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.

Coordination Chemistry – Ch. 5

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.

Introduction

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 complexes

As we have done for diatomic and

triatomic molecules containing s- and p-block elements, one

can generate a molecular orbital

diagram by considering the

overlap of ligand orbitals (s,p) with

those on the transition metal (d).

Metal complex molecular orbitals

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 bondingin 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 propertiesof transition metal complexes?

Crystal field theory

Isolated ligand environment octahedral field

Crystal field theory

[Fe(H2O)6]3+

Let’s take a closer look at the splitting of d orbitals in an octahedral field

Crystal field theory

L :

LL

L

: :

:

L

::

L

L

: ::

:

L

L L

dxy, dxz, dyz

dz 2dx - y 22

eg orbitals t2g orbitals

high energy low energy

Crystal field stabilization energy (CFSE) is the energy “gained” by putting valence electrons in the lower d-orbital set (t2g for octahedral complexes)

Crystal field theory

Ti(H2O)63+ Cr(H2O)6

3+ Cu(H2O)62+

What defines the magnitude of Δo?

Crystal field theory

Compare Δo across a set of metal complexes – what do these numbers mean?

Crystal field theory

What are the colors of these compounds?

Red light = 620 nm ≈ 16,000 cm-1

Blue light = 430 nm ≈ 23,000 cm-1

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 factorsthat influence the magnitude of Δo?

Consider the following octahedral complexes – where do the electrons reside?

Crystal field theory

Ti(H2O)63+ V(H2O)6

3+ Cr(H2O)63+ Mn(H2O)6

3+

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?

Crystal field theory

What impact does this have on where electrons reside in Mn(H2O)63+?

Strong field ligands Weak field ligands

4d and 5d transition metal complexes : high or low spin?

Crystal field theory

: :M

[Co(H2O)6]2+[Co(CN)6]4-

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.

Crystal field theory

I– < Br– < Cl– < NO3– < F– < OH– < H2O

< py < NH3 < NO2– < en < CN– < CO

How can we rationalize these trends?

Some ligands are anomalously high in the spectrochemical series,considering their weak Lewis basicity. Why?

Crystal field theory