12th Chemistry Coordination Chemistry

The magnetic behaviour of the complexes can be explained using Valence Bond Theory (VB Theory).

[Cr(NH₃)₆]³⁺ – Paramagnetic

[Ni(CN)₄]²⁻ – Diamagnetic

1. Most of the elements exhibit two types of valence namely primary valence and secondary valence and each element tends to satisfy both the valences. In modern terminology, the primary valence is referred to as the oxidation state of the metal atom and the secondary valence as the coordination number. For example, according to Werner, the primary and secondary valences of cobalt are 3 and 6 respectively.
2. The primary valence of a metal ion is positive in most of the cases and zero in certain cases. They are always satisfied by negative ions. For example, in CoCl₃·6NH₃, the primary valence of Co is +3 and is satisfied by 3Cl⁻ ions.
3. The secondary valence is satisfied by negative ions, neutral molecules, positive ions or the combination of these. For example, in CoCl₃·6NH₃, the secondary valence of cobalt is 6 and is satisfied by six neutral ammonia molecules, whereas in CoCl₃·5NH₃, the secondary valence of cobalt is satisfied by five neutral ammonia molecules and a Cl⁻ ion.
4. According to Werner, there are two spheres of attraction around a metal atom/ion in a complex. The inner sphere is known as the coordination sphere and the groups present in this sphere are firmly attached to the metal. The outer sphere is called the ionisation sphere. The groups present in this sphere are loosely bound to the central metal ion and hence can be separated into ions upon dissolving the complex in a suitable solvent.

1. The ligand → metal bond is covalent in nature. It is formed by sharing of electrons between the central metal atom and the ligand.
2. Each ligand should have at least one filled orbital containing a lone pair of electrons.
3. In order to accommodate the electron pairs donated by the ligands, the central metal ion present in a complex provides required number of vacant orbitals.
4. These vacant orbitals of the central metal atom undergo hybridisation, the process of mixing of atomic orbitals of comparable energy to form equal number of new orbitals called hybridised orbitals with same energy.
5. The vacant hybridised orbitals of the central metal ion linearly overlap with filled orbitals of the ligands to form coordinate covalent sigma bonds between the metal and the ligand.
6. The hybridised orbitals are directional and their orientation in space gives a definite geometry to the complex ion.
7. In the octahedral complexes, if the (n−1)d orbitals are involved in hybridisation, then they are called inner orbital complexes or low spin complexes or spin paired complexes. If the nd orbitals are involved in hybridisation, then such complexes are called outer orbital or high spin or spin free complexes.
8. The complexes containing a central metal atom with unpaired electron(s) are paramagnetic. If all the electrons are paired, then the complexes will be diamagnetic.
9. Ligands such as CO, CN⁻, en, and NH₃ present in the complexes cause pairing of electrons present in the central metal atom. Such ligands are called strong field ligands.
10. Greater the overlapping between the ligand orbitals and the hybridised metal orbital, greater is the bond strength.

Coordination Number, Hybridisation and Geometry

The Crystal Field Stabilization Energy (CFSE) is defined as the energy difference of electronic configurations in the ligand field (E_LF) and the isotropic field / barycentre (E_iso).

Here,

• n_t₂g is the number of electrons in t_2g orbitals.
• n_eg is the number of electrons in e_g orbitals.
• n_p is the number of electron pairs in the ligand field.
• n′_p is the number of electron pairs in the isotropic field (barycentre).