In the previous Unit we learnt that the transition metals

the metal atoms are bound to a number of anions or

neutral molecules. In modern terminology such

chemistry of coordination compounds is an important

and challenging area of modern inorganic chemistry.

New concepts of chemical bonding and molecular

structure have provided insights into the functioning of

vital components of biological systems. Chlorophyll,

compounds of magnesium, iron and cobalt respectively.

Variety of metallurgical processes, industrial catalysts

and analytical reagents involve the use of coordination

compounds. Coordination compounds also find many

applications in electroplating, textile dyeing and

medicinal chemistry.

After studying this Unit, you will be able to

? appreciate the postulates of

Werner?s theory of coordination

compounds;

? know the meaning of the terms:

coordination entity, central atom/

ion, ligand, coordination number,

coordination sphere, coordination

polyhedron, oxidation number,

homoleptic and heteroleptic;

? learn the rules of nomenclature

of coordination compounds;

? write the formulas and names

of mononuclear coordination

compounds;

? define different types of isomerism

in coordination compounds;

? understand the nature of bonding

in coordination compounds in

terms of the Valence Bond and

Crystal Field theories;

? learn the stability of coordination

compounds;

? appreciate the importance and

applications of coordination

compounds in our day to day life.

Objectives

Coordination

Compounds

his ideas about the structures of coordination compounds. He prepared

and characterised a large number of coordination compounds and

studied their physical and chemical behaviour by simple experimental

series of compounds of cobalt(III) chloride with ammonia, it was found

that some of the chloride ions could be precipitated as AgCl on adding

excess silver nitrate solution in cold but some remained in solution.

9.1 Werner?s

Theory of

Coordination

Compounds

Unit

9

Chemistry 238

These observations, together with the results of conductivity

measurements in solution can be explained if (i) six groups in all,

either chloride ions or ammonia molecules or both, remain bonded to

the cobalt ion during the reaction and (ii) the compounds are formulated

as shown in Table 9.1, where the atoms within the square brackets

form a single entity which does not dissociate under the reaction

number of groups bound directly to the metal ion; in each of these

examples the secondary valences are six.

Note that the last two compounds in Table 9.1 have identical empirical

termed as isomers. Werner in 1898, propounded his theory of

coordination compounds. The main postulates are:

(valences)-primary and secondary.

negative ions.

neutral molecules or negative ions. The secondary valence is equal to

the coordination number and is fixed for a metal.

characteristic spatial arrangements corresponding to different

coordination numbers.

In modern formulations, such spatial arrangements are called

coordination entities or complexes and the ions outside the square

bracket are called counter ions.

He further postulated that octahedral, tetrahedral and square planar

geometrical shapes are more common in coordination compounds of

square planar, respectively.

239 Coordination Compounds

(i) Secondary 4 (ii) Secondary 6

(iii) Secondary 6 (iv) Secondary 6 (v) Secondary 4

On the basis of the following observations made with aqueous solutions,

assign secondary valences to metals in the following compounds:

Solution

Both double salts as well as complexes are formed by the combination

of two or more stable compounds in stoichiometric ratio. However, they

dissociate into simple ions completely when dissolved in water. However,

a small community in the French province of Alsace.

His study of chemistry began in Karlsruhe (Germany)

and continued in Zurich (Switzerland), where in his

doctoral thesis in 1890, he explained the difference in

properties of certain nitrogen containing organic

substances on the basis of isomerism. He extended vant

Hoff?s theory of tetrahedral carbon atom and modified

it for nitrogen. Werner showed optical and electrical differences between

complex compounds based on physical measurements. In fact, Werner was

the first to discover optical activity in certain coordination compounds.

He, at the age of 29 years became a full professor at Technische

Hochschule in Zurich in 1895. Alfred Werner was a chemist and educationist.

His accomplishments included the development of the theory of coordination

compounds. This theory, in which Werner proposed revolutionary ideas about

how atoms and molecules are linked together, was formulated in a span of

only three years, from 1890 to 1893. The remainder of his career was spent

gathering the experimental support required to validate his new ideas. Werner

became the first Swiss chemist to win the Nobel Prize in 1913 for his work

on the linkage of atoms and the coordination theory.

