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Color of Transition Color of Transition Metal Ions in Water Metal Ions in Water SolutionSolution

Dr Dragica MinićDr Dragica Minić

August 17, 2005August 17, 2005

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Position of Transition Metals in the Periodic Table

The elements in the periodic table are often divided into four categories:

The transition metals are the metallic elements that serve as a bridge, or transition, between the two sides of the table.

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Let me consider the first transition metal series, elements from Let me consider the first transition metal series, elements from scandium (Z=21) to copper (Z=29). This metals either have scandium (Z=21) to copper (Z=29). This metals either have incompletely filled incompletely filled d subshellsd subshells or readily give rise to cations that or readily give rise to cations that have incompletely filled have incompletely filled d subshellsd subshells..

Along this series the added electrons are placed in theAlong this series the added electrons are placed in the 3d 3d orbitalsorbitals according to Hund's rule: according to Hund's rule: to to ..

1. irregulation:1. irregulation:electron configuration of chromium (Z=24) is electron configuration of chromium (Z=24) is

and not ;and not ;

2. irregulation:2. irregulation:electron configuration of copper (Z=29) iselectron configuration of copper (Z=29) is

and not .and not .

1 5[Ar] 4s 3d

2 4[Ar] 4s 3d

1 10[Ar] 4s 3d

2 9[Ar] 4s 3d

2 1[Ar]4s 3d1 9[Ar]4s 3d

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The reason is that slightly greater stability is associated with the half-The reason is that slightly greater stability is associated with the half-filled filled and completely filled and completely filled susubbshells.shells.

Electrons in the same subshells have equal energy but different Electrons in the same subshells have equal energy but different spatial distribution. Consequently, their shielding of one another is spatial distribution. Consequently, their shielding of one another is relatively small and the electronrelatively small and the electronss are more strongly attracted by the are more strongly attracted by the nucleus when they have the nucleus when they have the configuration.configuration.

Consequently the orbital diagram for Cr isConsequently the orbital diagram for Cr is

and orbital diagram for Cu is and orbital diagram for Cu is

53d

103d

53d

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Transition metals have a distinct tendency to form complex ions often with neutral molecules water. Many transition metal ions and complex ions and anions containing transition metals are distinctively colored. The origin of the color is electronic transition involving d electrons.

In anhidrous form is blue

but in the hydrated form is pink.

2CoCl

2 2CoCl 6H O

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Colors of some of the first-row transitionColors of some of the first-row transition metal ions in water metal ions in water solution:solution:

3+ 3+ 2+ 2+ 2+ 2+ 2+Ti , Cr , Mn , Fe , Co , Ni , Cu .

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Colors of aqueous solutions of compounds containing vanadium in different oxidations states (V, IV, III and II).

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For explanation of the color transition metals ions in water For explanation of the color transition metals ions in water solution existing in complex form, we need to consider the solution existing in complex form, we need to consider the bonding in complex ions of transition metals;bonding in complex ions of transition metals;

There are three theories:There are three theories:

The The valence-bond theoryvalence-bond theory

TheThe crystal field theory crystal field theory

The The ligand-field theoryligand-field theory. .

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Crystal Field TheoryCrystal Field Theory

This theory tried to describe the effect of the electrical field of This theory tried to describe the effect of the electrical field of neighboring ions on the energies of the valence orbitals of an ion in a neighboring ions on the energies of the valence orbitals of an ion in a crystal.crystal.

Question: Question: What effect will the surrounding ligands have on the energies five What effect will the surrounding ligands have on the energies five d d

orbitalsorbitals of metal’s atoms? of metal’s atoms?

Answer:Answer: d d oorbitalsrbitals of of ligands have various orientations and in absence external ligands have various orientations and in absence external

disturbance have the same endisturbance have the same eneergy.rgy.

When such metal ion is in the center of octahedron surrounding by six When such metal ion is in the center of octahedron surrounding by six lone pair of electrons lone pair of electrons of ligands, of ligands, two types of electrostatic interaction two types of electrostatic interaction exist:exist:

TThe attraction between positive metal ion and negatively charged he attraction between positive metal ion and negatively charged ligand ligand (this force holds the ligands to metal in complex)(this force holds the ligands to metal in complex)..

TThe repulsion between lonhe repulsion between lonee pairs on ligand and electron in pairs on ligand and electron in d d orbitalsorbitals metals. metals.

