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Coordination Compound

Transition metals form complex or coordination compounds. Coordination compounds are extensively found in animals, plants and minerals. For example, chlorophyll a green pigment responsible for photosynthesis is a coordination compound of magnesium. A red pigment present in blood and hemoglobin, acting as an oxygen carrier, is a complex compound of iron.

A coordination compound contains a central metal atom or ion surrounded by a number of oppositely charged ions or neutral molecules. There is a coordinate bond between the metal atom and these ions or molecules.

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Nomenclature of Coordination Compounds

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A few common rules were given by the IUPAC regarding the naming convention of coordination compounds which are given below:

1. Order of naming ions

The positive ion is named first followed by the name of the negative ion. In the case of ionic compounds, the name of the cation is mentioned first followed by the name of the anion, e.g., K2PtCl6 is named as potassiumhexchloroplatinate (IV) in which cation potassium is written first followed by the anion Chloro. In the case of non ionic compound the name is written in one word.
For example, [Ni (NH3)4Cl2] is named as tetraamminedichloronickel (II).

2. Naming of coordination sphere


The name of the legends are written first followed by the name of the central ion and the oxidation number of the central metal atom is expressed in roman numerals just after the name of the central atom. Or the sequence can be written as [Name of legend] [Name of central metal atom] [Oxidation number of metal in roman].
For example, [Co(NH3)6]3+, the coordination sphere is named as Hexaamminecobalt (III) ion.

3. Naming of Ligand

The central metal ion is surrounded by positive or negative or neutral ligands. If a ligand carries a negative charge the name has a characteristic ending word "O". For example, SCN - Thiocynate, HS - hydrogen sulphido etc. For a ligand carrying a positive charge, the name has the characteristic ending of "ium". For example, NO2- Nitrosonium, NO2 + Nitronium etc. For neutral ligands no characteristic ending is used.
For example, H2O Aqua, etc.

4. Order of naming ligands

According to the IUPAC conventions the names of ligands surrounding the central metal atom are written in alphabetical order of preference irrespective of whether they are negative or neutral. For example, in the complex [Co(NH3)4Cl(NO2)], the ligands are named in the order ammine, chloro and nitro. The prefixes di, tri, etc., are not considered while determining the alphabetical order.

5. Numerical prefixes to indicate number of ligands

The number of each kind of ligand is specified by the prefixes di, tri, tetra, etc., i.e., CO (en)3]Cl3 is named as Tris (ethylendiamine) cobalt (III) chloride.

Numerical Prefixes

Number
Prefix
Number Prefix Number Prefix
1
mono 5 penta
(pentakis)
9 nona
(ennea)
2 di(bis) 6 hexa
(hexakis)
10 deca
3 tri
(tris)
7 hepta 11
undeca
4 tetra
(tetrakis)
8 octa 12 dodeca

6. Ending of name of central atom

If the complex ion is a cation, the metal is named the 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. So, Co in a complex anion will be cobaltate and Pt is called platinate.

7. Oxidation state of central atom or ion

The oxidation state is designated by Roman numerals (I, II, III, etc.) in the bracket at the end of the name of the complex without a gap between the two.
E.g. Pt(NH3)2Cl4 $\alpha$ diamminetetrachloro platinum (IV), Fe(CO)5- pentacarbonyliron(0), etc.

8. Naming of ambidentate ligands

The ligands which can coordinate with more than one atom are called ambi-dentate ligands, and in these ligands the point of attachment is indicated by putting the symbol of an atom through which it is coordinating with the rest. Example of this is thiocyanate, SCN- which can get attached to either the sulfur atom or the nitrogen atom, similar NO2- ion.

9. Naming of Geometrical isomers and optical isomers

In these isomers, the term cis is used to show similar groups at adjacent position & trans is for similar groups at opposite positions.

Isomers

In the case of optical isomers Dextro & laevo rotatory optically active compounds are designated either by (+) & (-) or by d- & l- respectively.

