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A coordination compound is composed of a central metal atom or an ion. It is surrounded by a certain number of oppositely charged ions or neutrally charged molecules.
- Coordination compounds are also known as coordination complexes.
- These molecules or ions rebond to the ion or the metal atom through a coordinate bond.
- Even when dissolved in water, they don't separate into simple ions.
- Ligands (also known as complexing agents) are molecules or ions that are attached to the central atom.
- They are pervasive in terms of structures and reactions.
- Coordination compounds consist of haemoglobin, vitamin B12, pigments, catalysts and chlorophyll.
- It also includes dyes which are used in preparing organic substances.
- They are neutral molecules, also known as anions, that are bonded to the central atom.
- The atoms are linked by a covalent bond.
Key Terms: Coordination Compounds, Double Salt, Ligands, Chelate, Chelation, Isomerism, Ion, Metal Carbonyls, Valence Bond Theory, Atom, Negative charge, Positive charge, Geometrical isomerism, Optical isomerism
What are Coordination Compounds
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Coordination compounds are chemical compounds in which metal atoms are bonded to several atoms or molecules. The bonding of atoms takes place by sharing of the electron.
- Coordination compounds are surrounded by an array of molecules, which are known as ligands.
- It includes transition metal that belongs to the d group of the periodic table.
- The atom bonded to the central atom is called the donor atom.
- A covalent bond is formed between ligands and the central atom.
- This type of bonding is different from normal covalent bonds.
- The number of donor atoms linked to the central atom is known as the coordination number.
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Important terms involving Coordination Compounds
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Some of the important terminology used in coordination compounds are as follows:
Coordination Entity
Coordination Entity is defined as the arrangement of the central atom to an array of other groups of atoms. The array of groups of atoms is called ligands. The central atom is metallic in nature. Some common examples of coordination entities include [CoCl3(NH3)3] and [Fe(CN)6]4-.
Coordination Number
Coordination Numbers are numbers that are determined on the basis of donor atoms. It is also known as ligancy. In this, the number of donor atoms where central atoms are carried in a coordination compound or in a crystal as its neighbours.
Coordination Sphere
Coordination Sphere is an important terminology used in coordination compound that is made of a central atom and its attached ligands. The formula of the coordination sphere is written in closed brackets.
Coordination Polyhedron
Coordination Polyhedron is defined as the arrangement or design of ligands that are directly attached to the central atom. Square planar, tetrahedral and octahedral are some common examples of coordination polyhedron.
Oxidation Number
The oxidation number is a type of number that is used to determine the charge associated with the central atom or ion. It depends upon the electron donated by the ligands.
Properties of Coordination Compounds
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The important properties of coordination compounds are as follows:
- Coordination Compounds are coloured compounds due to the presence of a lone pair of electrons.
- It absorbs light with the help of electronic transition.
- Iron complexes are green and pale green colours.
- If a coordination compound has unpaired electrons, it is paramagnetic in nature.
- In case any of the electrons in the coordination compound are paired, it is diamagnetic.
- Inner orbital or low spin complexes are those in which metal hybrid orbitals are formed by the hybridisation.
- It consists of hybridisation of (n-1) d, ns, and np-orbitals.
- Some common example includes [Fe(CN)6]4-, [CO(NH3)6]3+, [Cr(NH3)6]3+, [Fe(CN)6]2+, [Fe(H20)6]2+, and [(MnCCN)].
- Outer orbital or high spin complexes are also formed by the hybridisation.
- It includes hybridisation of ns, np, and nd-vacant orbitals.
- Some common example includes [MnF6]3-, [FeF6]3-, [Ni(NH3)6]2+, [Ni(H20)6]2+.
Double Salt and Coordination Complexe
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Double salts are formed when two salts in a stoichiometric ratio crystallize together from their saturated solution. When dissolved in water, they remain stable in solid form but dissociate into constituent ions.
Example of Double SaltExample 1: Consider the following scenario: FeSO4(NH4) 2SO4.6H2O (Mohr's salt) Example 2: Consider the following scenario: Fe2 is the central metal ion in K4[Fe(CN)6. |
Coordination Complex
Coordination Complex is a chemical compound made of one or multiple metal centres bounded by ligands. They are sometimes neutral in nature, and some are charged molecules.
- The coordination complex is stabilized by neighbouring counter-ions.
- It mainly consists of d elements of the periodic table.
