Transcript Transition metal chemistry

Mysteries of polarized light




Enantiomers have identical properties except in one respect:
the rotation of the plane of polarization of light
Modern symbols are (+) and (-)
Days of yore d and l (dextrose)
Racemic mixture contains equal portions of the (+) and (-)
Transition metal ions and
spectroscopy
The color of a complex corresponds to
wavelengths of light that are not absorbed
by the complex. The observed color is
usually the complement of the color
absorbed. If all wavelengths of light are
absorbed, a complex appears black. If no
wavelengths of light are absorbed, a
complex appears white (colorless).
The artist’s wheel
Valence bond reprise




Valence bond theory is the simplest approach
to an orbital picture of covalent bonds
Each covalent bond is formed by an overlap of
atomic orbitals from each atom
The individual orbital identity is retained
The bond strength is proportional to the
amount of orbital overlap
Valence bond picture in complexes


In the conventional covalent bond, each atomic
orbital brings one electron with it
In the coordination complex, the ligand provides
both, while the metal orbital is empty
Geometry and hybridization

The original atomic orbitals are mixed together and
transformed into a new set of hybrid orbitals that
match the directional requirements for bonding
Coordination Geometry
number
Hybrid
orbitals
Example
2
Linear
sp
[Ag(NH3)2]+
4
Tetrahedral
sp3
[CoCl4]2-
4
Square Planar
dsp2
[Ni(CN)4]2-
6
Octahedral
d2sp3 or sp3d2
[Cr(H2O)6]3+
Electron configurations and
geometry


Electronic configuration of Co2+ is [Ar]3d7
Empty 4s and 4p orbitals are used for bonding in
tetrahedral complex
3d

4s
4p
Three unpaired d electrons mean that the Co2+ is
paramagnetic
Metal
electrons
ligand
electrons
Square planar




Electronic configuration of Ni2+ is 3d8
Square planar geometry is dsp2
Use of one d orbital forces pairing of the Ni d
electrons
Ni(CN)42- is diamagnetic
Octahedral complexes


Two options: d2sp3 or sp3d2
Same or different?

Low spin Co(CN)63- diamagnetic
3d

4s
4p
High spin CoF63- paramagnetic
3d
4s
4p
4d
Let’s spin



Why are some complexes high-spin and others
low spin?
Valence bond theory can describe the bonding
in complexes which is consistent with
observed magnetic properties; it cannot
explain why the ligands dictate one over the
other
Enter the crystal field theory…
The crystal field theory



The ligands are
considered negative
charges
The central ion is a
positive charge
The effect of the
electrostatic interactions
on the energies of the d
orbitals form the basis
of the theory
Relative positions of ligands and d
orbitals


dxy etc interact least with the ligands
dx2-y2 and dz2 interact most with the ligands in an
octahedral field
Orbitals
“miss”
the
ligands
Orbitals
“hit” the
ligands
Crystal field splitting


The orbitals that interact more strongly with the
ligands are raised in energy (electrostatic repulsion)
more than those that interact less strongly
The result is a splitting of the levels
Splitting and spectroscopy


Electrons in the incompletely filled d orbitals can be
excited from lower occupied to higher unoccupied
orbitals
The frequency of the absorption is proportional to the
crystal field splitting: Δ = hc/λ
Splitting and spectroscopy


Electrons in the incompletely filled d orbitals can be
excited from lower occupied to higher unoccupied
orbitals
The frequency of the absorption is proportional to the
crystal field splitting: Δ = hc/λ
Coat of many colours


Transition metal ions exhibit colours that vary
strongly with the type of ligand used
Spectrochemical series orders the ligands according
to the degree of crystal field splitting achieved

An absorption peak of 500 nm corresponds to
a crystal field splitting of
(6.626 x1034 J .s)(3.00 x108 m / s)
19

3
.
98
x
10
J
9
500 x10 m

On a molar basis
  (3.98 x10 19 J / ion )(6.02 x10 23 ion / mol )
 240kJ / mol
Spectrochemical series of ligands



Weak field
I-<Br-<Cl-<F-<H2O<NH3<en<CN Strong field
When the d orbitals are empty (d0) or full (d10),
the complexes are colourless – no d – d
transitions
The theory successfully accounts for observed
optical and magnetic properties
Comparison of Co(CN)63- andCoF63


Opposition of electron-electron repulsion and lower energy of
lower lying orbitals
High-spin complex: Δ is lower than P (electrons unpaired,
repulsion dominates)
Low-spin complex: Δ is higher than P (electrons pair, lower
energy of the lower orbitals)
Important note


Low-spin, high-spin dichotomy only occurs for
d4 – d7.
d1 – d3 and d8 – d10 only have one
configuration
Crystal field splitting in square
planar and tetrahedral complexes



Tetrahedral is inverse of octahedral
Δ is lower than in octahedral because of fewer ligands –
all complexes high-spin
Crystal field splitting in square planar is between the
high-lying d x  y and the d xy orbital
Square planar is favoured for d8 configuration
2

2