Writing formula of a complex by the name

For writing the formula of a complex compound the following steps have to be followed:
  1. The central atom is listed first.
  2. The ligands are then listed in alphabetical order. In the case of an abbreviated ligand, the first letter of the abbreviation is used to determine the position of the ligand in alphabetical order.
  3. The formula of the coordination compound, whether charged or not, is enclosed in square brackets.
  4. There should be no space between the names of ligands and the metal.
  5. When the formula of a charged coordinate entity is to be written without that of counter ion, the charge is indicated outside the square bracket as a right super script with the number before the sign.
  6. Charge on the complex ion =Oxidation state of metal + charges on ligands (with sign).
  7. As the molecule of coordination compound is neutral, the total positive charge should be equal to the total negative charge.
For example, Tetraammineaquachloridecobalt (III) chloride- In this the anion is=Cl-

  • Cation [Co(NH3)4(H2O)Cl]x where x is the charge on the complex ion.
  • So, x = oxidation state of cobalt + charge on the ligand
  • x = +3+0+0+(-1) = +2 as the H2O & NH3 are neutral group so carry no charge & chloride group carry -1 charge.
  • Charge on anion = -1 & on the complex ion = +2. Since the overall complex molecule should be neutral, two anions can neutralize the charge of the cation.
  • Hence the formula of given compound is [Co(NH3)4(H2O)Cl]Cl2.
Similarly,
  • hexaammineiron(III) nitrate [Fe(NH3)6](NO3)3
  • ammonium tetrachlorocuprate(II) (NH4)2[CuCl4]
  • sodium monochloropentacyanoferrate(III) Na3[FeCl(CN)5]
  • potassium hexafluorocobaltate(III) K3[CoF6]

Bonding in Coordination Compounds

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The first theory was called Werner's theory of co-ordination compounds.
Metals possess two types of valencies.
  1. Primary valency or ionisable valency. It is also referred to as the oxidation state.
  2. Secondary valency which a metal atom or cation exercises towards neutral molecules or negative groups (ligands) in the formation of complex ions. The secondary valency is also called the coordination number.
    Example:
    In [Pt(NH3)6]Cl4 secondary valency of Pt is 6
  • Primary valencies are satisfied by negative ions, secondary valencies may be satisfied by negative ions or neutral molecules.
  • Ligands satisfying secondary valencies are directed towards fixed positions in space giving a definite geometry to the complex but the primary valencies are non-directional.
  • Six valencies are directed towards a regular octahedron while four are directed towards either a tetrahedral manner or square planar.

Coordinate Complex

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Coordination compounds are the compounds in which the central metal atom is linked to a number of positive or negative ions or neutral molecules by coordinate bonds and the donor atoms, molecules or ions which donate a pair of electrons to the central metal atom or ion and form a coordinate bond called ligand.

For example nickel tetra carbonyl, [Ni(CO)4] in which CO molecules are linked to the central nickel atom by coordinate bonds by donating lone pair of electrons is a coordinate compound or [Fe(CN)6]4-, [Cu(NH3)4]2+, etc.

Nickel Tetra Carbonyl


Types of complexes


There are three types of complexes:

1. Cationic complex

A complex in which the complex carries a net positive charge is called cationic complex. For example, [Co (NH3)6]3+

Cationic Complex

2. Anionic complex

A complex in which the complex carries a net negative charge is called anionic complex. For example, [Fe (CN)6]4-

Anionic Complex

3. Neutral Complex

A complex carrying no net charge is called a neutral complex or simply a complex. For exmaple, [Mn2(CO)10]

Neutral Complex

Isomerism in Coordination Compounds

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Two or more chemical compounds with identical chemical formula but different structures are called isomers and this phenomena is known as isomerism. Coordination compounds show two main types of isomerism.

Classification of Isomerism

(a) Structural Isomerism


This type of isomers are different in structure. It can be subdivided into the different types which are as follows.

