transition metals - ms. suchy's science site · spectrochemical series • molecular orbital...

Transition Metals

Monday09/21/15

Monday, September 21, 15

Agenda

Start Topic 13.2 - Colored Complexes

Topic 13.1 - First Row Transition Elements handout (this will be classwork for Wednesday & Thursday)

We will go over homework Thursday in class

Last 10 minutes of class - discuss/research ideas for IA


The Periodic Table - The Transition Metals

First-row d-block elementsColored complexes

Ms. Thompson - HL ChemistryWooster High School


Topic 13.2Colored complexes

• The d-sublevel splits into two sets of orbitals of different energy in a complex ion

• Complexes of d-block elements are colored, as light is absorbed when an electron is excited between the d orbitals

• The color absorbed is complementary to the colored observed.


Colored complexes

Nature of science

• Models and theories - the color of transition metal complexes can be explained through the use of models and theories based on how electrons are distributed in d-orbitals.

• Transdisciplinary - Color is linked to symmetry can be explored in the sciences, architecture and the arts.


Colored complexes

Energy of d orbitals

• In an isolated atom, d orbitals have the same energy but in a complex ion, they split into two sublevels. The electronic transitions between these sublevels leads to an absorption and emission of photons of visible light, which are responsible for the color of the complex.


Colored complexes

Theories on complexes

• Valence bond theory (VBT)• Developed by Linus Pauling (1930’s) that had hybridization as its basis. Rarely

used• Crystal field theory (CFT)

• Based on electrostatic model. Cannot explain order of ligands in the spectrochemical series

• Molecular orbital theory (MOT)• Considers covalent interactions between transition metal centers and ligands.

• Ligand field theory (LFT)• Extension of CFT but not based on electrostatic model. Combination of CFT

and MOT models - though bonding description is more detailed and considers frontier orbitals.

• Angular overlap model• Relative sizes of orbital energies are estimated in molecular orbital

calculations.


Colored complexes

Theories on complexes

• These models help us explain and understand characteristics of transition metal complexes• i.e. color, electronic spectra, and magnetic properties

• For this course - we will only consider the crystal field theory (CFT)


Colored complexes

Crystal field theory (CFT)

• d-sublevel consists of five d-orbitals• Three lie 45º to the cartesian plane and two point along

cartesian plane


Colored complexes


• Based on electrostatic model where ligands are considered point charges that surround the metal cation, Mn+.• If electrostatic field created by ligands is:

• isotropic (spherically symmetrical) the energies of the d orbitals will remain degenerate and increase uniformly.

• octahedral then the d orbitals will split into two sets of degenerate energy.

isotropic set (t2g) octahedral set (eg)


Colored complexes


• isotropic set (t2g)• Decrease in energy and become stabilized• Stabilization arises from electron density lying 45º to the

cartesian axes.• octahedral set (eg)

• Increase in energy and become destabilized• Destabilization arises from electron density directed along

the cartesian axes.• The energy separation between the two split degenerate sets of

orbitals is defined as ∆o, the crystal field splitting energy.


Colored complexes

Factors that affect crystal field splitting energy

• identity of the metal ion• oxidation state of the metal ion• nature of ligands• geometry of the complex ion


Colored complexes

Identity of the metal ion

• The identity of the metal ion can influence the extent of the crystal field splitting. In general, ∆o increases descending a group with the metal in the same oxidation state.

Group 9 complex ∆o / cm-1

[Co(NH3)6]3+ 22900

[Rb(NH3)6]3+ 34100

[Ir(NH3)6]3+ 41100


Colored complexes

Oxidation state of the metal ion

• For a given metal, ∆o increases as the oxidation state increases. • As the charge on the metal increases, the distances between the metal

and ligands decrease resulting in a better overlap between the metal orbitals and the ligand orbitals.

Complex ∆o / cm-1

[Co(NH3)6]2+ 10200

[Rb(NH3)6]3+ 22900


Colored complexes

Nature of the ligands

• Ligands may have different charge densities. The greater the charge density, the greater the crystal field splitting.

• R. Tsuchida suggested ligands can be arranged into a spectrochemical series, based on order of increasing ∆o.

Complex ∆o / cm-1

[Co(H2O)6]3+ 18200

[Co(NH3)6]3+ 22900


Colured complexes

Explanation of the color of transition complexes{Transition elements

Transition elements contain d orbitals. Mg,

Na, and Ca do not contain d orbitals but s orbitals so they do

not form colored complexes


Colored complexes

Explanation of the color of transition complexes

• [Cu(H2O)6]2+ is blue in color due to the Cu2+ ion transmitting visible wavelengths from 400-550nm and absorbing wavelengths 550 onwards.


Coloured complexes

Explanation of the colour of transition

complexes

• White light contains all wavelengths in visible spectrum.

• The colour wheel can be used to determine the colour of light transmitted and the complementary colour of absorbed light.


Coloured complexes

Why wavelengths of visible light are absorbed when passed through a solution with a transition

element

• When ligands bond to the central metal ion, there is repulsion between the electrons in the ligands and the electrons in the d orbitals of the metal ion

• This repulsion causes the five d orbitals to split into two different sets; two with higher energy and three with lower energy.

• The energy difference between the two sets of d orbitals corresponds to the wavelength of visible light.


Coloured complexes


element

• Ions of transition elements have incomplete d orbitals • Electrons can transition from the lower set to the higher set of

d orbitals.• In [Cu(H2O)6]2+, the ∆o required to promote an electron to the

higher set of d orbitals corresponds to a wavelength of 650-750nm.


Coloured complexes


element

In [Cu(H2O)6]2+, the ∆o required to promote an electron to the higher set of d orbitals corresponds to a wavelength of

650-750nm.Monday, September 21, 15

Topic 13.2Coloured complexes

➡The d-sublevel splits into two sets of orbitals of different energy in a complex ion

➡Complexes of d-block elements are coloured, as light is absorbed when an electron is excited between the d orbitals

➡The colour absorbed is complementary to the coloured observed.


transition metals - ms. suchy's science site · spectrochemical series • molecular orbital...

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