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Approximately 50 percent of enzymes in our body are coordination compounds and many life saving medicines are made up of them. From the vibrant colour of transition metals complexes to the red colour of blood, these compounds play a very important role in both industrial and biological processes. These are special compounds formed when a central metal atom combines with small atoms or ions called ligands. NCERT notes of Coordination Compounds help you understand topics like coordination spheres, the role of ligands in stabilising metal ions, Valence Bond Theory, and Crystal Field Theory.
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These comprehensive notes cover all important concepts of Coordination compounds in a simple and exam-friendly format. Students can easily download the PDF by clicking the button given below.
Notes of Coordination compounds given below covers all concepts of NCERT textbook and explains them in a concise and easy-to-understand manner. All the essential topics like ligands, coordination number, isomerism, IUPAC naming rules and bonding theories like VBT, CFT are explained in a step by step and detailed manner. These notes are useful for last minute revision as well as exam preparation.
The postulates from Werner’s theory are listed below:
The central metal or the metal atoms within a coordination compounds will always shows two types of valency. Among these two types, one is known as primary valency and another one is secondary valency.
The primary valency denotes the oxidation state of the atom, and the secondary valency will show the coordination number.
The secondary valences is not variable i.e. constant for every metal atom. Or we can say that the coordination number of any particular atom is fixed.
The metal atom will fulfil both its primary as well as secondary valencies. Generally, a negative ion i.e. anion, will satisfies the primary valency. And on the other hand, among these two i.e. negative ion or neutral molecules both can satisfy secondary valencies.
Coordination Entity
A coordination entity comprises a central metal atom or ion bonded with a fixed number of ions or molecules.
For example:
Other examples of coordination entities are as follows:
Central atom/ion
Central metal atom/ ion to which one or more neutral molecules or ions are linked by coordinate bonds in a definite geometrical arrangement around it. It is also referred as Lewis's acid.
Ligands
The ions or molecules that are bound to the central atom or ion within the coordination entity are called ligands. They can be simple ions like Cl–, or small molecules like H2O or NH3, they can also be larger molecules like
Coordination number:
It is the total number of coordinate bonds through which the central metal atom/ion is attached to ligands.
Coordination sphere:
The central metal atom/ion and the ligands which are directly attached to it, are enclosed in square brackets and is collectively known as the coordination sphere. It is non-ionisable.
Coordination polyhedron:
It is the spatial arrangement of the ligands around the central metal/atoms or ions. The most common coordination Polyhedra are octahedral, square planar, and tetrahedral.
Oxidation number of the central atom:
It is the number that represents the charge on the central metal atom, if all the ligands are removed along with the electron pairs that are shared with the central atom.
Homoleptic and heteroleptic complexes
Homoleptic complex are those complexes in which the metal atom/ion is bound to only one kind of ligand.
Heteroleptic complex are those complexes in which a metal atom/ion is bound to more than one kind of ligand.
Rules for writing the formula of coordination compounds:
Formula of the cation whether simple or complex must be written first followed by anion.
Rules for naming coordination compounds:
Naming of ligands:
Isomers are two or more compounds that have the same chemical formula but a different arrangement of atoms.
The two principal types of isomerism are:
1). Stereoisomerism
Coordination compounds having different positions and arragements of ligands in space. They are further divided into two categories
a). Geometrical Isomerism
This isomerism is due to ligands occupying different positions around the central metal atom or ion. The ligands occupy positions either adjacent or opposite to one another. This type of isomerism is also known as cis-trans isomerism. When two same ligands are at right angle(90o), the form is cis- form and when they are present diagonally at 180o to each other, the form is termed as trans- from. Geometrical isomerism is very common in coordination number 4 and 6 complexes.
Another type of geometrical isomerism occurs in octahedral coordination entities of the type
b). Optical Isomerism
Optical isomers are mirror images that cannot be superimposed on one another. These are called as enantiomers. The molecules or ions that cannot be superimposed are called chiral. The two forms are called dextro (d) and laevo (l) depending upon the direction they rotate the plane of polarised light in a polarimeter (d rotates to the right, l to the left). Optical isomerism is common in octahedral complexes involving didentate ligands.