1. Ionization Isomerism

The isomers which form different ions in solution, although, they have same the composition, are called ionization isomers.

For example, pentaamminebromocobalt (III) sulfate, [CoBr(NH3)5]SO4 & pentaamminesulfatocobalt(III) bromide, [Co(SO4)(NH3)5]Br

In the former, the bromide ion is coordinated to the Co3+ ion and the sulfate ion is outside the coordination sphere; in the latter the sulfate ion occurs within the coordination sphere, and the bromide ion is outside it.

2. Coordination Isomerism

This type of isomerism is shown by compounds in which both cation and the anion are complexes. The isomers differ as a result of different groups being coordinated about a particular coordination center.

Example: [Co(NH3)6][Cr(CN)6] = hexaaminecobalt (III) hexacyanochromate (III)
[Cr(NH3)6][Co(CN)6] = hexaaminechromium (III) hexacyanocobalt (III)


3. Linkage Isomerism

This type of isomerism occurs with ambidentate ligands. These ligands are capable of coordinating in more than one way. The best known cases involve the monodentate ligands SCN- & NCS- and NO2- & ONO-.

Linkage isomers

[Co(NO2)(NH3)5]2+ & [Co(ONO)(NH3)5]2+ are linkage isomers of each other as ligand NO2- are attached with two different donor atoms in both isomers.

4. Hydrate / Solvate Isomerism

Compounds which have the same composition but differ in the number of water molecules present as ligands and as a free solvent molecule in the crystal lattice, are called solvate isomers. For example, all three complexes are different in the number of water molecules in coordination sphere.

Metal complex Name
Color
Number of ionic chlorine
[Cr(H2O)6]Cl3 hexaaquochromium(III) chloride Violet Three ionic chlorine
[Cr(H2O)5Cl]Cl2.H2O pentaaquachlorochromium(III) chloride monohydrate Blue green
Two ionic chlorine
[Cr(H2O)4Cl]Cl2.2H2O tetraaquadichlorochromium(III) chloride dehydrate
Green
One ionic chlorine


(b) Stereo / Space Isomerism


These isomers differ only in the spatial arrangement of atoms or groups about the central atom. It's possible only in the complexes having coordination number four or greater than four. The two type of stereo-isomerism are described as follows.

1. Geometrical Isomerism

This isomerism arises due to the difference in the geometrical arrangement of the ligands around the central ion. When the same kind of ligand occupies positions adjacent to each other called cis-isomer. When identical ligands occupy positions opposite to each other called trans-isomer. It is very common in coordination complexes with co-ordination number of 4 and 6.

Isomerism in Complexes with coordination number 4

Isomerism with Coordination Number Four


These complexes can either be tetrahedral or square planner geometry. Tetrahedral dont showing this isomerism as the relative position of the atoms is the same with respect to each other.

The square planner shows this kind of isomerism as follows.

Compound type
No. of isomers
Ma2b2
2 (cis- and trans-)
Mabcd
3 (cis- and trans-)

Here a, b, c, d is monodentate ligands.

Ma2b2 type


For example, cis-PtCl2(NH3)2 , trans- PtCl2(NH3)2

Isomers with Ma2b2 Type

Ma3b3 type

Some example are [Co(NH3)3Cl3], [Co(NH3)3(NO2)3], etc. The isomers are called facial (fac) when all the same ligands occupy the same face on the octahedran and the other is called meridional (mer).