- They are catalytic or stoichiometric in nature.
Types of Coordination Complex
The different type of coordination complex are as follows:
Cationic Complexes
Cationic Complexes are complexes where coordination sphere of the compounds carries the positive charge. This way sphere will act as a cation. Some example of cationic complexes include [Co(NH3)6]Cl3
Anionic Complexes
Anionic Complexes are complexes where coordination sphere of the compounds carries the negative charge. This way sphere will act as a anion. Some example of anionic complexes include K4[Fe(CH)6]
Neutral Complexes
Neutral Complexes are complexes where coordination sphere of the compounds carries both negative and positive charge. This way sphere will act as a cation as well as anion. Some example of neutral complexes include [Ni(CO)4].
Homoleptic Complexes
Homoleptic Complexes are type of coordination complex which consists of a similar type of ligands. Some example includes K4[Fe(CN)6]
Heteroleptic Complexes
Heteroleptic complexes consist of different kind of ligands molecules. Some example includes [Co(NH3)5Cl]SO4
Mononuclear Complexes
Mononuclear Complexes are type of complexes where the sphere has a single transition metal ion. Some example includes K4[Fe(CN)6]
Polynuclear Complexes
Polynuclear Complexes are type of complexes where more than one transition metal ion is present.
What are Ligands in Coordination Compounds
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Ligands are atoms or groups that are directly attached to central atoms. They are Lewis bases that donate an electron pair to the metal atom and form coordinate bonds with it.
- For instance an individual can consider, H2O, CO, NO2- and so on.
- A ligand may have a neutral charge, a positive charge, or a negative charge.
- It result in the formation of dative bond between ligands and transition metal.
Types of Ligands
Ligands can be classified into three categories based on the amount of charge present:
1. Anionic Ligands
Anionic ligands are type of ligands that posses negative charge over the ligand. CN- (cyanide), NO2- (nitrito-N), NO3- (nitrito), X (halido), and other anionic ligands
2. Cationic Ligands
Cationic ligands are type of ligands that posses positive charge over the ligand. NO2+ (nitrosonium), NO+ (nitronium), N2F5+ (Hydrazenium), and other cationic ligands
3. Neutral Ligands
Neutral Ligands are type of ligands that posses neutral charge over the ligand. NH3 (ammine), H2O (aqua), CO (carbonyl), and other neutral ligands
Ligands are defined as follows based on the number of coordinating atoms present:
1. Monodentate Ligands
Monodentate ligands are those that only have one coordinating atom. For example, X stands for halido, NH3 stands for ammine, H2O stands for water, and CN stands for cyanide.
2. Diddentate Ligands
Diddentate refers to a ligand that has two coordinating sites. H2NCH2CH2NH2 (ethane-1, 2-diamine) and C2O42-are two examples (oxalate).
3. Polydentate Ligands
Polydentate refers to a ligand that contains several donor sites. N(CH2CH2NH2)3 is an example (Nitrilotriethylamine, a tetradentate ligand)
4. Ambidentate Ligands
This is a ligand that can bind to two separate atoms in its structure. For instance, the NO2 ion may coordinate to a central metal atom/ion either via nitrogen or oxygen.
Chelate, Chelating ligands and Chelation
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The development of a cyclic structure around the central metal atom is caused by di- and polydentate ligands. Chelates are cyclic metal complexes, and the ligands that produce chelates are known as chelating ligands, and the process is known as chelation.
Denticity
The number of coordinating atoms present per ligand is known as denticity.
Coordination number
The number of coordinate bonds formed by the central metal atom with the ligands is described as the coordination number. Consider the following scenario: Fe has coordination number 6 in K4[Fe(CN)6].
Oxidation number
The charge left on a given atom after all other atoms in a complex is extracted as ions are known as the central atom's oxidation number. Roman numerals are used to represent the oxidation number.
IUPAC Nomenclature of Coordination Compounds
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The steps to naming a complex compound are as follows:
- The cations are often called first, followed by the anions.
- After that, the ligands are classified alphabetically.
- In the case of polydentate ligands, the prefixes di, tri, tetra, penta, and so on.
- It is used to denote the number of ligands of that form present.
- The name of the metal atom is then written in Roman numerals, followed by its oxidation state.
- Finally, the anion is given a name.
