Shri Sachhidanand Shikshan Sanstha’s Department of Chemistry Seminar On Crystal Field Theory & its Application to Octahedral Complexes Saturday, 24th December 2011 Crystal Field Theory The 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 complex prussian blue (Artist’s Colour) was discovered by Berlin Colour maker. In 1799 Tassaert discovered Cobalt Ammine Coplexes. In 1893 Werner gave Co-ordination theory based on primary and secondary valency. In 1927 Sidwick introduced concept of co-ordinate bond and EAN. Alfred Werner Modern Theories Bonding: of Metal Ligand VBT given by Pauling & Slater in 1935 CFT given by Brethe in 1929 & further developed by 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 neutral molecules (H2O, NH3 etc.) as point dipoles. The electrons of ligand does not enter metal orbital. Thus there is no orbital 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= Oxidation State of Metal Ion L= Ligand n+ M L L L x Interaction of ligand with d – orbitals of metal ion g g Hypothetical Situation of d - orbital Splitting of d – orbitals eg orbitals t2g orbitals Factors Affecting on CFSE 1)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-1 b) 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-1 d) 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 CFSE 2)Nature of ligand a) 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 Strong field suppose to be degenerate and therefore distribution Ligands (violet, low spin) of electrons takes place according to Hund’s rule. These are High spin Complexes . Weak field Ligands (red, high spin) Factors Affecting on CFSE 2)Nature of ligand :c) Distribution of electron in High spin and Low spin Complexes Strong field Weak field Strong field Weak field d1 d2 d5 d4 1 u.e. 5 u.e. 2 u.e. 1 u.e. Weak field d3 d6 0 u.e. 4 u.e. d7 1 u.e. d9 d8 2 u.e. Strong field 1 u.e. 3 u.e. d10 0 u.e. 0 u.e. Factors Affecting on CFSE 2)Nature of ligand : When the common ligand are arranged in the order of their increasing splitting power the series is obtained called Spectrochemical series. Application of CFT 1) 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 electron e.g. [ Ti (H2O)6]3+ complex absorb green radiation at 5000 A0 , hence transmitted the radiation of purple colour due to d – d transition of electron 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 d1 d2 d5 d4 1 u.e. 5 u.e. 2 u.e. 1 u.e. Weak field d3 d6 0 u.e. 4 u.e. d7 1 u.e. d9 d8 2 u.e. Strong field 1 u.e. 3 u.e. d10 0 u.e. 0 u.e.
