

● Bonding in Metal Carbonyls

● Crystal Field Theory

● Geometric and Optical Isomerism

● Importance and Applications of Coordination Compounds

● Introduction and Werner’s Theory of Coordination Compounds

● Isomerism in Coordination Compounds

● Nomenclature of Coordination Compounds

● Valence Bond Theory in Coordination Compounds

Central Atom/Ion

The atom/ion in a coordination entity that is bound to a fixed number

of ions/groups in a definite geometrical arrangement is the central

atom/ion. For example: In the coordination entities [NiCl2(H2O)4],

[CoCl(NH3)5]2+ and [Fe(CN)6]3–, the central atoms/ions are Ni2+,

Co3+ and Fe3+, respectively. These central atoms/ ions are also Lewis

acids.

Ligands

Ligands are the ions or molecules bound to the central atom/ion in a

coordination entity. These ions may be simple ions like Cl– or small

molecules such as NH3 or H2O. They may also be larger molecules

like H2NCH2CH2NH2 or N(CH2CH2NH2)3 or macromolecules such

as proteins.

Unidentate Ligands

In a lot of cases, only one atom in the ligand is bound to the metal ion.

In these cases, the ligand is unidentate. Examples of unidentate

ligands are H2O, NH3 or Cl–.

Bidentate Ligands

Ligands that bind to the metal ion through two donor atoms are

bidentate or didentate ligands. Examples are H2NCH2CH2NH2

(ethane-1,2-diamine) and C2O42– (oxalate).

Polydentate Ligands

Polydentate ligands are those that can bind to the metal ion through

several donor atoms. Examples are N(CH2CH2NH2)3 and

Ethylenediaminetetraacetate ion (EDTA4–). EDTA is an important

hexadentate ligand that can bind to a central metal ion through two

nitrogen and four oxygen atoms.

Ethylenediaminetetraacetic acid [Source: Wikipedia]

A di or polydentate ligand that uses two or more of its donor atoms to bind to a

single metal ion is a chelate ligand. The number of such donor groups is the

denticity of the ligand. These complexes are chelate complexes and are more

stable than other complexes containing unidentate ligands.

Ambidentate Ligands

Ambidentate ligands can ligate through two different atoms. Examples

are NO2– and SCN– ions where the NO2– ion can ligate with a central

metal atom/ion through nitrogen or oxygen and SCN– can ligate

through sulfur or nitrogen.

More Important Terms in Coordination Chemistry

Coordination Number

The number of ligand donor atoms to which the metal is directly

bound is the coordination number (CN) of the metal ion in the

complex. For example, the CN of Pt and Ni in the complex ions

[PtCl6]2– and [Ni(NH3)4]2+ are 6 and 4, respectively.

Likewise, the CN of Fe and Co in the complex ions [Fe(C2O4)3]3– and

[Co(en)3]3+ is 6 because both C2O42– and en (ethane-1,2-diamine) are

bidentate ligands.

Note: The CN of the central atom/ion is determined only by the

number of sigma bonds between the central atom/ion and the ligand,

not the Pi bonds.

Coordination Sphere

The coordination sphere consists of the central atom/ion and the

ligands attached to it. The ionizable groups, called counter ions are

written outside the bracket. For example, in the complex K4[Fe(CN)6],

[Fe(CN)6]4– is the coordination sphere and K+ is the counter ion.

Coordination Polyhedron

It refers to the geometric pattern or spatial arrangement of the ligands

directly attached to the central atom/ion. The most common polyhedra

are tetrahedral, octahedral and square planar. Examples:

[Co(NH3)6]3+ is octahedral, [Ni(CO)4] is tetrahedral and [PtCl4]2– is

square planar.

Oxidation Number of Central Atom

It is the charge the central atom would carry if all the ligands are

removed along with the electron pairs that are shared with it. It is

represented as the name of the coordination entity followed by the

oxidation number written in Roman numerals in parenthesis. For

example, in [Cu(CN)4]3–, the oxidation number of copper is +1 and is

Cu(I).

Homoleptic and Heteroleptic Complexes

Homoleptic complexes are complexes where a metal is bound to only

one kind of donor group whereas in heteroleptic complexes the metal

is bound to more than one kind of donor group. Examples –

[Co(NH3)6]3+ is homoleptic and [Co(NH3)4Cl2]+ is heteroleptic.

Solved Example on Coordination Chemistry

Question: Match the columns.

