Oxidation states PDF

Preview

Summary

Oxidation states notes...

Description

Summary of Lecture 7 : Oxidation states, electronic configurations and crystal field theory - How to determine the oxidation state and d-configuration for TM/TM complexes. - Ionic model of bonding – and its limitations - Introduction to Crystal Field Theory - Derived d-orbital splitting diagram for an octahedral complex eg

dxy dxz dyz t2g dz2 dx2-y2 Lecture 8 - Transition Metal Chemistry 4 This lecture develops crystal field theory and applies it to octahedral and tetrahedral complexes. At the end of this lecture you should be able to: 1. Show the energy change of 3d orbitals in an octahedral field, be able to label the d-orbital levels and attach the symmetry labels eg and t2g. 2. Apply the methods for the determination of the oxidation state, 3d configuration and the construction of d-orbital splitting diagrams to determine the ground state electron configurations of simple octahedral complexes. 3. Understand what is meant by the crystal field splitting parameter, Δo. 4. Calculate the crystal field stabilization energy for octahedral complexes. 5. Work out the number of unpaired electrons from d-orbital splitting diagrams. 6. Draw the electronic configurations for high and low spin octahedral complexes with d4 d7 configurations. 7. Understand what factors influence the size of Δo and predict high spin or low spin configurations for Fe(II) and Fe(III) complexes. 8. Show the energy change of 3d orbitals in an tetrahedral field, be able to label the dorbital levels and attach the symmetry labels e and t2. 9. Understand what is meant by the crystal field splitting parameter, Δt. 10. Compare the splitting diagrams for octahedral and tetrahedral ligand fields, know that Δt ≈ 4/9 Δo. 11. Apply the methods for the determination of the oxidation state, 3d configuration and the construction of d-orbital splitting diagrams to determine the ground state electron configurations of simple tetrahedral complexes.

Reading: d-Block Chemistry - Winter (Oxford Chemistry Primer). Chapter 5.

1 Octahedral Splitting d1 - d3 and d8- d10 Configurations 3d +0.6Δo -0.4Δo t2g Spherical field Octahedral field along x,y,z axes Stabilized relative to spherical field What is the crystal field stabilization energy (CFSE)for [Cr(OH2)6]3+? From lecture 7 Cr(III) 3d3 CFSE = 3x -0.4Δo = -1.2Δo What is the crystal field stabilization energy (CFSE) for [Ni(OH2)6]Cl2 and what is the number of unpaired electrons in this compound? dxy dxz dyz d z2 d x2-y2 e Destabilized relative to spherical field

g

2 Octahedral Splitting d4-d7 Configurations 1. Example d4 configuration:

+0.6Δo -0.4Δo +0.6Δo -0.4Δo We have to pair up two electrons. This costs us energy +P RELATIVE to the symmetrical field. CFSE = -1.6Δo+ P Number of unpaired spins = 2 LOW SPIN

CFSE = -0.6Δo Number of unpaired spins = 4 HIGH SPIN 2. The electronic configuration adopted depends on the magnitude of Δo. The pairing up between two electrons is constant. Big Δo will outweigh the pairing energy and the complex will be low spin. A small Δo will lead to high spin complexes.

3 What Factors Influence the Size of Δo? The oxidation state of the metal.

1. 2. The ligand: The greater the oxidation state of the metal, the greater Δo.

CN- > bipy > en = NH > H O > Cl- 32 Δo big Δo small 3. Determine (a) Which of the following complexes are high spin and which are low spin. (b) The number of unpaired d electrons by drawing octahedral splitting diagrams. K3[Fe(CN)6] K4[Fe(CN)6] [Fe(H2O)6]Cl3 [Fe(H2O)6]SO4

4 Tetrahedral Splitting 1. We need to look at the effect of a tetrahedral ligand field on the 3d orbitals. 2. Draw a tetrahedron in a cube: z

3. Inspect the shapes of the 3d orbitals to see which interact the greatest with the ligand electrons. x y

Tetrahedral Splitting 5 4. Inspection reveals that although no 3d orbitals point directly at the ligands, dxy, dxz and dyz have more interaction with the ligands than dz2 and dx2-y2 do. 5. Splitting diagram: dxy dxz dyz Δt -0.6Δt e t2 +0.4Δt

3d

Spherical field Tetrahedral field 6. Differences to octahedral diagram: (i) Inverted energy order 2. (ii) e and t2 symmetry labels NOT eg and t2g - no centre of symmetry in Td. 3. (iii) Δt < Δo (Δt ≈ 4/9Δo) dz2 dx2-y2

6 Tetrahedral Splitting 5. For the tetrahedral complexes in this course, Δt is never large enough to offset the pairing energy, P. Consequently, all tetrahedral complexes in this course are high spin. 6. Example: Draw the 3d orbital splitting diagram for: (i) [NH4]2[NiCl4]. (ii) [Cu(py)4]+ Show which orbitals the 3d electrons go into for each complex. Determine the number of unpaired 3d electrons in each complex....