Coordination Chemistry – Ch. 5
Big-picture perspective:
The interactions of the d orbitals with their surrounding chemical environment (ligands)
influences their energy levels, and this coupled with the variable number of electrons (and
incomplete filling of the d block) imparts on transition metals a unique, rich, and diverse
chemistry. We will focus on developing and using a simple model for describing the bonding in
octahedral transition metal complexes – a common molecular geometry – and then extend these
concepts to non-octahedral complexes. This will allow us to describe many of the unique
characteristics that are observed for transition metal complexes while setting the stage for more
sophisticated models.
Learning goals:
• Determine oxidation states and assign d-electron counts for transition metals in complexes.
• Derive the d-orbital splitting patterns for octahedral, elongated octahedral, square pyramidal,
square planar, and tetrahedral complexes.
• For octahedral and tetrahedral complexes, determine the number of unpaired electrons and
calculate the crystal field stabilization energy.
• Know the spectrochemical series, rationalize why different classes of ligands impact the crystal
field splitting energy as they do, and use it to predict high vs. low spin complexes.
Introduction
Transition metals are central to life – biology, medicine, energy, technology,
many things that society relies on to function
The interactions of the d orbitals with their surrounding chemical environment
(ligands) influences their energy levels, and this coupled with the variable
number of electrons (and incomplete filling of the d block) imparts on
transition metals a unique, rich, and diverse chemistry.
Metal complexes
We can describe metal complexes in several ways
VERY important: Determining the d-electron count
In order to predict their structures and understand their chemical reactivity
and properties, we need to first consider the bonding in transition metal
complexes and how the electrons are distributed
Metal complex molecular orbitals
As we have done for
diatomic and
triatomic molecules
containing s- and pblock elements, one
can generate a
molecular orbital
diagram by
considering the
overlap of ligand
orbitals (s,p) with
those on the
transition metal (d).
Crystal field theory
We can simplify this process (go backwards in sophistication) by focusing
exclusively on the d orbitals, since they contain the valence electrons.
Crystal field theory is a simple way of describing the bonding
in transition metal complexes.
Here, we consider the metal ion acceptors (positive charge) in a
“field” of the ligand electron pair donors (negative charge).
Crystal field theory
What is the origin of the different colors and magnetic properties
of transition metal complexes?
Crystal field theory
Isolated  ligand environment  octahedral field
[Fe(H2O)6]3+
Crystal field theory
Let’s take a closer look at the splitting of d orbitals in an octahedral field
eg orbitals
L
:
L
t2g orbitals
:
L
dxy, dxz, dyz
L
dx2 - y2
dz 2
L
L:
:L
L
:
:
L
high energy
L
low energy
Crystal field theory
Crystal field stabilization energy (CFSE) is the energy “gained” by putting
valence electrons in the lower d-orbital set (t2g for octahedral complexes)
Ti(H2O)63+
Cr(H2O)63+
Cu(H2O)62+
Crystal field theory
What defines the magnitude of Δo?
Crystal field theory
Compare Δo across a set of metal complexes – what do these numbers mean?
What are the colors of these compounds?
Red light = 620 nm ≈ 16,000 cm-1
Blue light = 430 nm ≈ 23,000 cm-1
Crystal field theory
2nd and 3rd row transition metals (4d, 5d elements) always have larger values of
Δo than 1st row transition metals (3d elements) – why?
Crystal field theory
Increasing oxidation state of the metal also increases Δo – why?
Crystal field theory
Ligands influence the magnitude of Δo – why?
Crystal field theory
What are the consequences of these multiple factors
that influence the magnitude of Δo?
Consider the following octahedral complexes – where do the electrons reside?
Ti(H2O)63+
V(H2O)63+
Cr(H2O)63+
Mn(H2O)63+
Crystal field theory
There is a small energy penalty associated with pairing electrons
(electrostatic repulsion of electrons in the same orbital)
For 3d elements, p (pairing energy) is approximately constant: ____________
How does the magnitude of p compare with typical magnitudes of Δo?
What impact does this have on where electrons reside in Mn(H2O)63+?
Crystal field theory
Strong field ligands
M
:
[Co(CN)6]4-
Weak field ligands
:
[Co(H2O)6]2+
4d and 5d transition metal complexes : high or low spin?
Crystal field theory
The spectrochemical series places ligands in order of increasing field strength,
with a somewhat arbitrary cutoff between those we consider to be generally
strong field ligands and those we consider to be generally weak field ligands.
I–
<
<
Br–
py
<
<
Cl–
NH3 <
<
NO3–
NO2–
<
<
F–
en
<
<
OH–
CN–
How can we rationalize these trends?
<
<
H2O
CO
Crystal field theory
Some ligands are anomalously high in the spectrochemical series,
considering their weak Lewis basicity. Why?