Ligand Example

(a) Unidentate (1) EDTA4–

(b) Bidentate (2) SCN–

(c) Polydentate (3) C2O42–

(d) Ambidentate (4) NH3

Solution: a-4, b-3, c-1, d-2.

Geometric and Optical Isomerism

You have come across the concept of allotropes in Chemistry, haven’t

you? Two elements with similar characteristics? In this chapter, we

will cover a concept quite relatable to this. It is the concept of

isomerism. We will also look at coordination number and its relation

to isomerism. We will start with the basic concept of what isomerism

is.

What is Isomerism?

Isomers are compounds having the same chemical formula but

different structural arrangements. We find a number of isomers due to

the variety of bond types and complicated formulae of many

coordination compounds. In the below chapter, we will discuss the

types of isomerism that these compounds exhibit.

1) Optical Isomerism

Optical isomers are those two compounds with the same chemical

formula such that their mirror images are not superimposable on each

other. Depending on the direction they rotate the plane of polarised

light in a polarimeter, we have two forms of optical isomers. They are:

● Dextro Isomer: This rotates to right.

● Laevo Isomer: This rotates to left.

Let us now look at the concept of geometric isomerism.

2) Geometric Isomerism

In case of disubstituted complexes, the substituted groups could either

be adjacent or opposite to each other. This leads to geometric

isomerism. Thus, we can prepare square planar complexes such as

[Pt(NH3)4Cl2] in two forms, cis and trans. When the chlorine atoms

are adjacent to each other, we get the cis form. While when two

chlorine atoms are opposite, we get the trans-form.

More about Geometric Isomerism and Coordination Number

● We find this type of isomerism in heteroleptic complexes. The

main reason behind this would be the multiple geometric

arrangements of ligands around the central metal atom.

● Coordination compounds with the coordination number of 4

and 6 mainly exhibit this type of isomerism.

● Square planar complexes are coordination compounds with

coordination number 4 having [MX2L2] type formula, where X

and L are unidentate ligands. The two ligands X could either be

adjacent to each other in a cis isomer or opposite to each other

to form a trans isomer.

Square planar complexes with MABXL type formula show

three isomers-two cis and one trans.

● Tetrahedral geometry does not display these isomers. However,

octahedral complexes do show cis and trans isomerism. In

complexes with formula [MX2L4] type, we can have the X

ligands in the arrangement of cis or trans to each other.

● We also observe this type of isomerism when bidentate ligands

L–L [e.g., NH2 CH2 CH2 NH2 (en)] are present in complexes

with [MX2(L–L)2] type formula.

● There is another type of geometrical isomerism that we find in

octahedral coordination entities with [Ma3b3] type formula. An

example is [Co(NH3)3(NO2)3].

Solved Example for You

Q: What are facial and meridional isomers?

Ans: Facial isomers are those where the three donor atoms of the same

ligands occupy adjacent positions at the corners of an octahedral face.

They have the ligands in the cis arrangement. We get the meridional

isomers when the positions of the ligands are around the meridian of

the octahedron. One pair is in trans arrangement here.

Importance and Applications of Coordination Compounds

So, why are we discussing so much about coordination compounds? Is

it just in theory or does it have any applications? Well, you would be

surprised to know how important these coordination compounds are in

real life. In this chapter, we will look at the importance and

applications of coordination compounds. We will look at the practical

applications of these important compounds.

Review of Organic Compounds

Coordination compounds are a class of compounds that we know as

the complex compounds. This is because of the chemistry that

involves these compounds. We have known enough about these

compounds already, including their structures and isomers etc.

We know that transition metals have this special property of forming

coordination complexes. This is due to the high charge to mass ratio

and also the availability of d-orbitals. The advances in coordination

chemistry provide various complex compounds that we use in various

industries. Coordination compounds are a common application in

various industries. These include mining & metallurgy, medical

sciences etc. to name a few.

Browse more Topics under Coordination Compounds

● Bonding in Metal Carbonyls

● Crystal Field Theory

● Definition of Some Important Terms Pertaining to

Coordination Compounds

● Geometric and Optical Isomerism

● Introduction and Werner’s Theory of Coordination Compounds

● Isomerism in Coordination Compounds

● Nomenclature of Coordination Compounds

● Valence Bond Theory in Coordination Compounds

Examples and Types

Many of the biological compounds are coordination complexes. You

surely know of haemoglobin, chlorophyll, and vitamin B-12. Don’t

you? What do you think these are? These are nothing but complex

compounds.

There are numerous other coordination compounds that play an

important role in biological processes. Our body produces and

consumes many complex compounds during these physiological

processes.