(1866-1919)

Example 9.1

Chemistry 240

A coordination entity constitutes a central metal atom or ion bonded

is a coordination entity in which the cobalt ion is surrounded by

three ammonia molecules and three chloride ions. Other examples

In a coordination entity, the atom/ion to which a fixed number

of ions/groups are bound in a definite geometrical arrangement

around it, is called the central atom or ion. For example, the

The ions or molecules bound to the central atom/ion in the

coordination entity are called ligands. These may be simple ions

such as proteins.

When a ligand is bound to a metal ion through a single donor

When a ligand can bind through two donor atoms as in

an important hexadentate ligand. It can bind through two

nitrogen and four oxygen atoms to a central metal ion.

When a di- or polydentate ligand uses its two or more donor

ligand. Such complexes, called chelate complexes tend to be more

stable than similar complexes containing unidentate ligands (for

reasons see Section 9.8). Ligand which can

ligate through two different atoms is called

can coordinate either through nitrogen or

through oxygen to a central metal atom/ion.

sulphur or nitrogen atom.

The coordination number (CN) of a metal ion in a complex can be

defined as the number of ligand donor atoms to which the metal is

9.2 Definitions of

Some

Important

Terms

Pertaining to

Coordination

Compounds

241 Coordination Compounds

It is important to note here that coordination number of the central

atom/ion is determined only by the number of sigma bonds formed by

the ligand with the central atom/ion. Pi bonds, if formed between the

ligand and the central atom/ion, are not counted for this purpose.

The central atom/ion and the ligands attached to it are enclosed in

The spatial arrangement of the ligand atoms which are directly

attached to the central atom/ion defines a coordination

polyhedron about the central atom. The most common

coordination polyhedra are octahedral, square planar and

shapes of different coordination polyhedra.

9.3 Nomenclature

of

Coordination

Compounds

The oxidation number of the central atom in a complex is defined

as the charge it would carry if all the ligands are removed along

with the electron pairs that are shared with the central atom. The

oxidation number is represented by a Roman numeral in parenthesis

following the name of the coordination entity. For example, oxidation

Complexes in which a metal is bound to only one kind of donor

which a metal is bound to more than one kind of donor groups,

Nomenclature is important in Coordination Chemistry because of the

need to have an unambiguous method of describing formulas and

writing systematic names, particularly when dealing with isomers. The

formulas and names adopted for coordination entities are based on the

recommendations of the International Union of Pure and Applied

Chemistry (IUPAC).

Chemistry 242

The formula of a compound is a shorthand tool used to provide basic

information about the constitution of the compound in a concise and

convenient manner. Mononuclear coordination entities contain a single

central metal atom. The following rules are applied while writing the formulas:

(i) The central atom is listed first.

(ii) The ligands are then listed in alphabetical order. The placement of

a ligand in the list does not depend on its charge.

(iii) Polydentate ligands are also listed alphabetically. In case of

abbreviated ligand, the first letter of the abbreviation is used to

determine the position of the ligand in the alphabetical order.

(iv) The formula for the entire coordination entity, whether charged or

not, is enclosed in square brackets. When ligands are polyatomic,

their formulas are enclosed in parentheses. Ligand abbreviations

are also enclosed in parentheses.

(v) There should be no space between the ligands and the metal

within a coordination sphere.

(vi) When the formula of a charged coordination entity is to be written

without that of the counter ion, the charge is indicated outside the

square brackets as a right superscript with the number before the

(vii) The charge of the cation(s) is balanced by the charge of the anion(s).

The names of coordination compounds are derived by following the

principles of additive nomenclature. Thus, the groups that surround the

central atom must be identified in the name. They are listed as prefixes

to the name of the central atom along with any appropriate multipliers.

The following rules are used when naming coordination compounds:

(i) The cation is named first in both positively and negatively charged

coordination entities.

(ii) The ligands are named in an alphabetical order before the name of the

central atom/ion. (This procedure is reversed from writing formula).

(iii) Names of the anionic ligands end in ?o, those of neutral and cationic

carbonyl for CO and nitrosyl for NO. These are placed within

enclosing marks ( ).

(iv) Prefixes mono, di, tri, etc., are used to indicate the number of the

individual ligands in the coordination entity. When the names of

dichlorobis(triphenylphosphine)nickel(II).

(v) Oxidation state of the metal in cation, anion or neutral coordination

entity is indicated by Roman numeral in parenthesis.

