Shri Sachhidanand Shikshan Sanstha’s Department of Chemistry Seminar OnCrystal Field Theory & its Application to Octahedral Complexes Saturday, 24th December 2011
Crystal Field TheoryThe above topic covered following points Introduction & Historical Development Assumptions of CFT Application to Octahedral Complex Factors Affecting CFSE Colour & Magnetic Properties of Complex
Introduction & Historical Development In 1704 first metal complexprussian blue (Artist’s Colour) wasdiscovered by Berlin Colour maker. In 1799 Tassaert discovered CobaltAmmine Coplexes. In 1893 Werner gave Co-ordinationtheory based on primary andsecondary valency. In 1927 Sidwick introduced concept Alfred Wernerof co-ordinate bond and EAN.
Modern Theories of Metal LigandBonding: VBT given by Pauling & Slater in 1935 CFT given by Brethe in 1929 & further developedby Van Vleck in 1932 LFT given by Van Vleck in 1935
Assumptions of CFT:The central Metal cation is surrounded by ligand which contain one or more lone pair of electrons.The ionic ligand (F-, Cl- etc.) are regarded as point charges and neutralmolecules (H2O, NH3 etc.) as point dipoles.The electrons of ligand does not enter metal orbital. Thus there is noorbital overlap takes place.The bonding between metal and ligand is purely electrostatic
Application of CFT to the formation of Octahedral complex: z L y L L M = Central Metal Ion n+ M n= Oxidation State of Metal Ion L= Ligand L L L x
Interaction of ligand with d – orbitals of metal ion g g Hypothetical Situation of d - orbital
Splitting of d – orbitalseg orbitals t2g orbitals
Factors Affecting on CFSE1)Nature of metal ion:a) Same metal ion with different charge e.g. [Co(H2O)6]3+ [Co(H2O)6]2+ Co3+ Co2+ Do=18,200 cm-1 > Do=9,300 cm-1b) Different metal ion with same charge e.g. [Co(H2O)6]2+ [Ni(H2O)6]2+ Co2+ (d7) Ni2+ (d8) Do=9,300 cm-1 > Do=8,500 cm-1.c) Different metal ion with different charge but same number of d – electrons e.g. [Cr (H2O)6]3+ [V(H2O)6]2+ Cr3+ (d3) V2+ (d3) Do= 17,400 cm-1 > Do= 12,400 cm-1d) Different metal ion with same charge but different principal quantum number. e.g. [Ir (NH3)6]3+ [Rh(NH3)6]3+ [Co(NH3)6]3+ Ir3+ (5d6) Rh3+ (4d6) Co3+ (3d6) n=5 n=4 n=3 Do= 41,000 cm-1 > Do= 34,000 cm-1 > Do= 23,000 cm-1
Factors Affecting on CFSE2)Nature of liganda) When the ligands are strong the energy gap between t2g and eg is more the distribution of electron does not takes place according to Hund’s rule. These are Low spin Complexes .b) When ligands are weak CFSE is relatively small hence five d- orbitals are Weak field suppose to be degenerate Strong field Ligands (red, high and therefore distribution Ligands (violet, low spin) spin) of electrons takes place according to Hund’s rule. These are High spin Complexes .
Factors Affecting on CFSE2)Nature of ligand :c) Distribution of electron in High spin and Low spinComplexes Strong field Weak field Strong field Weak field Strong field Weak field d1 d2 d3 d4 d5 d6 d7 1 u.e. 5 u.e. 0 u.e. 4 u.e. 1 u.e. 3 u.e. d8 d9 d10 2 u.e. 2 u.e. 1 u.e. 1 u.e. 0 u.e. 0 u.e.
Factors Affecting on CFSE2)Nature of ligand :When the common ligand are arranged in theorder of their increasing splitting power theseries is obtained called Spectrochemicalseries.
Application of CFT1) Colour of complexes :The transition metal complexes whose central metal ion contain partially filled d – orbitals are usually coloured in their solid and solution form.
d – d transition of electrone.g. [ Ti (H2O)6]3+ complex absorb green radiation at 5000 A0 , hencetransmitted the radiation of purple colour due to d – d transition ofelectron h =239kJ/mole
2) Magnetic Properties :a) in d1, d2, d3, d8, d9 complexes have same spin state and all are paramagnetic.b) The low spin d6 and d10 complexes are diamagnetic.c) In d4, d5, d6 and d7 the number of unpaired electron are different in high spin and low spin octahedral complexes . Strong field Weak field Strong field Weak field Strong field Weak field d1 d2 d3 d4 d5 d6 d7 1 u.e. 5 u.e. 0 u.e. 4 u.e. 1 u.e. 3 u.e. d8 d9 d10 2 u.e. 2 u.e. 1 u.e. 1 u.e. 0 u.e. 0 u.e.