Photosynthesis in plants requires chlorophyll for the process. This

chlorophyll is a magnesium-porphyrin complex. Many enzymes that

catalyse the life processes within our body are coordination

complexes. One such example is that of carboxypeptidase. It is a

coordination compound acting as an enzyme. It is necessary for

catalysing the process of digestion.

Applications of Coordination Compounds

● Coordination compounds have specific colours. Therefore, they

find a common place in industries for intense colourations.

Phthalocyanine is a class of coordination complexes that the

dyes and pigments industry extensively use. They use it to

impart specific colouration to fabrics.

● Some of the cyanide complexes find their use for electroplating

a protective layer on surfaces. There are complexes that find

the application of coordination compounds in photography.

● EDTA is another complex compound we use for the

determination of hardness of water. Uses of coordination

compounds also involve their application as catalysts. These

days, they are becoming increasingly popular in the polymer

industry as well.

● We apply the concept of coordination compounds in the

extraction of metals from their ores too frequently these days.

Extraction of nickel and cobalt involves uses a major use of

these compounds. These metals are extracted by

hydro-metallurgical processes requiring a lot of complex ions.

● As more and more coordination compounds are getting

synthesised, scientists and engineers are now having a wide

range of options for improving and optimising the processes

that require them.

Solved Example for You

Q: Why do we use coordination compounds to separate metals in

extractive metallurgy?

Ans: We generally use these compounds in the separation of metals

during the process of extractive metallurgy. This is because these

complex ions possess this specific property of selective precipitation

and solubility.

Introduction and Werner’s Theory of Coordination Compounds

Who is the father of coordination chemistry? Yes! The man in

question is none other than Werner. He is famous for his Werner’s

theory of coordination compounds. Are you aware of what this theory

is? In this chapter, we will study all about the theory and look at its

postulates and examples. 

What is Werner’s Theory?

In 1823, Werner put forth this theory to describe the structure and

formation of complex compounds or coordination compounds. It is

because of this theory that he got the Nobel prize and is known as the

father of coordination chemistry. Are you ready to learn the important

postulates of this theory?

Browse more Topics under Coordination Compounds

● Bonding in Metal Carbonyls

● Crystal Field Theory

● Definition of Some Important Terms Pertaining to

Coordination Compounds

● Geometric and Optical Isomerism

● Importance and Applications of Coordination Compounds

● Isomerism in Coordination Compounds

● Nomenclature of Coordination Compounds

● Valence Bond Theory in Coordination Compounds

Postulates of Werner’s Theory

The important postulates of Werner’s theory are:

● The central metal or the metal atoms in coordination

compounds show two types of valency. They are the primary

and secondary valency.

● The primary valency relates to the oxidation state and the

secondary valency relates to the coordinate number.

● The number of secondary valences is fixed for every metal

atom. It means that the coordination number is fixed.

● The metal atom works towards satisfying both its primary and

secondary valencies. A negative ion satisfies the primary

valency. On the other hand, a negative ion or neutral molecules

satisfy secondary valencies.

● The secondary valencies point towards a fixed position in

space. This is the reason behind the definite geometry of the

coordinate compound. For example, let us consider the case of

a metal ion having six secondary valencies. These arrange

octahedrally around the central metal ion. If the metal ion has

four secondary valencies, these arrange in either tetrahedral or

square planar arrangement around the central metal ion.

Therefore, we see that the secondary valency determines the

stereochemistry of the complex ion. On the other hand, the

primary valency is non-directional.

Learn the Nomenclature of Coordination Compounds here.

Examples Based on Postulates of Werner’s Theory

Werner’s theory is responsible for the formation of structures of

various cobalt amines. We will look at its explanation now. Cobalt has

a primary valency (oxidation state) of three and exhibits secondary

valency (coordination number) of 6. We represent the secondary

valencies by thick lines and the primary valency by broken lines.

1) CoCl3.6NH3 Complex: In this compound, the coordination number

of cobalt is 6 and NH3 molecules satisfy all the 6 secondary valencies.

Chloride ions satisfy the 3 primary valencies. These are

non-directional in character. These chloride ions instantaneously

precipitate on the addition of silver nitrate. The total number of ions,

in this case, is 4, three chloride ions and one complex ion.

2) CoCl3.5NH3 complex: In this compound, cobalt has the

coordination number of 6. However, we see that the number of

NH3molecule decreases to 5. The chloride ion occupies the remaining

one position. This chloride ion exhibits the dual behaviour as it has

primary as well as secondary valency.