2). Structural isomerism
Coordination compounds having different ligands within their coordination spheres.
a). Linkage Isomerism
Linkage isomerism arises in a coordination compound containing ambidentate ligand. A simple example is provided by complexes containing the thiocyanate ligand, NCS–, which may bind through the nitrogen to give M–NCS or through sulphur to give M–SCN. Jørgensen discovered such behaviour in the complex [Co(NH3)5(NO2)]Cl2, which is 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 bound through nitrogen (–NO2).
b). Coordination Isomerism
This type of isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex. An example is provided by [Co(NH3)6][Cr(CN)6], in which the NH3 ligands are bound to Co3+ and the CN– ligands to Cr3+. In its coordination isomer [Cr(NH3)6][Co(CN)6], the NH3 ligands are bound to Cr3+ and the CN– ligands to Co3+.
c). Ionisation isomerism
Complexes that give different ions in solution.
d). Solvate isomerism
Complexes that differ in number of water molecules present as ligands (inside the coordination sphere) and as free molecules (outside the coordination sphere).
Werner 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?
To explain the nature of bonding in coordination compounds various theories are proposed such as Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular Orbital Theory (MOT).
According to this theory, the metal atom or ion under the influence of ligands can use its (n-1)d or nd orbitals along with its ns and np for hybridisation to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar and so on. These hybridised orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding. The different types of hybridisation and their respective shapes are given below.
Coordination Number | Type of Hybridisation | Shape |
4 | sp3 | Tetrahedral |
4 | dsp2 | Square Planar |
5 | sp3d | Trigonal Bipyramidal |
6 | sp3d2 | Octahedral |
6 | d2sp3 | Octahedral |
Magnetic Properties of Coordination Compound
The transition metals have a unique ability to form magnets. Metal complexes show paramagnetism due to unpaired electrons as the last electrons will reside in the d orbitals. By considering only monometallic complexes, they have unpaired electrons or an odd number of electrons in which each electron has a magnetic moment associated with spin angular momentum and causes destabilization.
Limitations of Valence Bond Theory
Some limitations of VBT are given below:
Crystal Field Theory (CFT) is not a basic theory but an electrostatic model. As the theory is based on the electrostatic model of hard spheres and the interaction is done in a purely electrostatic way. The central atom is having a positive charge and ligands have a negative charge and thus this negative charge approaches towards the positive charge and due to ligands point charge degeneracy has been created. This means that all the five “d” orbitals in an isolated gaseous metal atom or ion will have the same energy or we can be told that they are degenerate. However, when this negative field is formed due to ligands in a complex, this will become asymmetrical and then the degeneracy of d orbitals is lifted. It will result in the splitting of the d orbitals. The pattern of splitting will depend on the nature of the crystal field.
In an octahedral coordination entity with six ligands surrounding the metal atom/ion, there will be repulsion between the electrons in metal d orbitals and the electrons (or negative charges) of the ligands. Such a repulsion is more when the metal d orbital is directed towards the ligand than when it is away from the ligand. Thus, the
The crystal field splitting,
Such a series is termed as spectrochemical series. It is an experimentally determined series based on the absorption of light by complexes with different ligands. Let us assign electrons in the d orbitals of metal ion in octahedral coordination entities. Obviously, the single d electron occupies one of the lower energy
Calculations show that
In tetrahedral coordination entity formation, the d orbital splitting is inverted and is smaller as compared to the octahedral field splitting. For the same metal, the same ligands and metal-ligand distances, it can be shown that
Applications of CFT
These are the various applications of crystal field theory.
Limitations of Crystal Field Theory
The coordination compound is made up of a ligand and a metal ion, and ligands are responsible for the coloration of the complex compounds. This means that different types of ligands show different colors.
The energy is required to remove the electron from a lower energy state to a higher energy state.
A higher energy state absorbs the color of shorter wavelengths.
We know that the metals of complex compounds are basically from d- orbitals or transition elements which have half-filled or unfilled d orbitals which pull oy the electron from the lower state and forward it to a higher energy level which causes the d-d transition of the metal atom thus radiates the color depending on the type of ligands.
For example, considering the two ligands one is a strong field ligand and the other is a weak field ligand.
The complex [Cr(NH3)6]3+ has strong-field ligands, so it will absorb relatively high-energy photons. This corresponds to the blue-violet light zone, which will give it a yellow color.
The other complex is of weak-field ligands, the [Cr(H2O)6]3+ ion, which absorbs lower-energy photons. This corresponds to the yellow-green portion of the visible spectrum and leads to giving deep violet color.
Discussing other examples based upon the high and low spin is as follows:
The iron(II) complex [Fe(H2O)6]SO4 gives blue-green due to the high-spin complex which absorbs the photons in the red wavelengths.
