Crystal & Ligand field theory
© Drs. XO, PHB, CWR
Crystal & Ligand Field Theory (10.2.1, 10.3)
CHEM 241
Fall 2014
TM − p.1
Ligand field theory (the MO version of crystal field theory) includes two main components: 1. What holds the complexes together:
The set of ligands are held to a metal ion by largely electrostatic forces
(although there really is a high degree of covalency). The forces arise from the positive charge on the metal ion and the electric dipoles of the ligands, whose negative ends are associated with the donor atom lone pairs. See Chem 341.
2. What gives the complexes their unique properties - colour and magnetic properties: This is connected to the effect of the electric field of the ligands on the metal d orbitals and the electrons in them. This effect is a repulsion, so it does not explain the bonding. But, it does explain the d orbital energies in a complex.
In what follows, we concentrate on the second component only.
© Drs. XO, PHB, CWR
Octahedral Ligand Field (10.3.1 too detailed)
Any electron in a d orbital is repelled by the ligand lone pairs
(like charges…) and therefore has its energy increased relative to what it would be in the free metal ion.
CHEM 241
Fall 2014
TM − p.2
The ligand lone pairs
(negatively charged) are located on the x,y,z coordinate axes.
IN ADDITION:
An electron in a dx2-y2 or dz2 orbital is closer to the ligands than if it were in a dxy, dxz or dyz orbital, and so is more repelled by the lone pairs. These two metal orbitals (dx2-y2 & dz2) orbitals are therefore at higher energy.
eg set (higher E) orbitals along x,y,z
(aimed at Ls)
t2g set (lower E) between x,y,z
Figures from: Kotz & Treichel,
Chemistry & Chemical Reactivity,
Thomson
© Drs. XO, PHB, CWR
CHEM 241
Fall 2014
TM − p.3
Oh Ligand Field – Splitting (10.3.2) eg t2g
Relative to the free ion, both sets of d orbitals are at higher energy, but the eg set are higher by an energy difference ∆o
Figures
Crystal & Ligand Field Theory (10.2.1, 10.3)
CHEM 241
Fall 2014
TM − p.1
Ligand field theory (the MO version of crystal field theory) includes two main components: 1. What holds the complexes together:
The set of ligands are held to a metal ion by largely electrostatic forces
(although there really is a high degree of covalency). The forces arise from the positive charge on the metal ion and the electric dipoles of the ligands, whose negative ends are associated with the donor atom lone pairs. See Chem 341.
2. What gives the complexes their unique properties - colour and magnetic properties: This is connected to the effect of the electric field of the ligands on the metal d orbitals and the electrons in them. This effect is a repulsion, so it does not explain the bonding. But, it does explain the d orbital energies in a complex.
In what follows, we concentrate on the second component only.
© Drs. XO, PHB, CWR
Octahedral Ligand Field (10.3.1 too detailed)
Any electron in a d orbital is repelled by the ligand lone pairs
(like charges…) and therefore has its energy increased relative to what it would be in the free metal ion.
CHEM 241
Fall 2014
TM − p.2
The ligand lone pairs
(negatively charged) are located on the x,y,z coordinate axes.
IN ADDITION:
An electron in a dx2-y2 or dz2 orbital is closer to the ligands than if it were in a dxy, dxz or dyz orbital, and so is more repelled by the lone pairs. These two metal orbitals (dx2-y2 & dz2) orbitals are therefore at higher energy.
eg set (higher E) orbitals along x,y,z
(aimed at Ls)
t2g set (lower E) between x,y,z
Figures from: Kotz & Treichel,
Chemistry & Chemical Reactivity,
Thomson
© Drs. XO, PHB, CWR
CHEM 241
Fall 2014
TM − p.3
Oh Ligand Field – Splitting (10.3.2) eg t2g
Relative to the free ion, both sets of d orbitals are at higher energy, but the eg set are higher by an energy difference ∆o
Figures