3) CoCl3.4NH3 complex: In this compound, two chloride ions exhibit

the dual behaviour of satisfying both Primary and Secondary

Valencies. This compound gives a precipitate with silver nitrate

corresponding to only one Cl– ion and the total number of ions, in this

case, is 2. Hence, we can formulate it as [CoCl2(NH3)4]Cl.

Werner’s Theory and Isomerism

Werner turned his attention towards the geometrical arrangements of

the coordinated groups around the central cation. He was successful in

explaining the cause behind optical and geometrical isomerism of

these compounds. Some examples are as follows:

1) [CoCl2(NH3)4]Cl complex: According to Werner, there are three

structures possible for this complex. These are planar, trigonal prism,

octahedral. The number of possible isomers is 3 for planar, 3 for

trigonal prism and 2 for octahedral structure.

However, as we could isolate only two isomers of the compound, he

concluded that geometrical arrangement of the coordinated group

around the central atom in this compound was octahedral. In the case

of several other complexes in which the coordination number of the

central atom was six, Werner was of the opinion that in all these cases

the six coordinated complex have octahedral geometry.

He also read the geometry of the complexes where the coordination

number of the central metal atom is 4. He gave two possible structures

for such compounds: Square Planar and Tetrahedral. Let us look at an

example of the same.

2) [PtCl2(NH3)2] complex: In this complex, the coordination number

of the metal is 4. According to Werner, this complex exists in two

isomeric forms, cis and trans. This shows that all the four ligands lie in

the same plane. Therefore, the structure should be a square planar or

tetrahedral.

Learn more about Isomerism and its types.

Solved Example for You

Q: What are the limitations of Werner’s Theory?

Ans: Like all the major theories, Werner’s Theory was not free from

limitations. The common limitations of the theory are:

● It could not explain the inability of all elements to form

coordination compounds.

● The Werners theory could not explain the directional properties

of bonds in various coordination compounds.

● It does not explain the colour, the magnetic and optical

properties shown by coordination compounds.

Q. What is IUPAC name of [Fe(NH3)4O2C2O4]Cl?

Learn the Nomenclature of Coordination Compound to find the

answer.

Isomerism in Coordination Compounds

Certain compounds such as coordinates have an identical formula but

different properties and different structure. Even though they have the

same formula they carry different color. You might be wondering

which are these compounds. To know more about isomerism in

coordination compounds, let’s dig deep into this interesting topic in

the section below.

Isomerism in Coordination Compounds

What are Ligands?

A metal ion in a solution cannot exist in isolation but in combination

with ligands such as solvent molecules or simple ions that form

complex ions or coordination compounds. These complexes have a

central atom or ion which is often a transition metal.

These complexes also have a cluster of ion or neutral molecules

surrounding ligands that bond with a coordinated bond to a central

metal atom or ion. Ligands act as Lewis bases (electron pair donor) or

as Lewis acids (electron pair acceptor).

Complex Ions

The ability of metallic elements to act as Lewis acids that form metal

complexes with a variety of Lewis bases is their most important

property. A metal complex has a central metal atom or ion that bonds

to one or more ligands. These are the complex ions. A coordinate

compound contains one or more metal complexes.

Why are Coordination Compounds Important?

Mentioned below are three reasons explaining why coordination

compounds are essential.

● The periodic table has most of the elements that are metals, and

almost all metals form complexes.

● Many industrial catalysts are metal complexes and important to

control reactions.

● Transition metal complexes are essential in biochemistry.

Isomers

Isomers are two or more different compounds having the same

formula. There are two principal types of isomerism in coordination

compounds:

1. Stereo isomerism in coordination compounds

2. Structural isomerism in coordination compounds

Coordination compounds

What are Stereoisomers?

Stereoisomers are the isomers that have the same atoms, the same set

of bonds that differ only in the relative orientation of these bonds.

These are subdivided as follows:

● Geometrical isomerism: Geometrical isomers are possible for

square, planar, and octahedral complexes, but not tetrahedral.

● Optical isomerism: This isomerism is possible for both

tetrahedral and octahedral complexes, but not for square and

planar.

Source: Wikipedia

What are Structural Isomers?

Structural isomers are the isomers that have the same molecular

formula, that differ in the bonding patterns and atomic organization.

