Lecture Twenty - Coordination chemistry and crystal field theory Flashcards
What gives gem stones their colour?
Trace amounts of transition metals.
Explain valance bond theory with regards to d orbitals.
Complex formation is explianed by donation of electron pair(s) (by lewis bases) to a metal ion (lewis acid) to form a coordinate covalent bond.
Hybrid orbitals of equal energy provide no explanation for the colour and magnetic properties of transition metal complexes.
Explain crystal field theory.
This is designed to explain colour and magenrism but not the nature of metal ligand bonding.
It describes how the energies of the d orbitals on the metal ion are affected as the ligands approach.
Important assumption:
Complexes result from electrostatic attractions between metal cation and negative charges or dipoles on ligands (i.e. not covalent in character).
What happens to the energy of the metal d orbitals when six ligands approach?
Consider that the ligands (balls of negative charge) can approach the metal centre (uniform positive charge) in two ways:
1) Directly along the x, y and z axis.
2) Between the axis.
If the ligands are positioned on the x, y and z axis, then there will be greater ligand - d-orbital repulsion as the electron densities from the metal and the ligand will repel one another.
If the ligands are positioned between the axis, there will be less repulsion as electrons from the metal centre and ligads can exist further from one another.
Two energy levels for d orbitals:
The dx^2-y^2 and dz^2 orbitals have a higher energie, as they have greater repulsion forces acting against the ligands electrons.
The dxy, dxz and dyz orbitals have lower energy as the electrons in the metal centre and the ligands repel one another less.
What are the steps to complex formation?
The five d-orbitals increase in energy relative to free Mn+ ion due to greater d-electrons - ligand repulsion.
However they are not equally affected.
Direct head to head repulsion leads to a higher energy (more unstable) situation for dz^2 or dx^2-y^2 orbitals.
Tangential (more side on) repulsion leads to a relative lower energy situation for the other d orbitals.
The energy difference is Δoct or Δo, where Δo = crystal field splitting energy.
What are the names given to the different energied d orbitals?
eg orbitals go up in energy by 3/5 Δo.
t2g orbitals go down in energy by 2/5 Δo.