(vi) If the complex ion is a cation, the metal is named same as the

element. For example, Co in a complex cation is called cobalt and

Pt is called platinum. If the complex ion is an anion, the name of

the metal ends with the suffix ? ate. For example, Co in a complex

243 Coordination Compounds

(vii) The neutral complex molecule is named similar to that of the

complex cation.

The following examples illustrate the nomenclature for coordination

compounds.

triamminetriaquachromium(III) chloride

a cation. The amine ligands are named before the aqua ligands

according to alphabetical order. Since there are three chloride ions in

the compound, the charge on the complex ion must be +3 (since the

compound is electrically neutral). From the charge on the complex

ion and the charge on the ligands, we can calculate the oxidation

number of the metal. In this example, all the ligands are neutral

molecules. Therefore, the oxidation number of chromium must be

the same as the charge of the complex ion, +3.

tris(ethane-1,2?diammine)cobalt(III) sulphate

Since it takes 3 sulphates to bond with two complex cations, the

charge on each complex cation must be +3. Further, ethane-1,2?

diamine is a neutral molecule, so the oxidation number of cobalt

diamminesilver(I) dicyanoargentate(I)

Write the formulas for the following coordination compounds:

(i) Tetraamineaquachloridocobalt(III) chloride

(ii) Potassium tetrahydroxozincate(II)

(iii) Potassium trioxalatoaluminate(III)

(iv) Dichloridobis(ethane-1,2-diamine)cobalt(III)

(v) Tetracarbonylnickel(0)

Write the IUPAC names of the following coordination compounds:

(i) Diamminechloridonitrito-N-platinum(II)

(ii) Potassium trioxalatochromate(III)

(iii) Dichloridobis(ethane-1,2-diamine)cobalt(III) chloride

(iv) Pentaamminecarbonatocobalt(III) chloride

(v) Mercury tetrathiocyanatocobaltate(III)

Example 9.2

Solution

Example 9.3

Solution

Chemistry 244

Intext Questions

(i) Tetraamminediaquacobalt(III) chloride

(ii) Potassium tetracyanonickelate(II)

(iii) Tris(ethane?1,2?diamine) chromium(III) chloride

(iv) Amminebromidochloridonitrito-N-platinate(II)

(v) Dichloridobis(ethane?1,2?diamine)platinum(IV) nitrate

(vi) Iron(III) hexacyanoferrate(II)

Isomers are two or more compounds that have the same chemical

formula but a different arrangement of atoms. Because of the different

arrangement of atoms, they differ in one or more physical or chemical

properties. Two principal types of isomerism are known among

coordination compounds. Each of which can be further subdivided.

(i) Geometrical isomerism (ii) Optical isomerism

(i) Linkage isomerism (ii) Coordination isomerism

(iii) Ionisation isomerism (iv) Solvate isomerism

Stereoisomers have the same chemical formula and chemical

bonds but they have different spatial arrangement. Structural isomers

have different bonds. A detailed account of these isomers are

given below.

This type of isomerism arises in heteroleptic

complexes due to different possible geometric

arrangements of the ligands. Important examples of

this behaviour are found with coordination numbers

4 and 6. In a square planar complex of formula

as depicted in Fig. 9.2.

Other square planar complex of the type

MABXL (where A, B, X, L are unidentates)

You may attempt to draw these structures.

Such isomerism is not possible for a tetrahedral

geometry but similar behaviour is possible in

9.4 Isomerism in

Coordination

Compounds

Co

Cl

+

Co

Cl

Cl

+

245 Coordination Compounds

This type of isomerism also

arises when didentate ligands

are present in complexes of formula

Another type of geometrical

isomerism occurs in octahedral

coordination entities of the type

three donor atoms of the same

ligands occupy adjacent positions

at the corners of an octahedral

around the meridian of the

Why is geometrical isomerism not possible in tetrahedral complexes

having two different types of unidentate ligands coordinated with

the central metal ion ?

Tetrahedral complexes do not show geometrical isomerism because

the relative positions of the unidentate ligands attached to the central

metal atom are the same with respect to each other.

Solution

Optical isomers are mirror images that

cannot be superimposed on one

another. These are called as

that cannot be superimposed are

upon the direction they rotate the

plane of polarised light in a

the left). Optical isomerism is common

in octahedral complexes involving

didentate ligands (Fig. 9.6).

