Spectrochemical series

Spectrochemical Series. Crystal Field Splitting. Strong Field Ligands and Weak Field Ligands.

1. Spectrochemical Series :- INTRODUCTION CRYSTAL FIELD SPLITTING CRYSTAL FIELD SPLITTING PARAMETER OCTAHEDRAL SPLITTING TETRAHEDRAL SPLITTING STRONG AND WEAK LIGANDS

2. Introduction:- Spectrochemical series is a list of ligands arranged in order of their field strength. The ligands cyanide and co are considered as strong field ligands and the halides are weak field ligands. Ligands such as water (h2o) and ammonia (nh3) are medium field ligand. The spectrochemical series places ligands in order of increasing (Δ). (Δ) is known as Crystal Field Splitting parameter (CFSP) which tells us about splitting of d orbital.

3. CRYSTAL FIELD SPLITTING:- In order to understand the importance of spectrochemical series , we need to understand Crystal Field Theory which explains how ligands affect the energy levels of degenerate orbitals. In this theory ligands are considered to be point negative charges and metal ions are assumed to occupy appropriate atomic orbitals of the metal.

4. It considers two types of interactions as follow:- Electrostatic attraction between the positive metal ion nucleus and the negatively charged electrons of the ligands. Electrostatic repulsion between valence shell of the metal and the ligand electrons. Electrostatic attraction will result in overall decrease of energy and repulsion will result in overall increase in energy .

5. Crystal Field Splitting Parameter:- The size of split is called crystal field splitting parameter or (CFSP) and is represented by Δ. This is affected by type of ligands as Strong Field Ligand repel the metal electrons very effectively so in this case large splits will produce. Weak Field Ligands only partly repel the electrons result in smaller splits. On the other hand size and number of energy splits also depend on geometry of the complex. In order to understand splitting we will now study Octahedral splits and Tetrahedral splits.

6. Octahedral Splits:- If we look at the diagram of octahedral complex have six ligands and are symmetrical and there are 5 d orbitals:- dxy,dyz,dxz,dx2-y2 and dz2. (co(f6)). Co

7. Now electrons in octahedral fields will have two possible energy levels. Five different orbitals can be simplified into two set of orbitals: HIGH ENERGY eg. LOWER ENERGY t2g. Now ligand strength will effect how electrons fill these orbitals. Filling OF Orbitals:- When surrounded by weak field ligands electrons fill the orbitals as normal: one electron fills each orbital before a second joins in. The Complexes have high spin. Strong field ligands push electrons to lower energy orbitals so that t2g orbitals fill before starting eg. These complexes have low spin.

8. Tetrahedral splits:- In case of tetrahedral there are four ligands attached to central metal atom. Two energy levels eg lower and t2g higher energy level here splitting will be according to diagram:-

9. LIGANDS :- Strong Field Ligand Strong field ligands result in greater splitting of the crystal field. They form complexes with low spin. They are diamagnetic in nature. C,N and P are usually strong ligands. Example : Co and cyanide ion. Weak Field ligand Weak field ligands include less splitting of the crystal fields. They form complexes with high spins. They are paramagnetic in nature. Halides, Oxygen and Sulphur are usually weak ligands. Example : chloride and fluoride ions.