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Electron in Electron in and and orbitals lying along orbitals lying along z, z, x,x, and and y axes, y axes,

experience a greater repulsion from ligands than electrons in other experience a greater repulsion from ligands than electrons in other orbitalsorbitals::

x2 23dy2x

3d

1111

and as result the equality energy of and as result the equality energy of 5 5 d orbitalsd orbitals is nullified to give is nullified to give two high-lying levelstwo high-lying levels and and three low-lying energy levels.three low-lying energy levels. The energy The energy difference between two sets difference between two sets d orbitalsd orbitals,, is called is called crystal field crystal field splittingsplitting; ; its magnitude depends on the metal and the nature of the its magnitude depends on the metal and the nature of the ligandsligands..

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A substance appears colored because it absorbs light at one or more A substance appears colored because it absorbs light at one or more wave-lengths in the visible part of the electromagnetic spectrum (wave-lengths in the visible part of the electromagnetic spectrum (400400 to to 700700 nm) and reflects or transmits the others. nm) and reflects or transmits the others.

Each wavelength of light in this region appears as a different color. A Each wavelength of light in this region appears as a different color. A combination of all colors appears white, absence of lightwaves combination of all colors appears white, absence of lightwaves appears black.appears black.

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Relationship between wavelenght absorbed and color observed

Wavelenght absorbed, nmWavelenght absorbed, nm Color observedColor observed

400 (violet)400 (violet)450 (blue)450 (blue)

490 (blue-green)490 (blue-green)570 (yellow green)570 (yellow green)

580 (yellow)580 (yellow)600 (orange)600 (orange)

650 (red)650 (red)

Greenish yellowGreenish yellowOrange-yellowOrange-yellow

RedRedPurplePurple

Dark blueDark blueBlueBlue

GreenGreen

When we say that the hydrated cupric ion is blue, we mean that each ion absorbs a photon a wavelength of about 600 nm (orange light), the transmitted light appears blue to our eyes.

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QuantumQuantum--mechanicalmechanical descriptiondescription

Absorption of light may occur Absorption of light may occur when the frequency of the when the frequency of the incoming photon, multiplied by incoming photon, multiplied by the Plank constant, is equal to the Plank constant, is equal to the difference in energy between the difference in energy between these two levelthese two levelss..

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ExampleExample::

Hydrated cupric ion absorbs photon whose frequency is about Hydrated cupric ion absorbs photon whose frequency is about Hz Hz or or 600600 nm. nm.

The energy change involved in the electron transition that The energy change involved in the electron transition that occurs in the cupric ion is:occurs in the cupric ion is:

When we say that the hydrated cupric ion is When we say that the hydrated cupric ion is blueblue, we mean that , we mean that each ioneach ion absorbs a photon wavelength of about 600 nm absorbs a photon wavelength of about 600 nm (orange light), (orange light), the transmitted light appears the transmitted light appears blueblue to our eyes. to our eyes.

34 14 -1 -19(6.63 10 J s)(5 10 s ) 3 10 JE h

2+2 6Cu[H O]

145 10

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Example Example

Ti has an outer configuration of , so that will be a Ti has an outer configuration of , so that will be a

ionion. This means that in its ground state, one electron will . This means that in its ground state, one electron will occupy the lower group of occupy the lower group of d orbitalsd orbitals, and the upper group will be , and the upper group will be emptyempty, after absorption of energy, after absorption of energy the lower groups the lower groups d orbitalsd orbitals will will be empty.be empty.

3+2 6Ti[H O]

2 24s 3d3+Ti

1d

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ion absorbs light in the visible region; the wavelenght corresponding to maximum absorption is 498 nm.

Crystal field splitting is:

This is the energy required to excite one ion.

-34 8-19

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(6.63 10 Js)(3 10 m/s)3.99 10 J=240 kJ/mol

498 10 m

hch

3+2 6Ti[H O]

3+2 6Ti[H O]

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The d-orbital splitting in this case is 240 kJ per mole which corresponds to light of blue-green color; absorption of this light promotes the electron to the upper set of d orbitals, which represents the exited state of the complex. If we illuminate a solution of with white light, the blue-green light is absorbed and the solution appears violet in color.

3+2 6Ti[H O]

1919

                                                                                                      

                                                                           

Position of Transition Metals in the Periodic Table

The elements in the periodic table are often divided into four categories:

The transition metals are the metallic elements that serve as a bridge, or transition, between the two sides of the table.