In a coordination

entity of the type

activity (Fig. 9.7).

Example 9.4

Chemistry 246

Linkage isomerism arises in a coordination compound containing

ambidentate ligand. A simple example is provided by complexes

nitrogen to give M?NCS or through sulphur to give M?SCN. Jørgensen

obtained as the red form, in which the nitrite ligand is bound through

oxygen (?ONO), and as the yellow form, in which the nitrite ligand is

This type of isomerism arises from the interchange of ligands between

cationic and anionic entities of different metal ions present in a complex.

This form of isomerism arises when the counter ion in a complex salt

is itself a potential ligand and can displace a ligand which can then

become the counter ion. An example is provided by the ionisation

Out of the following two coordination entities which is chiral

(optically active)?

The two entities are represented as

Solution

Example 9.5

Solution

Example 9.6

247 Coordination Compounds

This form of isomerism is known as ?hydrate isomerism? in case where

water is involved as a solvent. This is similar to ionisation isomerism.

Solvate isomers differ by whether or not a solvent molecule is directly

bonded to the metal ion or merely present as free solvent molecules

in the crystal lattice. An example is provided by the aqua

(grey-green).

Intext Questions

draw the structures for these isomers:

isomers.

Werner was the first to describe the bonding features in coordination

compounds. But his theory could not answer basic questions like:

(i) Why only certain elements possess the remarkable property of

forming coordination compounds?

(ii) Why the bonds in coordination compounds have directional

properties?

(iii) Why coordination compounds have characteristic magnetic and

optical properties?

Many approaches have been put forth to explain the nature of

Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular

Orbital Theory (MOT). We shall focus our attention on elementary

treatment of the application of VBT and CFT to coordination compounds.

According to this theory, the metal atom or ion under the influence of

to yield a set of equivalent orbitals of definite geometry such as octahedral,

tetrahedral, square planar and so on (Table 9.2). These hybridised orbitals

are allowed to overlap with ligand orbitals that can donate electron pairs

for bonding. This is illustrated by the following examples.

9.5 Bonding in

Coordination

Compounds

Chemistry 248

2+

hybridised

orbitals of Ni

[NiCl ]

(high spin complex)

4

2?

Four pairs of electrons

It is usually possible to predict the geometry of a complex from

the knowledge of its

magnetic behaviour on

the basis of the valence

bond theory.

In the diamagnetic

octahedral complex,

is in +3 oxidation state

and has the electronic

hybridisation scheme is as

shown in diagram.

3+

hybridised

orbitals of Co

[Co(NH ) ]

(inner orbital or

low spin complex)

3 6

3+

Six pairs of electrons

3+

hybridised

orbitals of Co

[CoF ]

(outer orbital or

high spin complex)

6

3?

Six pairs of electrons

hybrid orbitals. Thus, the complex has octahedral geometry and is

diamagnetic because of the absence of unpaired electron. In the formation

In tetrahedral complexes

are hybridised to form four

equivalent orbitals oriented

tetrahedrally. This is illustrated

Here nickel is in +2

oxidation state and the ion

has the electronic

hybridisation scheme is as

shown in diagram.

paramagnetic since it contains two unpaired electrons. Similarly,

zero oxidation state and contains no unpaired electron.

249 Coordination Compounds

dsp hybridised

orbitals of Ni

2

2+

[Ni(CN) ]

(low spin complex)

4

2?

Four pairs of electrons

in diagram:

Each of the hybridised orbitals receives a pair of electrons from a

cyanide ion. The compound is diamagnetic as evident from the absence

of unpaired electron.

It is important to note that the hybrid orbitals do not actually exist.

In fact, hybridisation is a mathematical manipulation of wave equation

for the atomic orbitals involved.

The magnetic moment of coordination compounds can be measured

by the magnetic susceptibility experiments. The results can be used to

obtain information about the structures adopted by metal complexes.

A critical study of the magnetic data of coordination compounds of

metals of the first transition series reveals some complications. For

free ions and their coordination entities is similar. When more than

octahedral hybridisation is not directly available (as a consequence of

which leaves two, one and zero unpaired electrons, respectively.

The magnetic data agree with maximum spin pairing in many cases,