Principles of Chemical Science_Transition Metals Crystal Field Theory Part I - Lec28
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Course
Chemistry
Institution
Lecture 28: Transition Metals: Crystal Field Theory Part I
1. Introduction to Crystal Field Theory
2. Crystal Field Theory: Octahedral Case
3. Spectrochemical Series: the Ligand to the Metal Matters
Reading For Today: 16.8-16.11 in 4th and 5th editions
Reading for Lecture #29: same as above
Topic: I. Introduction to Crystal Field Theory
II. Crystal Field Theory: Octahedral Case
III. Spectrochemical Series
I. Introduction to Crystal Field Theory
Crystal field and ligand field theories were developed to explain the special features of
transition metal coordination complexes, including their beautiful colors and their
magnetic properties. Coordination complexes are often used as contrast agents for
magnetic resonance imaging (MRI) and other types of imaging.
Basic idea behind theories:
When a metal ion with a given oxidation number (Mn+, where M is a metal and
n+ is its oxidation number) is placed at the center of a coordination sphere defined by a
set of ligands, the energy levels of the d orbitals housing the metal electrons are
f rom those in the free metal ion.
Crystal field theory is based on an ionic description of the metal-ligand bond.
Ligand field theory includes covalent and ionic aspects of coordination. It is a more
powerful description of transition metal complexes. It is, however, beyond the scope of
this course. (Take 5.03 if you are interested in this topic).
Crystal Field Theory considers ligands as point charges and considers the
repulsion between the negative point charges and the d-orbitals, and even though this
theory is simple, a number of properties of transition metals can be explained.
• Ligand (L) point charges are directed right toward the dz2 and dx2-y2 orbitals of
metal (Mn+), resulting in a large repulsion.
• The dz2 and dx2-y2 orbitals are destabilized, and they are destabilized by the same
amount. dz2 and dx2-y2 are .
• The dz2 and dx2-y2 orbitals are destabilized relative to dxy, dyz, and dxz.
Z
Z
Z
L-
L-
L- Mn+ L- Y Y Y
X X
X
L- L-
dyz
dxy
dxz
• Ligand point charges are directed in between dxy, dxz, and dyz orbitals (not
directly toward them), resulting in repulsion.
• The dxy, dxz, and dyz orbitals are stabilized relative to dz2 and dx2-y2 orbitals, and
they are stabilized by the same amount.
• dxy, dxz, and dyz orbitals are degenerate with respect to each other.
2
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