Example of IUPAC Nomenclature of Coordination CompoundsExample: [Cu(NH3)4]SO4 Tetra ammine copper (II) sulphate, for example :-
Consider the following scenario: |
The video below explains this:
IUPAC Nomenclature of Coordination Compounds Detailed Video Explanation:
Isomerism in Coordination Compounds
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Isomerism can be divided into two categories which are as follows:
Stereoisomerism
Stereoisomerism occurs as a result of atoms or groups being arranged differently in space. The following are two related forms of stereoisomerism:
Geometrical Isomerism
Geometrical isomerisms occurs in heteroleptic complexes as a result of various ligand geometrical arrangements. The two ligands X can be positioned adjacent to each other in a cis isomer, or opposite to each other in a trans isomer.
- In a square planar complex of formula [MX2L2] (where X and L are unidentate).
- Two ligands X can be directed cis or trans to each other in octahedral complexes of formula [MX2L4].
- Geometrical isomers of Pt(NH3)2Cl2), for example, are shown below:
Geometrical isomers of [Co(NH3)4Cl2], for example, are shown below:
![Geometrical isomers of [Co(NH3)4Cl2]](https://images.collegedunia.com/public/image/347b009972d6e22ce2279bba6f829ba0.png)
Optical Isomerism
The existence of non-superimposable mirror images causes it. Enantiomers are mirror copies that are not superimposable. When exposed to plane-polarized light, the enantiomers react differently.
- Dextrorotatory (d) or (+) is the enantiomer that rotates plane polarised light in a clockwise direction.
- Levorotatory (l) or () is the enantiomer that rotates plane polarised light in an anticlockwise direction.
The optical isomers of [Co(en)3 ] 3+are shown below:
![The optical isomers of [Co(en)3 ] 3+](https://images.collegedunia.com/public/image/9b7edf3a1767ba39db8c0ed304b153b6.png)
Structural Isomerism
Structural Isomerism occurs as a result of the structural differences between coordination compounds. The following are four distinct types of structural isomerism:
Ionisation isomerism
Ionisation isomerism refers to isomers that have the same molecular formula but produce different ions in solution. It include [Cr(NH3)Br]SO4 and [Cr(NH3)(SO4)]Br.
- It is caused by the exchange of ions between the metal ion's coordination sphere and ions outside the coordination sphere.
- In an aqueous solution, these two isomers produce different ions, for example,
Linkage isomerism
Linkage isomerism refers to isomers that have the same molecular formula but different linking atoms. The presence of ambident ligands causes this change.
- Linkage isomers include [CO(NH3)5(NO2)]2+ and [CO(NH3)5 (ONO)]2+.
- Coordination isomerism occurs when the cation and anion are both complex.
- Isomerism results from the full exchange of the coordination sphere.
Coordination isomers
Coordination isomers includes [Pt (NH3)4] [Ni (CN)4] and [Ni (NH3)4] [Pt (CN)4].
Hydration isomerism
Hydration isomers of each other are isomers with the same molecular formula but different water molecules of hydration. For example:- [Cr (H2O)5Cl]Cl2, for example.
- Hydration isomers are H2O and [Cr(H2O)4Cl2]Cl.2H2O.
Werner Theory
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Werner's Theory is a theory that involves determining the number of ligand-metal bonds. The arrangement of ligand-metal bonds takes place in transition metal cation. Alfred Werner proposed the Theory to formulate the design of coordination compounds.
- In this, the status of primary and secondary valency will be acquired.
- The method is used for the synthesis of coordination molecules.
- The elements act like lewis acid and are made up of a core central atom.
Postulates of Werner Theory
There are some postulates of Werner's Theory which are as follows:
- There are two types of valencies in coordination compounds, according to Werner's Theory of coordination compounds:
- Primary valencies: Ionizable valencies that are fulfilled by anions and determine the charge on complexions.
- Secondary valencies: These are no ionizable valencies that are fulfilled by ligands and specify the metal atom's coordination number.
- The ions that are attached to secondary linkages tend to exhibit spatial arrangements.
Valence Bond Theory
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Under the influence of ligands in Valence Bond Theory, a metal atom or ion may use its (n-1)d, ns, np or ns, np, nd orbitals for hybridisation, yielding a collection of identical orbitals of definite geometry such as octahedral, tetrahedral, square planar, and so on.
- These hybridised will overlap with ligand orbitals.
- They are capable of donating electron pairs for bonding.
