Introduction
We have already seen the ability of transition elements to form coordination compounds. This property is however not restricted to transition elements alone. It is also exhibited by certain other metals although to a lesser extent.
Co-ordination Compound
Co-ordination compounds are compounds in which the central metal atom is linked to a number of ions or neutral molecules by co-ordinate bonds i.e., donation of electron pairs.
Difference between a Double Salt and a Complex Ion
Double salt: Ionizes completely in solution.
Ligand
The donor atoms, molecules or anions which donate an electron pair to the metal atom.
Example: NH3, H2O and CN-
Types of Co-ordination Compounds
Co-ordination Number (C.N)
The number of co-ordinate bonds formed with the central metal ion by the ligand.
Examples: [Ag(CN)2]- C.N = 2
Co-ordination Sphere
The central atom and the ligands which are directly attached, are enclosed in square brackets called the coordination sphere.
Oxidation Number or State
It is a number that represents an electric charge which an atom or ion actually has or appears to have when combined with other atoms.
Formula Writing and Nomenclature of Co-ordination Compounds
Rules for formula writing:
1) Formula of the cation must be written first followed by anion.
2) The Co-ordination sphere is written in square brackets.
Rules for Naming Co-ordination Compounds
Order of naming ions
The cation is named first (whether simple or complex) then the anion.
Isomerism in Coordination Compounds
Definition
Two or more substances having the same molecular formula but different structural or spatial arrangement are called isomers.
Structural Isomerism
Ionization isomerism
Compounds, which give different ions in solution although they have same composition are called ionization isomers.
Stereo Isomerism or Space Isomerism
It arises because of the different positions and arrangements of ligands in space around the metal ion.
Optical Isomerism
Optical isomers rotate the plane of polarised light in opposite directions. The two isomers are mirror images of each other. The two mirror images are non superimposable, do not possess a plane of symmetry. They are called dextro and laevo (d and l) rotatory depending upon the direction in which plane polarised light is rotated.
Bonding in Co-ordination Compounds
The first theory was called the Werner's theory of co-ordination compounds.
Determination of the Structure of a Complex Using Werner's Theory
Example: In the complex between CoCl3 and NH3 the ionizable chloride ions are found by precipitation with AgNO3. The remaining Cl and NH3 are present around the central Co in such directions so as to minimize repulsion and are linked by secondary valencies.
Valence Bond (VB) Theory
The metal ligand bond arises by donation of pair of electrons by ligands to the central metal atom.
Octahedral Complexes
Tetrahedral Complexes
The tetrahedral arrangement of four ligands surrounding the metal ions.
Square Planar Complexes
Inner and Outer Orbital Complexes
In the octahedral structure the central metal atom uses inner (n -1) d - orbitals or outer (n)d-orbitals for hybridization. This results in Inner orbital complex - involving (n-1)d orbitals for d2sp3 hybridization.
Drawbacks of Valence Bond Theory
1) It is a qualitative approach describing bonding in coordination compounds.
2) The theory fails to explain the optical absorption spectra and magnetic properties of coordination compounds.
Crystal Field Theory
Bonding in a complex ion is due to electrostatic interactions between the positively charged nucleus of the central metal ion and electrons in the ligands i.e., attractive as well as repulsive interactions.
Orientation of d-orbitals and Crystal Field Splitting
The five d-orbitals can be divided into two groups depending upon the nature of their orientation in space.
Crystal Field Splitting in Tetrahedral Complexes
The tetrahedral arrangement of four ligands surrounding the metal ions.
Crystal Field Splitting in Octahedral Complexes
The octahedral arrangement of six ligands surrounding the central metal ion is as shown in the figure.
Magnitude of Crystal Field Splitting
Different ligands differ in their ability to produce a splitting of the d-orbitals.
Stability of Co-ordination Compounds in Solution
Although a complex ion does not dissociate generally, it may sometimes dissociate to a small extent.
Factors Affecting Stability of a Complex Ion
1) Charge on the central metal ion: Greater the charge, more the stability.
2) Basic nature of ligand: More the basic nature, more is the stability.
Preparation of Co-ordination Compounds
By substitution method
In this method a stronger ligand replaces a weaker ligand.
Applications of Co-ordination Compounds
Estimation of hardness in water:
Ca2+ and Mg2+ ions water can be estimated using EDTA
Organometallic Compounds
Organometallic compounds and Grignard reagent.
Synthesis of Organometallic Compounds
Organometallic compounds and Grignard reagent.
Applications of Organometallic Compounds
Homogenous catalysis
Many reactions in solution are catalyzed by organometallic compounds or transition metal complexes.
Example: Wilkinson's catalyst
(Ph3P)3RhCl - Catalyst in hydrogenation of alkenes.
Conclusion
Transition elements are known for their ability to form many complex compounds. The complex compounds in which the metallic ion is surrounded by two or more ions or molecules are called coordination compounds, also known as complex compounds. The chapter has covered the nomenclature, isomerism, bonding, applications and stability of coordination compounds and organometallic compounds.