[M(L - L)2b2] type

This type of isomerism also arises when bidentate ligands L – L [e.g., NH2CH2CH2NH2(en)] are present in complexes of formula [M(L - L)2b2]

Isomera with M(L  L)2b2 Type

2. Optical Isomerism

This type of isomerism is exhibited by the chiral molecule (molecules which dont have a plane of symmetry). Optical isomers are mirror images of each other. They have similar physical and chemical properties but differ in rotating the plane of plane polarized light.
Isomer which rotates the plane polarized light to the right is called dextro rotatory (d-form) and the isomer which rotates the plane polarized light to the left is called laevo rotatory (l-form). Complex with a coordination number 4 and 6 show this type of isomerism.Isomerism in complexes with coordination number 4

The tetra coordinated square planner complexes dont show this as they contain a plane of symmetry. So, tetrahedral complexes which contain unsymmetrical bidentate ligands, M (AB)2 show optical activity.

For example-bis(glycinato)nickel (II) shows this. Boron(II), Zn(II) & Cu(II) can also form this kind of complexes. The tetrahedral complexes having four different groups to central atom dont show optical activity because these complexes are labile & can be isolated.

Isomerism in complexes with coordination number 6

The different kind of Hexa coordinated complexes containing at least one cheating ligand show optical activity. Some examples are discussed below.
d & l isomers of [Pt(Py)2(NH3)2Cl2]2+ & [Pt(py)NH3NO2Cl Br]

isomerism with Coordination Number Six

[M(AA)3] type

Octahedral with three bidentate ligand show the optical activity. Complexes containing hexadentate ligands such as EDTA also show optical isomerism. For example-[Co(EDTA)]- can resolved in pair of Enantiomers.


Isomer with M(AA)3 Type

[M(AA)2XY] or [M(AA)2 X2]- type

These compounds also show optical activity where AA is bidentate ligand & X, Y are monodentate ligands. In this example trans isomer is optically inactive while cis is optically active.

Isomers with M(AA)2XY Type

Applications of Coordination Compounds

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The coordination compounds are of great importance as these compounds constitute in minerals, plants & also in animals.
  1. Dyes and Pigments: From the earliest times coordination compounds have been in vogue in the form of dyes and pigments. For example, madder dye which is red, was used by the ancient Greeks and others. It is a complex of Hydroxyanthraquinone.
  2. Qualitative analysis: The formation of the complex substances using suitable reagents is very effectively used in qualitative analysis.
  3. Color Tests: Since many complexes are highly colored they can be used as colorimetric reagents, e.g., formation of red 2,2'-bipyridyl and l,l0-phenanthroline complexes as a test for Fe(II)
  4. Gravimetric Analysis: The chelating ligands are occasionally utilized to form insoluble complexes, e.g., Ni(DMG)2 and Al(oxine)3.
  5. Complexometric Titrations and Masking Agents: An example of this is the use of EDTA in the volumetric determination of a wide variety of metal ions in solution, e.g., Zn2+, Pb2+, Ca2+,Co2+, Ni2+, Cu2+, etc. By careful adjustment of the pH and using suitable indicators, mixtures of metals can be analyzed, as in the case of analyzing Bi3+ in the presence of Pb2+
  6. Sequestering Agents: Used as masking agents or for ligands "sequestering", i.e., effective removal of avoidable ions from solutions for industrial processing, e.g., EDTA is basically used for softening water. Addition of EDTA to water is used in boilers, etc., to prevent "scaling" or build up of insoluble calcium salts.
  7. Metal extraction: Certain metals can be extracted by the leaching action by forming of stable complexes. For example, Ag and Au as complexes of cyanide ion.
  8. Bio-Inorganic Chemistry: Complexes like hemoglobin, chlorophyll, vitamin B12, etc., are naturally occurring. EDTA and other complex agents have been used to speed the elimination of harmful radioactive and other toxic elements from the body (e.g. Pb2+). In these cases a soluble metal chelate is formed.
  9. Chemo-therapy: The platinum complex cis-[Pt(NH3)2Cl2] known as cisplatin is used as an antitumor agent in the treatment of cancer.
  10. Electroplating: Many complexes are used as electrolyte in electroplating. For silver plating the complex K[Ag(CN)2] is used.
  11. In photography: Developed film is fixed by washing it with a solution of sodium thiosulphate.
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