The table below illustrates how different numbers of orbitals can be combined to produce different forms of hybridization:
As a result, the valence bond theory's fundamental concepts are as follows:
- Hybridisation of orbitals
- Bonding between the ligands and the metal atoms/ion
- Relationship between the magnetic moment observed and the bond form
Limitations of Valence Bond Theory
Various limitations of valence bond theory are as follows:
- It provides no information about the complexes' spectral properties.
- The theory does not include a comprehensive analysis of magnetic data.
- It can't tell the difference between strong and weak ligands.
- Valence bond theory explains little about the colour of coordination compounds.
- It does not have a quantitative interpretation of coordination compounds' thermodynamic or kinetic stabilities.
Crystal Field Theory
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Ligands are treated as point charges in crystal field theory (CFT), and the interaction between them and the metal ion is solely electrostatic. In an isolated gaseous metal atom/ion, all five d-orbitals have the same energy, indicating that they are degenerate.
- In the presence of the ligand region, the degeneracy is lost.
Limitations of Crystal Field Theory
The limitations of crystal field theory are as follows:
- The theory ignores the partially covalent nature of the bonding between the ligand and the central atom.
- It also lacks the ability to clarify the relative strengths of ligands, such as why H2O is a stronger ligand than OH.
Assumptions of Crystal Field Theory
The assumptions of crystal field theory are as follows:
- The ligands are believed to be point charges.
- The interaction between the point charges and the central metal's electrons is electrostatic in nature.
- The 5d-orbitals in a gaseous metal ion have the same energy, indicating that they are degenerate.
Metal carbonyls
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Metal carbonyls are homoleptic complexes in which carbon monoxide (CO) serves as the ligand. Consider the following scenario: Ni(CO)4 is a type of nickel. The following are the structures of several essential metal carbonyls:
Bonding in Metal Carbonyl
Metal carbonyls have both s and p character in their metal-carbon bonds. The metal-carbon (M-C) bond is formed when CO, as a ligand, binds to metal atoms via the carbon atom. It is a weak donor.
- The M–C bond is formed when a lone pair of electrons are donated to a metal's vacant orbital.
- It is formed when a pair of electrons is donated to carbon monoxide's empty antibonding* orbital.
- The synergic effect refers to the characteristic property of back bonding that stabilizes the metal-ligand interaction.
Properties of Metal Carbonyl
Various properties of metal carbonyl are as follows:
- They are solids at room temperature and pressure except for Ni(CO)4 and Fe(CO)5,
- Mononuclear carbonyls are flammable and poisonous.
- They may be colourless or light in colour.
Applications of Coordination compounds
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Some of the applications of coordination compounds are as follows:
- In hard water, EDTA is used to calculate Ca2+ and Mg2+.
- With EDTA, the ions Ca2+ and Mg2+ form stable complexes.
- The decomposition of metal coordination compounds may be used to purify metals.
- Impure nickel like [Ni(CO)4] is converted and decomposed to produce pure nickel.
- [Ni(DMG)2] is used in analytical chemistry.
- Ni in chocolates is detected using the 2+ complex.
- Cisplatin, a cis isomer of [Pt(Cl)2(NH3)2], is used to treat cancer in medicine.
- [Ag(CN)2]– and [Au(CN)2]– can be used to electroplate metals with gold or silver in a smooth.
- Chlorophyll is a magnesium coordination compound that is responsible for photosynthesis.
- Haemoglobin, the red pigment in blood that serves as an oxygen carrier, is also an iron coordination compound.
Things To Remember
- Coordination compounds are molecular compounds that are formed by combination of two or more molecular compounds.
- A central atom (or cation) is arranged to a sufficient number of anions or neutral molecules in compound coordination.
- The solution and solid-state are usually used to preserve their identity.
- In coordination compounds, metals have two kinds of valencies: primary and secondary valencies.
- In terms of structural structures, compounds with the same molecular formula are known as isomers.
- The metal-carbon (M – C) bond in metal carbonyl has both the - and -bond character.
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Sample Questions
Ques. Anwer the following questions:
(A) How can you figure out what a coordination compound's composition is
(B) Some of the uses for coordination compounds? (2 Marks)
Ans. (A)By using bonding theories to discover its hybridization.
(B) The use of coordination compounds as catalysts, which change the rate of chemical reactions, is a common application. In the manufacture of polyethene and polypropylene, for example, complex metal catalysts play an important role.