Structural isomerism is subdivided as below:

Source: Wikipedia

Coordination Isomerism

Here the compounds contain complex anionic and cationic parts

occurring by the interchange of some ligands from the cationic part to

the anionic part. For example,

[Co(NH3)6] [Cr(C2O4)3] isomer 1

[Co(C2O4)3] [Cr(NH3)6] isomer 2

Ionization Isomerism

Here, isomers occur because of the formation of different ions in a

solution. For example,

[PtBr(NH3)3]O2NO2 anion in solution

[Pt(NO2)(NH3)]Br anion in solution

However, both anions are necessary to balance the charge of the

complex. Moreover, the difference in both isomers is one ion directly

attaches to the central metal but the other does not.

Hydrate Isomerism

A well-known and the best example of this isomer is chromium

chloride, Cr Cl3.6H2O, which may contain 4, 5, or 6 co-ordinate

water molecules.

[CrCl2(H2O)4]Cl.H2O bright green

CrCl(H2O)5]Cl2.H2O grey-green

Cr(H2O)6]Cl3 violet

Solved Question For You

Question: What are the applications of coordination compounds?

Ans: Coordination compounds have the following applications.

● Dyes and pigments: Ancient Greeks used a red madder dye for

various purposes. A recent example is copper phthalocyanine,

which is blue.

● Analytical chemistry: Some coordination compounds are used

as reagents in the laboratory for color tests, gravimetric

analysis and complexometric titration and masking agents.

● Sequestering agents: We can get rid of objectionable ions in

industrial processes with coordination compounds.

● Extraction of metals: Leaching of metals is done from their

ores by the formation of stable complexes. For example, Ag

and An as complexes of cyanide ions.

● Bio-inorganic chemistry: Naturally occurring complexes are

hemoglobin, chlorophyll, vitamin B12, etc.

Nomenclature of Coordination Compounds

Just like we know that nomenclature of organic compounds is

important, naming the various coordination or complex compounds is

also very important. Complex nomenclature is an extensive study and

as chemistry students, you must be well-versed with the same. In this

chapter, we will see how the various complex compounds get their

names. We will look at the few simple rules to follow for the same.

Complex Nomenclature

Why do we need to name the compounds? How would your teacher

call you and your best friend if you both didn’t have any names?

Similarly, the naming of coordination compounds(complex

nomenclature) is important to provide an unambiguous method to

represent and describe formulas and names of coordination

compounds systematically.

Also, it becomes very important while you deal with isomers. We

follow a few rules of the International Union of Pure and Applied

Chemistry (IUPAC) system while naming these compounds. In a

nutshell, the rules are as follows:

In Coordination Compounds,

● Within the coordination entities, we list down the central

atom/ion first followed by the ligands.

● After this, we list down the ligands in alphabetical order.

● We write the formula of coordination entity in square brackets.

● Ligand abbreviations are to be enclosed in parentheses.

● We must not keep any space between the ligands and the metal

within a coordination sphere.

● The charge on the cation(s) should be equal to the charge of the

anion(s).

Now, let us look at these rules of complex nomenclature in greater

details, along with suitable examples.

Browse more Topics under Coordination Compounds

● Bonding in Metal Carbonyls

● Crystal Field Theory

● Definition of Some Important Terms Pertaining to

Coordination Compounds

● Geometric and Optical Isomerism

● Importance and Applications of Coordination Compounds

● Introduction and Werner’s Theory of Coordination Compounds

● Isomerism in Coordination Compounds

● Valence Bond Theory in Coordination Compounds

Rules of Complex Nomenclature

Rule 1

While naming a coordination compound, we always name the cation

before the anion. This rule does not count the fact of whether the

complexion is cation or anion. Let us understand this with the help of

an example.

Na[Co(NH)4 (Cl)2] → Na is to be named first followed by [Co(NH)4

(Cl)2]

Sodium tetramminedichlorocoblatate(I)

[Co(NH)4 (Cl)2]SO4 → [Co(NH)4 (Cl)2] is to be named first

followed by SO4

Tetraamminedichlorocobalt(0) sulphate.

Rule 2

When we see that there are multiple types of ligands present in any

coordination compound, we name the ligands in alphabetic order after

by the name of central metal atom/ion.

● Name of the anionic ligands ends with ‘o’.

For example, Chloride → Chlorido, Nitrate → Nitrito

● For neutral ligands, their common name is used as such e.g.