On the other hand, the complex iron(II) K4[Fe(CN)6] gives pale yellow which absorbs photons in violet wavelengths.
In general, strong-field ligands can cause a large split in the energies of d orbitals, so ligands are generally yellow, orange, or red because they absorb higher-energy violet or blue light.
On the other hand, weak-field ligands are often blue-green, blue, or indigo because they absorb lower-energy yellow, orange, or red light.
400-nm Violet light if absorbed → Green-yellow colour will be observed
430-nm Blue light if absorbed → Orange colour will be observed
450-nm Blue light if absorbed → Yellow colour will be observed
490-nm Blue-green light if absorbed → Red colour will be observed
570-nm Yellow-green light if absorbed → Violet colour will be observed
580-nm Yellow light if absorbed → Dark blue colour will be observed
600-nm Orange light if absorbed → Blue colour will be observed
650-nm Red light if absorbed → Green colour will be observed
Compounds that contain at least one carbon-metal bond are called organometallic compounds. Zeise, in 1830, prepared the first organometallic compound by the action of ethylene on a solution of potassium chloroplatinate(II). In the last four decades, enormous work has been done in this field and many fascinating compounds have been synthesized and investigated. Grignard reagent, RMgX is a familiar example of organometallic compounds where R is an alkyl group. Diethyl zinc [Zn(C2H5)2], lead tetraethyl [Pb(C2H5)4], ferrocene [Fe(C5H5)2], dibenzene chromium[Cr(C6H6)2], metal carbonyls are other examples of organometallic compounds. The compounds of metalloids such as germanium and antimony and mon-metallic elements such as boron and silicon are also included under this classification.
Organometallic compounds may be classified in three classes:
Sigma(σ) bonded complexes: These complexes contain a metal and carbon atom attached with a sigma bond e.g. Tetramethyl Tin, Trimethyl aluminium etc.
Bonding in Trimethyl aluminium is shown below
Pi(π) bonded complexes: These complexes contain a metal and carbon atom attached with a Pi bond. e.g. Ferrocene, Dibenzene Chromium etc. Bonding in Ferrocene and Dibenzene Chromium is shown below:
Complexes containing both
Coordination compounds are of great importance. These compounds are widely present in the mineral, plant and animal worlds and are known to play many important functions in the area of analytical chemistry, metallurgy, biological systems, industry and medicine.
Slected questions from previous year exams are given below:
Question: The diamagnetic species is:
[At.
(1)
(2)
(3)
(4)
Answer:
1)
The atomic number of Ni is 28
Ground state electronic configuration of Ni is
In complex
So, the electronic configuration becomes
In
Therefore it is diamagnetic in nature.
2)
Ni is in +2 oxidation state, so the electronic configuration becomes
3)
Fe atomic number 26
Overall charge -3
So, Fe oxidation state
Low spin
So, it not diamagnetic cause it has one unpaired electron.
4)
Co atomic number 27 and the charge is -3
So,
Hence, the correct answer is option (1).
Question: Write the electronic configuration of
Answer:
In an octahedral field, the d-orbitals split into:
Low-spin
All five electrons occupy the lower-energy
Electronic configuration:
Hence, the answer is
Question: Write IUPAC names of the following coordination entity :
Answer:
Central metal: Ni (nickel)
Ligands:
Oxidation state of Ni
Let oxidation state of
Since the complex ion is anionic (charge
Nickel
IUPAC Name: Tetracyanidonickelate(II) ion
Hence, the answer is Tetracyanidonickelate(II) ion
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Coordination compounds are complex structures formed by the coordination of metal ions with ligands, which are molecules or ions that can donate electron pairs to the metal. These compounds are characterized by the presence of a central metal atom surrounded by a number of ligands, and they exhibit unique properties in terms of color, solubility, and reactivity.
Studying coordination compounds is crucial because they play significant roles in biological systems, catalysis, and materials science. Understanding their properties and behaviors helps students comprehend complex chemical interactions and applications relevant to various fields.
Coordination compounds differ from other compounds in their structural composition, properties, and stability. The presence of a central metal atom and its interaction with ligands leads to unique electron configurations, which affect their chemical behavior differently than typical ionic or covalent compounds
Coordination compounds are found in various applications, including in medicine, fertilizers, dyes, and catalysts in chemical reactions.
A ligand is a molecule or ion that donates a pair of electrons to a central metal atom or ion to form a coordination complex. Ligands can vary in size and charge; they can be simple ions like chloride or complex molecules like ethylenediamine.
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