Ques. What exactly do you mean when you say "coordination compound"? (2 Marks)
Ans. A coordination compound consists of a metal atom or ion in the middle, surrounded by a variety of oppositely charged ions or neutral molecules. A coordinate bond connects these ions or molecules to the metal atom or ion. When dissolved in water, they do not dissociate into simple ions.
Ques. Name different topics that came under coordination compounds? (4 Marks)
Ans. The topics that fall under the topic, coordination compounds are listed below for the flexibility of students:
- Crystal Field Theory Bonding in Metal Carbonyls
- Definition of the Important Terms concerning Coordination Compounds
- Importance and Applications of the Coordination Compounds,
- Geometric and Optical IsomerismIsomerism in Coordination Compounds
- Introduction and Werner's Theory of Coordination Compounds
- Valence Bond Theory in Coordination Compounds
- Nomenclature of Coordination Compounds
Quest. How can you tell whether a compound is a coordination compound? (3 Marks)
Ans. The steps for determining a coordination compound's coordination number are as follows:
- Determine the chemical formula's core atom.
- Find the electron, molecule, or ion that is the closest to the central metal atom.
- Add the atoms of the closest atom/molecule/ions together.
- Find the total number of atoms that are closest to you.
Ques. With an example, what is a coordination compound? (2 Marks)
Ans. Nonmetal atoms or groups of atoms, known as ligands, surround a central metal atom in coordination compounds. Vitamin B12, for example, is made up of several nitrogen-containing ligands bound to a central metallic cobalt ion.
Ques. What characteristics do coordination compounds have? (2 Marks)
Ans. Colour, magnetic susceptibility, solubility and volatility, the ability to undergo oxidation-reduction reactions, and catalytic activity are all common physical and chemical properties of coordination compounds.
Ques. What's the distinction between primary and secondary Valency? (2 Marks)
Ans. The number of negative ions that are equal to the charge on the metal ion is the primary valency in coordination compounds. The number of ions of molecules coordinated to the metal ion, or the number of ligands connected or coordinated to metal ions, is the secondary valency.
Ques. Write IUPAC name of the complex [Co(en)2(NO2)Cl]+. What type of structural isomerism is shown by this complex? (2 marks)
Ans. Chlorobis(ethane-1, 2-diamine)nitrocobalt(III) is the IUPAC name of the complex [Co(en)2(NO2)Cl]+. This formula of the compound shows geometrical isomerism.
Ques. Using IUPAC norms, write the formula for the following complexes: (2 marks)
Hexaaquachromium (III) chloride
Sodium trioxalatoferrate (III)
Ans. a. Hexaaquachromium(III) chloride- [Cr(H2O)6]Cl3
- Sodium trioxalatoferrate(III)- Na3[Fe(C2O4)3]
Ques. When a coordination compound CrCl3⋅6H2O is mixed with AgNO3, 2 moles of AgCl are precipitated per mole of the compound. Write a structural formula of the complex. (2 marks)
Ans. We can say that for every mole of the compound the precipitated moles of AgCl are two. It shows that the number of ionisable chloride present in the compound is two. Therefore the structural formula can be said as [CrCl(H2O)5]Cl2.H2O
Ques. Indicate the types of isomerism exhibited by the following complexes : (3 marks)
[Co(NH3)5(NO2)]²+
[Co(en)3]Cl3 (en = ethylenediamine)
[Pt(NH3)2Cl2]
Ans.
- The types of isomerism exhibited by the following complexes are:
[Co(NH3)5NO2]²+ and [Co(ONO) (NH3)5]²+
- The types of isomerism exhibited by the following complexes are:
[Co(en)³]³+– Tris(ethane-1,2-diamine)cobalt (III) ion.
- The types of isomerism exhibited by the following complexes are:
Ques. (A) Which of the following is more stable and complex and why? (2 marks)
[Co(NH3)6]³+ and [Co(en)3]³+
(B) When a coordination compound CoCl3⋅6NH3 is mixed with AgNO3, 3 moles of AgCl are precipitated per mole of the compound. Write a structural formula of the complex. (2 marks)
Ans. (A) [Co(en)3]³+ tends to be a more stable complex than [Co(NH3)6]³+ because ethylene diammine is a bidentate ligand and the structure of the chelate is more stable. Hence chelating ligands tends to form a more balanced complex in comparison to non-chelating ligands.
(B) The structural formula is [Co(NH3)6]Cl3
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