H2NCH2CH2NH2 → ethylenediamine

H2O → aqua

NH3 → ammine

CO → carbonyl

N2 → dinitrogen

O2 → dioxygen

Rule 3

If the names of the ligands have a numerical prefix, then we use the

terms like bis, tris, tetrakis. This will represent the ligand to which

they refer being placed in parentheses. For example, we name

[NiCl2(PPh3)2] as dichloridobis(triphenylphosphine)nickel(II)

Rule 4

After naming the ligand in alphabetic order, we name the central metal

atom/ion.

● If the complex ion is a cation, we name the metal same as the

element.

● If the complex is an anion, the name of metal ends with the

suffix -ate for Latin name.

Rule 5

We give the oxidation state of the metal in the complex as a roman

numeral in parentheses.

Rule 6

We name the neutral complex molecule similar to that of the complex

cation.

Rule 7

There are some ligands like NO2, CN that are attached to the central

metal atom/ion through different atoms. We name them as follows:

Thus, M-NO2 → nitro

M-ONO → nitrito

M-SCN → thiocyanato

M-NCS → isothiocyanato

Solved Example for You

Q: Write the IUPAC name of [Fe(NH3)4O2C2O4]Cl.

Ans: In this complex part has a charge of +1. The ligand oxalato has a

charge of –2, so iron should be in +3 state meaning O2 to be neutral.

O2 behaves as a neutral ligand and the IUPAC name is

Tetraammineoxalatodioxygeniron (III) chloride.

Valence Bond Theory in Coordination Compounds

You must have studied that Werner’s theory for coordination

compounds failed. Do you remember why Werner’s theory for

coordination compounds failed? Well, it failed to explain many

critical aspects of valence electrons and directions in the coordination

compounds.

Then, how do we explain the structure of all these coordination

compounds? Yes, we use the Valence bond theory which came on to

replace the Werners theory. In this chapter, we will look at the valence

bond theory and its important postulates. In the end, we will also look

at the imitations of the valence bond theory.

What is the Valence Bond Theory?

One of the major drawbacks of Werner’s theory is its failure to

explain the coordination compounds’ directional properties. Valence

bond theory became successful in explaining the structure and bond

linkages in these coordination compounds.

So, what does the valence bond theory say? According to this theory,

the metal atom or ion under the influence of ligands can use its (n-1)d,

ns, np, nd orbitals for hybridisation. This would lead to a yield a set of

equivalent orbitals of definite geometry.

These include octahedral, tetrahedral, square planar and other such

geometrical arrangements. The hybridised orbitals can overlap with

the ligand orbitals. These orbitals are ready to donate valence

electrons / electron pairs for bonding.

Explanation of Valence Electrons in Valence Bond Theory

We will understand the valence bond theory with the help of an

example. Let us consider the diamagnetic octahedral complex

[Co(NH3)6]3+. Here, the cobalt ion has the electronic configuration of

3d6. The hybridization scheme is as follows,

Orbitals of Co+3ion:

d2sp3 hybridised orbitals of Co3+ is as follows,

d2sp3 hybrid,

We can see that the compound does not contain any unpaired valence

electrons. Therefore, it is diamagnetic in nature. All the six pairs of

electrons from NH3 molecules occupy the six hybridized orbitals.

Since, the inner d orbital (3d) takes place in hybridisation, the

complex, [Co (NH3)6]3+ is what we call the inner orbital or low spin

or spin paired complex. The paramagnetic octahedral complex

generally uses outer orbital (4d) in hybridisation (sp3d2). It is known

as outer orbital or high spin or spin-free complex.

Browse more Topics under Coordination Compounds

● Bonding in Metal Carbonyls

● Crystal Field Theory

● Definition of Some Important Terms Pertaining to

Coordination Compounds

● Geometric and Optical Isomerism

● Importance and Applications of Coordination Compounds

● Introduction and Werner’s Theory of Coordination Compounds

● Isomerism in Coordination Compounds

● Nomenclature of Coordination Compounds

Solved Example for You

Q: State the limitations of the valence bond theory.

Ans: The limitations of the valence bond theory are as follows:

● With the valence bond theory, we can not have a quantitative

interpretation of magnetic data. This is one major drawback of

the valence bond theory.

● The valence bond theory fails to explain the various colors that

the coordination compounds exhibit. This is also relatable to

the Werner theory.

● The valence bond theory does not give a quantitative

interpretation of the thermodynamic stabilities of the

coordination compounds. It does not also speak of the kinetic

stabilities of these compounds as well.

● The various predictions that the valence bond theory makes

regarding the tetrahedral and square planar structures of

4-coordinate complexes are not completely accurate.

● It does not draw any distinctive line between the weak and

strong ligands.